Android35个内容

Jetpack5个内容

Lifecycle1个内容

Android架构组件5个内容

一、Android Jetpack 介绍

1.1 Jetpack是啥

官方定义如下:

Jetpack 是一个由多个库组成的套件，可帮助开发者遵循最佳做法，减少样板代码并编写可在各种 Android 版本和设备中一致运行的代码，让开发者精力集中编写重要的代码。

JetPack更多是一种概念和态度，它是谷歌开发的非Android Framework SDK自带、但同时是Android开发必备的/推荐的SDK/开发规范合集。相当于Google把自己的Android生态重新整理了一番，确立了Android未来的开发大方向。

使用Jetpack有如下好处：

遵循最佳做法，Android Jetpack 组件采用最新的设计方法构建，具有向后兼容性，可以减少崩溃和内存泄露。
消除样板代码，Android Jetpack 可以管理各种繁琐的 Activity（如后台任务、导航和生命周期管理），以便您可以专注于打造出色的应用。
减少不一致，这些库可在各种 Android 版本和设备中以一致的方式运作，助您降低复杂性。

[图片上传失败...(image-e127e-1683613462826)]

Jetpack原意为喷气背包，Android背上Jetpack后就直冲云霄，这很形象了~

也就是，Jetpack是帮助开发者高效开发应用的工具集。那么这一工具包含了哪些内容呢？

1.2 Jetpack分类

分类如下图（现在官网已经找不到这个图了）：

[图片上传失败...(image-ba7c3e-1683613462826)]

Android Jetpack 组件覆盖以下 4 个方面：架构（Architecture）、基础（Foundation）、行为（Behavior）、界面（UI）。

真正的精华主要是Architecture，全称是Android Architecture Component（AAC），即Android架构组件。

其包括比较成功的Lifecycle、LiveData、ViewModel，同时也是我们使用MVVM模式的最好框架工具，可以组合使用，也可以单独使用。

以上基本都是官网的介绍，我们主要目标就是掌握AAC的组件，深入理解进而运用到MVVM架构中。

如题，我们学习Jetpack的重点就是AAC，这篇就从基础的Lifecycle讲起。

二、Lifecycle

Lifecycle，顾名思义，是用于帮助开发者管理Activity和Fragment 的生命周期，它是LiveData和ViewModel的基础。下面就先介绍为何及如何使用Lifecycle。

2.1 Lifecycle之前

官方文档有个例子来说明使用Lifecycle之前是如何生命周期管理的：

假设我们有一个在屏幕上显示设备位置的 Activity。常见的实现可能如下所示：

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n3808" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> class MyLocationListener {
public MyLocationListener(Context context, Callback callback) {
// ...
}

void start() {
// 连接系统定位服务
}

void stop() {
// 断开系统定位服务
}
}

class MyActivity extends AppCompatActivity {
private MyLocationListener myLocationListener;

@Override
public void onCreate(...) {
myLocationListener = new MyLocationListener(this, (location) -> {
// 更新UI
});
}

@Override
public void onStart() {
super.onStart();
myLocationListener.start();
// 管理其他需要响应activity生命周期的组件
}

@Override
public void onStop() {
super.onStop();
myLocationListener.stop();
// 管理其他需要响应activity生命周期的组件
}
}</pre>

虽然此示例看起来没问题，但在真实的应用中，最终会有太多管理界面和其他组件的调用，以响应生命周期的当前状态。管理多个组件会在生命周期方法（如 onStart() 和 onStop()）中放置大量的代码，这使得它们难以维护。

此外，无法保证组件会在 Activity 或 Fragment 停止之前启动myLocationListener。在我们需要执行长时间运行的操作（如 onStart() 中的某种配置检查）时尤其如此。在这种情况下，myLocationListener的onStop() 方法会在 onStart() 之前调用，这使得组件留存的时间比所需的时间要长，从而导致内次泄漏。如下：

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n3811" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> class MyActivity extends AppCompatActivity {
private MyLocationListener myLocationListener;

public void onCreate(...) {
myLocationListener = new MyLocationListener(this, location -> {
// 更新 UI
});
}

@Override
public void onStart() {
super.onStart();
Util.checkUserStatus(result -> {
//如果checkUserStatus耗时较长，在activity停止后才回调，那么myLocationListener启动后就没办法走stop()方法了，
//又因为myLocationListener持有activity，所以会造成内存泄漏。
if (result) {
myLocationListener.start();
}
});
}

@Override
public void onStop() {
super.onStop();
myLocationListener.stop();
}
} </pre>

即2个问题点：

activity的生命周期内有大量管理组件的代码，难以维护。
无法保证组件会在 Activity/Fragment停止后不执行启动

Lifecycle库则可以以弹性和隔离的方式解决这些问题。

2.2 Lifecycle的使用

Lifecycle是一个库，也包含Lifecycle这样一个类，Lifecycle类用于存储有关组件（如 Activity 或 Fragment）的生命周期状态的信息，并允许其他对象观察此状态。

2.2.1 引入依赖

1、非androidX项目 引入:

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n3823" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">implementation "android.arch.lifecycle:extensions:1.1.1"</pre>

添加这一句代码就依赖了如下的库:

[图片上传失败...(image-ccc12f-1683613462826)]

2、androidX项目 引入:

如果项目已经依赖了AndroidX:

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n3828" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">implementation 'androidx.appcompat:appcompat:1.2.0'</pre>

那么我们就可以使用Lifecycle库了，因为appcompat依赖了androidx.fragment，而androidx.fragment下依赖了ViewModel和 LiveData，LiveData内部又依赖了Lifecycle。

如果想要单独引入依赖，则如下：

在项目根目录的build.gradle添加 google() 代码库，然后app的build.gradle引入依赖，官方给出的依赖如下：

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n3832" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//根目录的 build.gradle
repositories {
google()
...
}

//app的build.gradle
dependencies {
def lifecycle_version = "2.2.0"
def arch_version = "2.1.0"

// ViewModel
implementation "androidx.lifecycle:lifecycle-viewmodel: $lifecycle_version" // LiveData implementation "androidx.lifecycle:lifecycle-livedata:$ lifecycle_version"
// 只有Lifecycles (不带 ViewModel or LiveData)
implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"

// Saved state module for ViewModel
implementation "androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version"

// lifecycle注解处理器
annotationProcessor "androidx.lifecycle:lifecycle-compiler: $lifecycle_version" // 替换 - 如果使用Java8,就用这个替换上面的lifecycle-compiler implementation "androidx.lifecycle:lifecycle-common-java8:$ lifecycle_version"
//以下按需引入
// 可选 - 帮助实现Service的LifecycleOwner
implementation "androidx.lifecycle:lifecycle-service: $lifecycle_version" // 可选 - ProcessLifecycleOwner给整个 app进程提供一个lifecycle implementation "androidx.lifecycle:lifecycle-process:$ lifecycle_version"
// 可选 - ReactiveStreams support for LiveData
implementation "androidx.lifecycle:lifecycle-reactivestreams: $lifecycle_version" // 可选 - Test helpers for LiveData testImplementation "androidx.arch.core:core-testing:$ arch_version"
}</pre>

看着有很多，实际上如果只使用Lifecycle，只需要引入lifecycle-runtime即可。但通常都是和 ViewModel、 LiveData 配套使用的，所以lifecycle-viewmodel、lifecycle-livedata 一般也会引入。

另外，lifecycle-process是给整个app进程提供一个lifecycle，会面也会提到。

2.2.2 使用方法

Lifecycle的使用很简单：

1、生命周期拥有者使用getLifecycle()获取Lifecycle实例，然后代用addObserve()添加观察者；
2、观察者实现LifecycleObserver，方法上使用OnLifecycleEvent注解关注对应生命周期，生命周期触发时就会执行对应方法；

2.2.2.1 基本使用

在Activity（或Fragment）中一般用法如下：

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n3844" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">public class LifecycleTestActivity extends AppCompatActivity {

private String TAG = "Lifecycle_Test";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_lifecycle_test);
//Lifecycle 生命周期
getLifecycle().addObserver(new MyObserver());
Log.i(TAG, "onCreate: ");
}
@Override
protected void onResume() {
super.onResume();
Log.i(TAG, "onResume: ");
}
@Override
protected void onPause() {
super.onPause();
Log.i(TAG, "onPause: ");
}
}</pre>

Activity（或Fragment）是生命周期的拥有者，通过getLifecycle()方法获取到生命周期Lifecycle对象，Lifecycle对象使用addObserver方法给自己添加观察者，即MyObserver对象。当Lifecycle的生命周期发生变化时，MyObserver就可以感知到。

MyObserver是如何使用生命周期的呢？看下MyObserver的实现：

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n3847" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">public class MyObserver implements LifecycleObserver {

private String TAG = "Lifecycle_Test";

@OnLifecycleEvent(value = Lifecycle.Event.ON_RESUME)
public void connect(){
Log.i(TAG, "connect: ");
}

@OnLifecycleEvent(value = Lifecycle.Event.ON_PAUSE)
public void disConnect(){
Log.i(TAG, "disConnect: ");
}
}</pre>

首先MyObserver实现了接口LifecycleObserver，LifecycleObserver用于标记一个类是生命周期观察者。然后在connectListener()、disconnectListener()上分别都加了@OnLifecycleEvent注解，且value分别是Lifecycle.Event.ON_RESUME、Lifecycle.Event.ON_PAUSE，这个效果就是：connectListener()会在ON_RESUME时执行，disconnectListener()会在ON_PAUSE时执行。

我们打开LifecycleTestActivity 然后退出，日志打印如下：

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n3850" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: normal; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">2020-11-09 17:25:40.601 4822-4822/com.hfy.androidlearning I/Lifecycle_Test: onCreate:

2020-11-09 17:25:40.605 4822-4822/com.hfy.androidlearning I/Lifecycle_Test: onResume:
2020-11-09 17:25:40.605 4822-4822/com.hfy.androidlearning I/Lifecycle_Test: connect:

2020-11-09 17:25:51.841 4822-4822/com.hfy.androidlearning I/Lifecycle_Test: disConnect:
2020-11-09 17:25:51.841 4822-4822/com.hfy.androidlearning I/Lifecycle_Test: onPause: </pre>

可见MyObserver的方法确实是在对应关注的生命周期触发时调用。当然注解中的value你也写成其它你关注的任何一个生命周期，例如Lifecycle.Event.ON_DESTROY。

2.2.2.2 MVP架构中的使用

如果是 在MVP架构中，那么就可以把presenter作为观察者：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3854" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">public class LifecycleTestActivity extends AppCompatActivity implements IView {
private String TAG = "Lifecycle_Test";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_lifecycle_test);
//Lifecycle 生命周期
// getLifecycle().addObserver(new MyObserver());

//MVP中使用Lifecycle
getLifecycle().addObserver(new MyPresenter(this));
Log.i(TAG, "onCreate: ");
}

@Override protected void onResume() {
super.onResume();
Log.i(TAG, "onResume: ");
}

@Override
protected void onPause() {
super.onPause();
Log.i(TAG, "onPause: ");
}

@Override
public void showView() {}
@Override public void hideView() {}
}

//Presenter
class MyPresenter implements LifecycleObserver {
private static final String TAG = "Lifecycle_Test";
private final IView mView;

public MyPresenter(IView view) {mView = view;}

@OnLifecycleEvent(value = Lifecycle.Event.ON_START)
private void getDataOnStart(LifecycleOwner owner){
Log.i(TAG, "getDataOnStart: ");

    Util.checkUserStatus(result -> {
            //checkUserStatus是耗时操作，回调后检查当前生命周期状态
            if (owner.getLifecycle().getCurrentState().isAtLeast(STARTED)) {
             start();
                mView.showView();
            }
        });

}
@OnLifecycleEvent(value = Lifecycle.Event.ON_STOP)
private void hideDataOnStop(){
Log.i(TAG, "hideDataOnStop: ");
stop();
mView.hideView();
}
}

//IView
interface IView {
void showView();
void hideView();
}</pre>

这里是让Presenter实现LifecycleObserver接口，同样在方法上注解要触发的生命周期，最后在Activity中作为观察者添加到Lifecycle中。

这样做好处是啥呢？当Activity生命周期发生变化时，MyPresenter就可以感知并执行方法，不需要在MainActivity的多个生命周期方法中调用MyPresenter的方法了。

所有方法调用操作都由组件本身管理：Presenter类自动感知生命周期，如果需要在其他的Activity/Fragment也使用这个Presenter，只需添加其为观察者即可。
让各个组件存储自己的逻辑，减轻Activity/Fragment中代码，更易于管理；

—— 上面提到的第一个问题点就解决了。

另外，注意到 getDataOnStart()中耗时校验回调后，对当前生命周期状态进行了检查：至少处于STARTED状态才会继续执行start()方法，也就是保证了Activity停止后不会走start()方法；

—— 上面提到的第二个问题点也解决了。

2.2.3 自定义LifecycleOwner

在Activity中调用getLifecycle()能获取到Lifecycle实例，那getLifecycle()是哪里定义的方法呢？是接口LifecycleOwner，顾明来思义，生命周期拥有者：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3867" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">/**

生命周期拥有者
生命周期事件可被自定义的组件用来处理生命周期事件的变化，同时不会在Activity/Fragmen中写任何代码
*/
public interface LifecycleOwner {
@NonNull
Lifecycle getLifecycle();
}</pre>

Support Library 26.1.0及以上、AndroidX的 Fragment 和 Activity 已实现 LifecycleOwner 接口，所以我们在Activity中可以直接使用getLifecycle()。

如果有一个自定义类并希望使其成为LifecycleOwner，可以使用LifecycleRegistry类，它是Lifecycle的实现类，但需要将事件转发到该类：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3870" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> public class MyActivity extends Activity implements LifecycleOwner {
private LifecycleRegistry lifecycleRegistry;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

        lifecycleRegistry = new LifecycleRegistry(this);
        lifecycleRegistry.markState(Lifecycle.State.CREATED);
    }       
    @Override

public void onStart() {
super.onStart();
lifecycleRegistry.markState(Lifecycle.State.STARTED);
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return lifecycleRegistry;
}
}</pre>

MyActivity实现LifecycleOwner，getLifecycle()返回lifecycleRegistry实例。lifecycleRegistry实例则是在onCreate创建，并且在各个生命周期内调用markState()方法完成生命周期事件的传递。这就完成了LifecycleOwner的自定义，也即MyActivity变成了LifecycleOwner，然后就可以和实现了LifecycleObserver的组件配合使用了。

补充一点，观察者的方法可以接受一个参数LifecycleOwner，就可以用来获取当前状态、或者继续添加观察者。若注解的是ON_ANY还可以接收Event，用于区分是哪个事件。如下：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3873" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> class TestObserver implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
void onCreated(LifecycleOwner owner) {
// owner.getLifecycle().addObserver(anotherObserver);
// owner.getLifecycle().getCurrentState();
}
@OnLifecycleEvent(Lifecycle.Event.ON_ANY)
void onAny(LifecycleOwner owner, Lifecycle.Event event) {
// event.name()
}
}</pre>

2.3 Application生命周期 ProcessLifecycleOwner

之前对App进入前后台的判断是通过registerActivityLifecycleCallbacks(callback)方法，然后在callback中利用一个全局变量做计数，在onActivityStarted()中计数加1，在onActivityStopped方法中计数减1，从而判断前后台切换。

而使用ProcessLifecycleOwner可以直接获取应用前后台切换状态。（记得先引入lifecycle-process依赖）

使用方式和Activity中类似，只不过要使用ProcessLifecycleOwner.get()获取ProcessLifecycleOwner，代码如下：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3878" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">public class MyApplication extends Application {
@Override
public void onCreate() {
super.onCreate();

//注册App生命周期观察者
ProcessLifecycleOwner.get().getLifecycle().addObserver(new ApplicationLifecycleObserver()); }

/**

Application生命周期观察，提供整个应用进程的生命周期
- Lifecycle.Event.ON_CREATE只会分发一次，Lifecycle.Event.ON_DESTROY不会被分发。
- 第一个Activity进入时，ProcessLifecycleOwner将分派Lifecycle.Event.ON_START, Lifecycle.Event.ON_RESUME。
而Lifecycle.Event.ON_PAUSE, Lifecycle.Event.ON_STOP，将在最后一个Activit退出后后延迟分发。如果由于配置更改而销毁并重新创建活动，则此延迟足以保证ProcessLifecycleOwner不会发送任何事件。
- 作用：监听应用程序进入前台或后台
  */
  private static class ApplicationLifecycleObserver implements LifecycleObserver {
  @OnLifecycleEvent(Lifecycle.Event.ON_START)
  private void onAppForeground() {
  Log.w(TAG, "ApplicationObserver: app moved to foreground");
  }
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
private void onAppBackground() {
Log.w(TAG, "ApplicationObserver: app moved to background");
}
}
}</pre>

看到确实很简单，和前面Activity的Lifecycle用法几乎一样，而我们使用ProcessLifecycleOwner就显得很优雅了。生命周期分发逻辑已在注释里说明。

三、源码分析

Lifecycle的使用很简单，接下来就是对Lifecycle原理和源码的解析了。

我们可以先猜下原理：LifecycleOwner（如Activity）在生命周期状态改变时（也就是生命周期方法执行时），遍历观察者，获取每个观察者的方法上的注解，如果注解是@OnLifecycleEvent且value是和生命周期状态一致，那么就执行这个方法。这个猜测合理吧？下面你来看看。

3.1 Lifecycle类

先来瞅瞅Lifecycle：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3885" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">public abstract class Lifecycle {
//添加观察者
@MainThread public abstract void addObserver(@NonNull LifecycleObserver observer);
//移除观察者
@MainThread public abstract void removeObserver(@NonNull LifecycleObserver observer);
//获取当前状态
public abstract State getCurrentState();

//生命周期事件，对应Activity生命周期方法
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY //可以响应任意一个事件
}

//生命周期状态.
（Event是进入这种状态的事件）
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;

//判断至少是某一状态
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}</pre>

Lifecycle 使用两种主要枚举跟踪其关联组件的生命周期状态：

Event，生命周期事件，这些事件对应Activity/Fragment生命周期方法。
State，生命周期状态，而Event是指进入一种状态的事件。Event触发的时机：

ON_CREATE、ON_START、ON_RESUME事件，是在LifecycleOwner对应的方法执行之后分发。
ON_PAUSE、ON_STOP、ON_DESTROY事件，是在LifecycleOwner对应的方法调用之前分发。这保证了LifecycleOwner是在这个状态内。

官网有个图很清晰：

[图片上传失败...(image-59d6ff-1683613462826)]

3.2 Activity对LifecycleOwner的实现

前面提到Activity实现了LifecycleOwner，所以才能直接使用getLifecycle()，具体是在androidx.activity.ComponentActivity中:

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3902" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//androidx.activity.ComponentActivity，这里忽略了一些其他代码，我们只看Lifecycle相关public class ComponentActivity extends androidx.core.app.ComponentActivity implements LifecycleOwner{ ...
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
...
@Override protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this); //使用ReportFragment分发生命周期事件
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
@CallSuper
@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
Lifecycle lifecycle = getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).setCurrentState(Lifecycle.State.CREATED);
}
super.onSaveInstanceState(outState);
mSavedStateRegistryController.performSave(outState);
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}</pre>

这里忽略了一些其他代码，我们只看Lifecycle相关。

看到ComponentActivity实现了接口LifecycleOwner，并在getLifecycle()返回了LifecycleRegistry实例。前面提到LifecycleRegistry是Lifecycle具体实现。

然后在onSaveInstanceState()中设置mLifecycleRegistry的状态为State.CREATED，然后怎么没有了？其他生命周期方法内咋没处理？what？和猜测的不一样啊。别急，在onCreate()中有这么一行：ReportFragment.injectIfNeededIn(this);,这个就是关键所在。

3.3 生命周期事件分发——ReportFragment

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3907" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//专门用于分发生命周期事件的Fragment
public class ReportFragment extends Fragment {

    public static void injectIfNeededIn(Activity activity) {
    if (Build.VERSION.SDK_INT >= 29) {
        //在API 29及以上，可以直接注册回调 获取生命周期            
         activity.registerActivityLifecycleCallbacks(                   
     new LifecycleCallbacks());

}
//API29以前，使用fragment 获取生命周期
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit(); manager.executePendingTransactions();
}
}

@SuppressWarnings("deprecation")
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
if (activity instanceof LifecycleRegistryOwner) {//这里废弃了，不用看
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
return;
}

if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);//使用LifecycleRegistry的handleLifecycleEvent方法处理事件
}
}
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
dispatch(Lifecycle.Event.ON_CREATE);
}
@Override
public void onStart() {
super.onStart();
dispatch(Lifecycle.Event.ON_START);
}
@Override
public void onResume() {
super.onResume();
dispatch(Lifecycle.Event.ON_RESUME);
}
@Override
public void onPause() {
super.onPause();
dispatch(Lifecycle.Event.ON_PAUSE);
}
...省略onStop、onDestroy

private void dispatch(@NonNull Lifecycle.Event event) {
if (Build.VERSION.SDK_INT < 29) {
dispatch(getActivity(), event);
}
}

//在API 29及以上，使用的生命周期回调
static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {
...
@Override
public void onActivityPostCreated(@NonNull Activity activity,@Nullable Bundle savedInstanceState) {
dispatch(activity, Lifecycle.Event.ON_CREATE);
}
@Override
public void onActivityPostStarted(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_START);
}
@Override
public void onActivityPostResumed(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_RESUME);
}
@Override
public void onActivityPrePaused(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_PAUSE);
}
...省略onStop、onDestroy
}
}</pre>

首先injectIfNeededIn()内进行了版本区分：在API 29及以上直接使用activity的registerActivityLifecycleCallbacks 直接注册了生命周期回调，然后给当前activity添加了ReportFragment，注意这个fragment是没有布局的。

然后，无论LifecycleCallbacks、还是fragment的生命周期方法最后都走到了 dispatch(Activity activity, Lifecycle.Event event)方法，其内部使用LifecycleRegistry的handleLifecycleEvent方法处理事件。

而ReportFragment的作用就是获取生命周期而已，因为fragment生命周期是依附Activity的。好处就是把这部分逻辑抽离出来，实现activity的无侵入。如果你对图片加载库Glide比较熟，就会知道它也是使用透明Fragment获取生命周期的。

3.4 生命周期事件处理——LifecycleRegistry

到这里，生命中周期事件的处理有转移到了 LifecycleRegistry 中：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3913" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//LifecycleRegistry.java
//系统自定义的保存Observer的map，可在遍历中增删
private FastSafeIterableMap<LifecycleObserver, ObserverWithState> mObserverMap = new FastSafeIterableMap<>();

public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);//获取event发生之后的将要处于的状态
moveToState(next);//移动到这个状态
}

private void moveToState(State next) {
if (mState == next) {
return;//如果和当前状态一致，不处理
}
mState = next; //赋值新状态
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
return;
}
mHandlingEvent = true;
sync(); //把生命周期状态同步给所有观察者
mHandlingEvent = false;
}

private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already" + "garbage collected. It is too late to change lifecycle state.");
}
while (!isSynced()) { //isSynced()意思是所有观察者都同步完了
mNewEventOccurred = false;
//mObserverMap就是在activity中添加observer后用于存放observer的map
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
...

     static State getStateAfter(Event event) {
    switch (event) {
        case ON_CREATE:
        case ON_STOP:
            return CREATED;
        case ON_START:
        case ON_PAUSE:
            return STARTED;
        case ON_RESUME:
            return RESUMED; 
       case ON_DESTROY:
            return DESTROYED;
        case ON_ANY:
            break;
    }
    throw new IllegalArgumentException("Unexpected event value " + event);
}</pre>

逻辑很清晰：使用getStateAfter()获取event发生之后的将要处于的状态（看前面那张图很好理解），moveToState()是移动到新状态，最后使用sync()把生命周期状态同步给所有观察者。

注意到sync()中有个while循环，很显然是在遍历观察者。并且很显然观察者是存放在mObserverMap中的，而mObserverMap对观察者的添加很显然就是 Activity中使用getLifecycle().addObserver()这里：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3916" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//LifecycleRegistry.java
@Override
public void addObserver(@NonNull LifecycleObserver observer)
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
//带状态的观察者，这个状态的作用：新的事件触发后遍历通知所有观察者时，判断是否已经通知这个观察者了
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
//observer作为key，ObserverWithState作为value，存到mObserverMap

    if (previous != null) {
        return;//已经添加过，不处理
    }
    LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
    if (lifecycleOwner == null) {
        return;//lifecycleOwner退出了，不处理
    }

//下面代码的逻辑：通过while循环，把新的观察者的状态连续地同步到最新状态mState。
//意思就是：虽然可能添加的晚，但把之前的事件一个个分发给你(upEvent方法)，即粘性
boolean isReentrance = mAddingObserverCounter != 0 || mHandlingEvent;
State targetState = calculateTargetState(observer);//计算目标状态
mAddingObserverCounter++;
while ((statefulObserver.mState.compareTo(targetState) < 0
&& mObserverMap.contains(observer))) {
pushParentState(statefulObserver.mState);
statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
popParentState();
// mState / subling may have been changed recalculate
targetState = calculateTargetState(observer);
}
if (!isReentrance) {
sync();
}
mAddingObserverCounter--;
}</pre>

用observer创建带状态的观察者ObserverWithState，observer作为key、ObserverWithState作为value，存到mObserverMap。接着做了安全判断，最后把新的观察者的状态连续地同步到最新状态mState，意思就是：虽然可能添加的晚，但会把之前的事件一个个分发给你，即粘性。

回到刚刚sync()的while循环，看看如何处理分发事件：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3919" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
Log.w(LOG_TAG, "LifecycleOwner is garbage collected, you shouldn't try dispatch " + "new events from it.");
return;
}
while (!isSynced()) {
mNewEventOccurred = false;
// no need to check eldest for nullability, because isSynced does it for us.
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}

    private boolean isSynced() {
    if (mObserverMap.size() == 0) {
        return true;
     }//最老的和最新的观察者的状态一致，都是ower的当前状态，说明已经同步完了
    State eldestObserverState = mObserverMap.eldest().getValue().mState;
    State newestObserverState = mObserverMap.newest().getValue().mState;
    return eldestObserverState == newestObserverState && mState == newestObserverState;
} 

   private void forwardPass(LifecycleOwner lifecycleOwner) {
    Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator = mObserverMap.iteratorWithAdditions();
    while (ascendingIterator.hasNext() && !mNewEventOccurred) {//正向遍历，从老到新
        Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
        ObserverWithState observer = entry.getValue();
        while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred && mObserverMap.contains(entry.getKey()))) {
            pushParentState(observer.mState); 
           observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));//observer获取事件
            popParentState();
        }
    }
}

private void backwardPass(LifecycleOwner lifecycleOwner) {
    Iterator<Entry<LifecycleObserver, ObserverWithState>> descendingIterator = mObserverMap.descendingIterator(); 
   while (descendingIterator.hasNext() && !mNewEventOccurred) {//反向遍历，从新到老
        Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
        ObserverWithState observer = entry.getValue();
        while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred && mObserverMap.contains(entry.getKey()))) {
            Event event = downEvent(observer.mState);
            pushParentState(getStateAfter(event));
            observer.dispatchEvent(lifecycleOwner, event);//observer获取事件
            popParentState();
        }
    }
}</pre>

循环条件是!isSynced()，若最老的和最新的观察者的状态一致，且都是ower的当前状态，说明已经同步完了。

没有同步完就进入循环体：

mState比最老观察者状态小，走backwardPass(lifecycleOwner)：从新到老分发，循环使用downEvent()和observer.dispatchEvent()，连续分发事件；
mState比最新观察者状态大，走forwardPass(lifecycleOwner)：从老到新分发，循环使用upEvent()和observer.dispatchEvent()，连续分发事件。

接着ObserverWithState类型的observer就获取到了事件，即observer.dispatchEvent(lifecycleOwner, event)，下面来看看它是如何让加了对应注解的方法执行的。

3.5 事件回调后方法执行

我们继续看下 ObserverWithState:

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3930" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> static class ObserverWithState {
State mState;
GenericLifecycleObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.getCallback(observer);
mState = initialState;
}

    void dispatchEvent(LifecycleOwner owner, Event event) {
        State newState = getStateAfter(event);
        mState = min(mState, newState);
        mLifecycleObserver.onStateChanged(owner, event);
        mState = newState;
    }
}</pre>

mState的作用是：新的事件触发后遍历通知所有观察者时，判断是否已经通知这个观察者了，即防止重复通知。

mLifecycleObserver是使用Lifecycling.getCallback(observer)获取的GenericLifecycleObserver实例。GenericLifecycleObserver是接口，继承自LifecycleObserver：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3933" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//接受生命周期改变并分发给真正的观察者public interface LifecycleEventObserver extends LifecycleObserver {
//生命周期状态变化
void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event);}</pre>

也就说，LifecycleEventObserver 给 LifecycleObserver 增加了感知生命周期状态变化的能力。

看看Lifecycling.getCallback(observer)：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3936" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> @NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
...省略很多类型判断的代码
return new ReflectiveGenericLifecycleObserver(object);
}</pre>

方法内有很多对observer进行类型判断的代码，我们这里关注的是ComponentActivity，所以LifecycleEventObserver的实现类就是ReflectiveGenericLifecycleObserver了：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3938" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;

ReflectiveGenericLifecycleObserver(Object wrapped) { 
   mWrapped = wrapped; 
   mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());//存放了event与加了注解方法的信息
}

@Override
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) {
    mInfo.invokeCallbacks(source, event, mWrapped);//执行对应event的观察者的方法
}

}</pre>

它的onStateChanged()方法内部使用CallbackInfo的invokeCallbacks方法，这里应该就是执行观察者的方法了。

ClassesInfoCache内部用Map存了所有观察者的回调信息，CallbackInfo是当前观察者的回调信息。

先看下CallbackInfo实例的创建，ClassesInfoCache.sInstance.getInfo(mWrapped.getClass())：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3942" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">//ClassesInfoCache.java
private final Map<Class, CallbackInfo> mCallbackMap = new HashMap<>();//所有观察者的回调信息
private final Map<Class, Boolean> mHasLifecycleMethods = new HashMap<>();//观察者是否有注解了生命周期的方法

    CallbackInfo getInfo(Class<?> klass) { 
   CallbackInfo existing = mCallbackMap.get(klass);//如果已经存在当前观察者回调信息 直接取 
   if (existing != null) {
        return existing;
    }
    existing = createInfo(klass, null);//没有就去收集信息并创建
    return existing;
}

    private CallbackInfo createInfo(Class<?> klass, @Nullable Method[] declaredMethods) {
    Class<?> superclass = klass.getSuperclass();
    Map<MethodReference, Lifecycle.Event> handlerToEvent = new HashMap<>();//生命周期事件到来 对应的方法
    ...
    Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);//反射获取观察者的方法
    boolean hasLifecycleMethods = false;
    for (Method method : methods) {//遍历方法 找到注解OnLifecycleEvent
        OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
        if (annotation == null) { 
           continue; //没有注解OnLifecycleEvent 就return
        } 
       hasLifecycleMethods = true;//有注解OnLifecycleEvent
        Class<?>[] params = method.getParameterTypes(); //获取方法参数
        int callType = CALL_TYPE_NO_ARG; 
       if (params.length > 0) { //有参数 
           callType = CALL_TYPE_PROVIDER;
            if (!params[0].isAssignableFrom(LifecycleOwner.class)) {
                throw new IllegalArgumentException(//第一个参数必须是LifecycleOwner 
                       "invalid parameter type. Must be one and instanceof LifecycleOwner"); 
           }
        } 
       Lifecycle.Event event = annotation.value(); 

    if (params.length > 1) { 
           callType = CALL_TYPE_PROVIDER_WITH_EVENT; 
           if (!params[1].isAssignableFrom(Lifecycle.Event.class)) { 
               throw new IllegalArgumentException(//第二个参数必须是Event
                        "invalid parameter type. second arg must be an event"); 
           }
            if (event != Lifecycle.Event.ON_ANY) {
                throw new IllegalArgumentException(//有两个参数 注解值只能是ON_ANY 
                       "Second arg is supported only for ON_ANY value"); 
           } 
       } 
       if (params.length > 2) { //参数不能超过两个
            throw new IllegalArgumentException("cannot have more than 2 params");
        } 
       MethodReference methodReference = new MethodReference(callType, method);
        verifyAndPutHandler(handlerToEvent, methodReference, event, klass);//校验方法并加入到map handlerToEvent 中
    }
    CallbackInfo info = new CallbackInfo(handlerToEvent);//获取的 所有注解生命周期的方法handlerToEvent，构造回调信息实例
    mCallbackMap.put(klass, info);//把当前观察者的回调信息存到ClassesInfoCache中 
   mHasLifecycleMethods.put(klass, hasLifecycleMethods);//记录 观察者是否有注解了生命周期的方法
    return info;
}</pre>

如果不存在当前观察者回调信息，就使用createInfo()方法收集创建
先反射获取观察者的方法，遍历方法找到注解了OnLifecycleEvent的方法，先对方法的参数进行了校验。
第一个参数必须是LifecycleOwner；第二个参数必须是Event；有两个参数注解值只能是ON_ANY；参数不能超过两个
校验方法并加入到map，key是方法，value是Event。map handlerToEvent是所有的注解了生命周期的方法。
遍历完，然后用 handlerToEvent来构造当前观察者回调信息CallbackInfo，存到ClassesInfoCache的mCallbackMap中，并记录观察者是否有注解了生命周期的方法。

整体思路还是很清晰的，继续看CallbackInfo的invokeCallbacks方法：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3955" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> static class CallbackInfo {
final Map<Lifecycle.Event, List<MethodReference>> mEventToHandlers;//Event对应的多个方法
final Map<MethodReference, Lifecycle.Event> mHandlerToEvent;//要回调的方法

    CallbackInfo(Map<MethodReference, Lifecycle.Event> handlerToEvent) { 
       mHandlerToEvent = handlerToEvent;
        mEventToHandlers = new HashMap<>(); 
       //这里遍历mHandlerToEvent来获取mEventToHandlers
        for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) { 
           Lifecycle.Event event = entry.getValue();
            List<MethodReference> methodReferences = mEventToHandlers.get(event); 
           if (methodReferences == null) { 
               methodReferences = new ArrayList<>();
                mEventToHandlers.put(event, methodReferences); 
           } 
           methodReferences.add(entry.getKey()); 
       }
    } 

   @SuppressWarnings("ConstantConditions") 
   void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) { 
       invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);//执行对应event的方法
        invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event,target);//执行注解了ON_ANY的方法
    } 

   private static void invokeMethodsForEvent(List<MethodReference> handlers,
            LifecycleOwner source, Lifecycle.Event event, Object mWrapped) {
        if (handlers != null) { 
           for (int i = handlers.size() - 1; i >= 0; i--) {//执行Event对应的多个方法
                handlers.get(i).invokeCallback(source, event, mWrapped);
            } 
       } 
   }

}</pre>

很好理解，执行对应event的方法、执行注解了ON_ANY的方法。其中mEventToHandlers是在创建CallbackInfo时由遍历mHandlerToEvent来获取，存放了每个Event对应的多个方法。

最后看看handlers.get(i).invokeCallback，即MethodReference中：

<pre spellcheck="false" class="md-fences mock-cm md-end-block" lang="" cid="n3958" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: var(--monospace); font-size: 0.9em; display: block; break-inside: avoid; text-align: left; white-space: pre-wrap; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(248, 248, 248); position: relative !important; border: 1px solid rgb(231, 234, 237); border-radius: 3px; padding: 8px 4px 6px; margin-bottom: 15px; margin-top: 15px; width: inherit; color: rgb(51, 51, 51); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"> static class MethodReference {
...
void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
try {
switch (mCallType) {
case CALL_TYPE_NO_ARG:
mMethod.invoke(target);//没有参数的
break;
case CALL_TYPE_PROVIDER:
mMethod.invoke(target, source);//一个参数的：LifecycleOwner
break;
case CALL_TYPE_PROVIDER_WITH_EVENT:
mMethod.invoke(target, source, event);//两个参数的：LifecycleOwner，Event
break;
}
}
...
}
...
}</pre>

根据不同参数类型，执行对应方法。

到这里，整个流程就完整了。实际看了这么一大圈，基本思路和我们的猜想是一致的。

这里借Android Jetpack架构组件（三）一文带你了解Lifecycle（原理篇）的图总结下：

[图片上传失败...(image-1cf88a-1683613462826)]

四、总结

本文先介绍了Jetpack和AAC的概念，这是Android官方推荐的通用开发工具集。其中AAC是架构组件，是本系列文章的介绍内容。接着介绍了AAC的基础组件Lifecycle，它能让开发者更好的管理Activity/Fragment生命周期。最后详细分析了Lifecycle源码及原理。

Jetpack的AAC是我们后续开发Android必备知识，也是完成MVVM架构的基础。Lifecycle更是AAC中的基础，所以完整掌握本篇内容十分必要。

文末

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全网最全：Jetpack AAC（-）解析

全网最全：Jetpack AAC（-）解析

Android35个内容

Jetpack5个内容

Lifecycle1个内容

Android架构组件5个内容

一、Android Jetpack 介绍

1.1 Jetpack是啥

1.2 Jetpack分类

二、Lifecycle

2.1 Lifecycle之前

2.2 Lifecycle的使用

2.2.1 引入依赖

2.2.2 使用方法

2.2.2.1 基本使用

2.2.2.2 MVP架构中的使用

2.2.3 自定义LifecycleOwner

2.3 Application生命周期 ProcessLifecycleOwner

三、源码分析

3.1 Lifecycle类

3.2 Activity对LifecycleOwner的实现

3.3 生命周期事件分发——ReportFragment

3.4 生命周期事件处理——LifecycleRegistry

3.5 事件回调后方法执行

四、总结

文末

推荐阅读更多精彩内容

全网最全：Jetpack AAC（-）解析

Android35个内容

Jetpack5个内容

Lifecycle1个内容

Android架构组件5个内容

一、Android Jetpack 介绍

1.1 Jetpack是啥

1.2 Jetpack分类

二、Lifecycle

2.1 Lifecycle之前

2.2 Lifecycle的使用

2.2.1 引入依赖

2.2.2 使用方法

2.2.2.1 基本使用

2.2.2.2 MVP架构中的使用

2.2.3 自定义LifecycleOwner

2.3 Application生命周期 ProcessLifecycleOwner

三、 源码分析

3.1 Lifecycle类

3.2 Activity对LifecycleOwner的实现

3.3 生命周期事件分发——ReportFragment

3.4 生命周期事件处理——LifecycleRegistry

3.5 事件回调后 方法执行

四、总结

文末

推荐阅读更多精彩内容

三、源码分析

3.5 事件回调后方法执行