Fragment的产生
Android 在 Android 3.0（API 级别 11）中引入了片段，主要是为了给大屏幕（如平板电脑）上更加动态和灵活的 UI 设计提供支持。但是大多数情况下，根据不同的使用情景我们并没有对平板应用使用fragment进行适配，所以更多的是我们把fragment作为一个可重复利用的模块化组件，利用它拥有自身生命周期回调，布局，行为等特性来对功能模块进行分离
fragment的平板适配
Fragment的生命周期
一般来说，Activity管理Fragment已经做得非常好了，比如，锁屏，回到屏幕的时候，Fragment都是跟随Activity发生改变的。Fragment包含与Activity类似的生命周期，比如onCreate，onStart，onResume，onPause，onStop，onDestroy，下面是引用自Google开发者的图片，关于一个Fragment被添加时（动态/静态）的生命周期的变化
Fragment的生命周期变化
我们来看看fragment的生命周期都有什么作用
//系统在向 Activity 添加片段时调用的方法，附加之后才能够通过getActivity()获取Activity的上下文。
public void onAttach(Context context)
public void onCreate(Bundle savedInstanceState)
//返回Fragment的布局
public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState)
// 在 Activity 的 onCreate() 方法已返回时调用。
public void onActivityCreated(@Nullable Bundle savedInstanceState)
//在Activity的onStart之后调用
public void onStart()
//在Activity的onResume之后调用
public void onResume()
//在Activity的onPause之前调用
public void onPause()
//在Activity的onStop之前调用
public void onStop()
// 在移除与片段关联的视图层次结构时调用。
public void onDestroyView()
// 在Activity的onDestroy之前调用
public void onDestroy()
// 在取消片段与 Activity 的关联时调用。
public void onDetach()
管理片段生命周期与管理 Activity 生命周期很相似。但是因为Fragment依附于Activity，所以很容易想象，关于Fragment创建的生命周期总是在Activity之后调用，关于Fragment销毁的生命周期总是在Activity之前调用。除此之外，在Activity需要重新创建的时候，fragment和activity一样可以通过onSaveInstanceState来保存状态，然后在onCreate，onCreateView或者onActivityCreated期间恢复状态。
Activity和Fragment的生命周期调用关系
对于Fragment的一些操作对生命周期的影响
// replace，FragmentA替换为FragmentB，在创建完B以后会先销毁A，再创建B的视图
FragmentB: onAttach
FragmentB: onCreate
——————————————————————————
FragmentA: onPause
FragmentA: onStop
FragmentA: onDestroyView
FragmentA: onDestroy
FragmentA: onDetach
——————————————————————————
FragmentB: onCreateView
FragmentB: onActivityCreated
FragmentB: onStart
FragmentB: onResume
// add，添加FragmentA
FragmentA: onAttach
FragmentA: onCreate
FragmentA: onCreateView
FragmentA: onActivityCreated
FragmentA: onStart
FragmentA: onResume
//remove，移除FragmentA
FragmentA: onPause
FragmentA: onStop
FragmentA: onDestroyView
FragmentA: onDestroy
FragmentA: onDetach
//attach，从detach到attach状态
FragmentA: onCreateView
FragmentA: onActivityCreated
FragmentA: onStart
FragmentA: onResume
//detach，从attach到detach状态
FragmentA: onPause
FragmentA: onStop
FragmentA: onDestroyView
注意：hide和show不会触发生命周期回调
给Fragment设置参数
使用setArguments，在创建Fragment的时候传递参数,然后在fragment的onCreate方法处获取参数，但是需要注意的是setArguments()方法必须在fragment创建以后，add之前调用。这样的好处是对Fragment的代码进行管理
//流行一种做法是使用静态方法newInstance()的方式来初始化Fragment
public static CircleFragmetn newInstance(String str){
Bundle bundle = new Bundle();
bundle.putString("data", str);
CircleFragmetn fragment = new CircleFragmetn();
fragment.setArguments(bundle);
return fragment;
}
没有UI的Fragment
可以添加没有UI的Fragment，使用add(Fragment,String)向Activity添加片段，由于它不与Activity布局的视图关联，所以不会触发onCreateView。通常没有UI的Fragment都是用于保存Activity的状态，或者借助setTargetFragment来控制其他fragment的UI变化，使用setRetainInstance()这个方法来告诉框架，当配置发生改变的时候，尝试保留这个fragment。SDK的APIdemos提供了范例
/*
* Copyright (C) 2010 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.apis.app;
import com.example.android.apis.R;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentManager;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;
/**
* This example shows how you can use a Fragment to easily propagate state
* (such as threads) across activity instances when an activity needs to be
* restarted due to, for example, a configuration change. This is a lot
* easier than using the raw Activity.onRetainNonConfiguratinInstance() API.
*/
public class FragmentRetainInstance extends Activity {
private static String TAG="FragmentRetainInstance";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// First time init, create the UI.
if (savedInstanceState == null) {
getFragmentManager().beginTransaction().add(android.R.id.content,
new UiFragment()).commit();
}
}
/**
* This is a fragment showing UI that will be updated from work done
* in the retained fragment.
*/
public static class UiFragment extends Fragment {
RetainedFragment mWorkFragment;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_retain_instance, container, false);
// Watch for button clicks.
Button button = (Button)v.findViewById(R.id.restart);
button.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
mWorkFragment.restart();
}
});
return v;
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
FragmentManager fm = getFragmentManager();
// Check to see if we have retained the worker fragment.
mWorkFragment = (RetainedFragment)fm.findFragmentByTag("work");
// If not retained (or first time running), we need to create it.
if (mWorkFragment == null) {
mWorkFragment = new RetainedFragment();
// Tell it who it is working with.
mWorkFragment.setTargetFragment(this, 0);
fm.beginTransaction().add(mWorkFragment, "work").commit();
}
}
}
/**
* This is the Fragment implementation that will be retained across
* activity instances. It represents some ongoing work, here a thread
* we have that sits around incrementing a progress indicator.
*/
public static class RetainedFragment extends Fragment {
ProgressBar mProgressBar;
int mPosition;
boolean mReady = false;
boolean mQuiting = false;
/**
* This is the thread that will do our work. It sits in a loop running
* the progress up until it has reached the top, then stops and waits.
*/
final Thread mThread = new Thread() {
@Override
public void run() {
// We'll figure the real value out later.
int max = 10000;
// This thread runs almost forever.
while (true) {
// Update our shared state with the UI.
synchronized (this) {
// Our thread is stopped if the UI is not ready
// or it has completed its work.
while (!mReady || mPosition >= max) {
if (mQuiting) {
return;
}
try {
wait();
} catch (InterruptedException e) {
}
}
// Now update the progress. Note it is important that
// we touch the progress bar with the lock held, so it
// doesn't disappear on us.
mPosition++;
max = mProgressBar.getMax();
mProgressBar.setProgress(mPosition);
}
// Normally we would be doing some work, but put a kludge
// here to pretend like we are.
synchronized (this) {
try {
wait(50);
} catch (InterruptedException e) {
}
}
}
}
};
/**
* Fragment initialization. We way we want to be retained and
* start our thread.
*/
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Tell the framework to try to keep this fragment around
// during a configuration change.
setRetainInstance(true);
// Start up the worker thread.
mThread.start();
}
/**
* This is called when the Fragment's Activity is ready to go, after
* its content view has been installed; it is called both after
* the initial fragment creation and after the fragment is re-attached
* to a new activity.
*/
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
// Retrieve the progress bar from the target's view hierarchy.
mProgressBar = (ProgressBar)getTargetFragment().getView().findViewById(
R.id.progress_horizontal);
// We are ready for our thread to go.
synchronized (mThread) {
mReady = true;
mThread.notify();
}
}
/**
* This is called when the fragment is going away. It is NOT called
* when the fragment is being propagated between activity instances.
*/
@Override
public void onDestroy() {
// Make the thread go away.
Log.d(TAG, "onDestroy");
synchronized (mThread) {
mReady = false;
mQuiting = true;
mThread.notify();
}
super.onDestroy();
}
/**
* This is called right before the fragment is detached from its
* current activity instance.
*/
@Override
public void onDetach() {
// This fragment is being detached from its activity. We need
// to make sure its thread is not going to touch any activity
// state after returning from this function.
Log.d(TAG, "onDetach");
synchronized (mThread) {
mProgressBar = null;
mReady = false;
mThread.notify();
}
super.onDetach();
}
/**
* API for our UI to restart the progress thread.
*/
public void restart() {
synchronized (mThread) {
mPosition = 0;
mThread.notify();
}
}
}
}
Fragment通过实现 onCreateOptionsMenu() 向 Activity 的应用栏添加菜单项。不过，为了使此方法能够收到调用，您必须在 onCreate() 期间调用 setHasOptionsMenu()，以指示片段想要向选项菜单添加菜单项（否则，片段将不会收到对 onCreateOptionsMenu() 的调用）。
锁屏带来的问题
最近我发现了一个我们可能在平常的使用中都没有注意到的问题，锁屏界面对生命周期的影响，很多时候我们的Activity都会直接在onResume或者onPause等地方做一些操作，但是大部分手机都存在锁屏界面，如果我们对锁屏的实现比较熟悉的话都知道其实锁屏界面也是一个Activity。如果我们的Activity处于前台的时候，我们进入了锁屏，那么在开启屏幕的时候（出现锁屏的时候），我们的Activity其实经历了一次onRestart—onStart—onResume—onPause—onStop的生命周期变化，这是因为当我们点开屏幕的时候，首先显示的是我们的Activity，然后系统会在我们的Activity上面打开锁屏应用，这样就导致我们在打开锁屏之前，Activity就经历了一次生命周期的变化，这一问题会留到将来学习KeyGuard的时候再进行深入研究
