这篇文章距离现在已经两年的时间了。当初自己刚毕业工作不久，才开始接触Android，有一天中午和同事一起吃饭的时候，一个大牛问我你思考过Activity的setContentView是怎么执行的么。当初就因为这个问题我接入到了Android源码。两年时间过去了现在回过头来看，感觉自己写得有很多的不足，本次再补充一下。
前言
这几天正在进行初级自定义组件的学习，一不小心想到了view到底是怎么加载到屏幕上面的。每一个Activity中都有一个方法setContentView,我们可以加载自己想要的界面布局，展示在手机屏幕上。但到底内部是怎么实现的呢？（PS:源码基于Android5.1，cm12.1）
Activity的onContentView
首先查看Activity的onContentView的方法：
//Activity.java
public void setContentView(int layoutResID) {
getWindow().setContentView(layoutResID);
initActionBar();
}
public void setContentView(View view) {
getWindow().setContentView(view);
initActionBar();
}
public void setContentView(View view, ViewGroup.LayoutParams params) {
getWindow().setContentView(view, params);
initActionBar();
}
Activity一共重载了三个setContentView方法，其中第一个setContentView(int layoutResID)方法是我们常用的。
public void setContentView(int layoutResID) {
//getWindow()获取activity内部对象mWindow并调用它的setContentView方法
getWindow().setContentView(layoutResID);
initActionBar(); //这是初始化actionBar，我们不关注它
}
public Window getWindow() {
return mWindow;
}
Activity的setContentView方法实际还是调用mWindow的setContentView方法，接下看我们试看看mWindow的相关代码。
mWindow对象
查看Activity源码，找到在attach方法中对mWindow做了赋值。
final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config) {
attachBaseContext(context);
mFragments.attachActivity(this, mContainer, null);
mWindow = PolicyManager.makeNewWindow(this);
mWindow.setCallback(this);
/…部分代码省略…/
}
那么Activity的attach方法是Activity生命周期的第一个方法，它是ActivityThread中performLaunchActivity方法调用的，这是通过AMS(ActivityManagerService)的startActivity调用ActivityTrack的startActivityMayWait来调用的。
attach字面意思就是“使依附；贴上；系上”，也就是点击activity进行启动的时候之执行的。
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
/*******部分代码省略********/
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}
try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
/*******部分代码省略********/
if (activity != null) {
Context appContext = createBaseContextForActivity(r, activity);
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
+ r.activityInfo.name + " with config " + config);
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor);
/*******部分代码省略********/
}
如述源代码当中就是在启动Activity的时候执行其attach：
- ApplicationThread#scheduleLaunchActivity
- ActivityThread#handleLaunchActivity
- ActivityThread#performLaunchActivity
- Activity#attach
PolicyManager获取Window对象
public final class PolicyManager {
private static final String POLICY_IMPL_CLASS_NAME =
"com.android.internal.policy.impl.Policy";
private static final IPolicy sPolicy;
static {
// Pull in the actual implementation of the policy at run-time
try {
Class policyClass = Class.forName(POLICY_IMPL_CLASS_NAME);
sPolicy = (IPolicy)policyClass.newInstance();
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
POLICY_IMPL_CLASS_NAME + " could not be loaded", ex);
} catch (InstantiationException ex) {
throw new RuntimeException(
POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
} catch (IllegalAccessException ex) {
throw new RuntimeException(
POLICY_IMPL_CLASS_NAME + " could not be instantiated", ex);
}
}
// Cannot instantiate this class
private PolicyManager() {}
// The static methods to spawn new policy-specific objects
public static Window makeNewWindow(Context context) {
return sPolicy.makeNewWindow(context);
}
/*******部分代码省略********/
}
PolicyManager.makeNewWindow方法实际是通过反射机制调用了"com.android.internal.policy.impl.Policy"的makeNewWindow方法。
public class Policy implements IPolicy {
private static final String TAG = "PhonePolicy";
private static final String[] preload_classes = {
"com.android.internal.policy.impl.PhoneLayoutInflater",
"com.android.internal.policy.impl.PhoneWindow",
"com.android.internal.policy.impl.PhoneWindow$1",
"com.android.internal.policy.impl.PhoneWindow$DialogMenuCallback",
"com.android.internal.policy.impl.PhoneWindow$DecorView",
"com.android.internal.policy.impl.PhoneWindow$PanelFeatureState",
"com.android.internal.policy.impl.PhoneWindow$PanelFeatureState$SavedState",
};
static {
// For performance reasons, preload some policy specific classes when
// the policy gets loaded.
for (String s : preload_classes) {
try {
Class.forName(s);
} catch (ClassNotFoundException ex) {
Log.e(TAG, "Could not preload class for phone policy: " + s);
}
}
}
public Window makeNewWindow(Context context) {
return new PhoneWindow(context);
}
/*******部分代码省略********/
}
Policy的makeNewWindow方法实际是返回一个PhoneWindow对象。
PhoneWindow.setContentView
public class PhoneWindow extends Window implements MenuBuilder.Callback {
/*******部分代码省略********/
@Override
public void setContentView(View view) {
setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
}
@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
//构造DecorView对象并赋值给mDecor，并进行mContentParent的初始化
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
view.setLayoutParams(params);
final Scene newScene = new Scene(mContentParent, view);
transitionTo(newScene);
} else {
//将附带params属性的view对象添加在mContentParent中
mContentParent.addView(view, params);
}
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
@Override
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
//将Resource对于的id等于layoutResID的xml布局文件，add到mContentParent中
mLayoutInflater.inflate(layoutResID, mContentParent);
}
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
/*******部分代码省略********/
}
setContentView主要做了两件事：
- 初始化整个界面（即：DecorView）
- 将setContentView的参数对于的View，add到mContentParent中。
addView
setContentView方法有两种在界面添加View的方法。
- 调用mContentParent的add方法，将目标View添加进去。
- 调用LayoutInfater.inflate方法将资源xml解析并转化为View，添加到mContentParent中。
installDecor
在看installDecor方法的源代码的时候，我先让大家看一个Android手机界面的布局文件的分析图。
Android手机界面的布局
PhoneWindow
PhoneWindow.java部分代码
protected DecorView generateDecor() {
return new DecorView(getContext(), -1);
}
private void installDecor() {
if (mDecor == null) {
//构造mDecor对象DecorView
mDecor = generateDecor();
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
}
if (mContentParent == null) {
//构造mContentParent
mContentParent = generateLayout(mDecor);
// Set up decor part of UI to ignore fitsSystemWindows if appropriate.
mDecor.makeOptionalFitsSystemWindows();
final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
R.id.decor_content_parent);
if (decorContentParent != null) {
//mDecorContentParent赋值R.id.decor_content_parent
mDecorContentParent = decorContentParent;
mDecorContentParent.setWindowCallback(getCallback());
if (mDecorContentParent.getTitle() == null) {
mDecorContentParent.setWindowTitle(mTitle);
}
/*******部分代码省略********/
}
mDecorContentParent为mDecor中的R.id.decor_content_parent
installDecor先构造mDecor，然后通过mDecor执行generateLayout()方法初始化mContentParent。
protected ViewGroup generateLayout(DecorView decor) {
/*******部分代码省略********/
View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
mContentRoot = (ViewGroup) in;
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
if (contentParent == null) {
throw new RuntimeException("Window couldn't find content container view");
}
/*******部分代码省略********/
return contentParent;
}
mContentRoot为decor的content，经测试(mContentRoot == mDecorContentParent)为true。
generateLayout(DecorView decor)方法构造出来的mContentParent为ID_ANDROID_CONTENT，即mDecor中的R.id.content。
从代码中可以看出显示获取当前窗口的根ViewGroup（mDecor），然后往这个ViewGroup中添加view。
最终我们要展示在Activity中的View已经构造好了，那么在Activity的onResume 方法之后，在 ActivityThread#handleResumeActivity 方法中会将该View通过WindowManager添加在Activity所挂在的Window上进行展现。
mDecor是什么可以参考博客：DecorView浅析
好了学习过程到此结束~！
下边介绍在我学习过程中膜拜的博客，感觉这些大牛就是点亮我前行的灯塔，哈哈哈。
Android View的加载过程
Android应用setContentView与LayoutInflater加载解析机制源码分析
android的窗口机制分析------UI管理系统
文章到这里就全部讲述完啦，若有其他需要交流的可以留言哦~！~！
