Fragment生命周期

这篇文章主要介绍fragment的生命周期而不是用法。

由于fragment不是独立的组件，需要被添加到Acitvity中，所以Fragment的生命周期和Activity存在一定的关联

和fragment生命周期有关的的几个重要回调方法：

onAttach(Activity)  fragment被添加到activity时被调用

onCreate(Bundle)  fragment创建时调用

onCreateView(LayoutInflater, ViewGroup, Bundle)  fragment初始化界面时调用

onActivityCreated(Bundle) 包含fragment的activity的onCreate方法执行完毕后调用

onStart()  fragment可见时，和activitiy的onStart相关联

onResume()  和activitiy的onResume相关联

onPause()  activity onPause或者fragment被移除时

onStop()  activity onStop或者fragment被移除时

onDestroyView() fragment被移除时，关联的view正在被移除时调用

onDestroy()，onDetach() fragment被移除时调用

API GUIDE给出的图示:

还有1个示意图对比了activity和fragment的生命周期

可以看出，fragment的onAttatch，onCreate，onCreateView，onActivityCreated是fragment的创建阶段，相当于Activity的创建阶段，onStart，onResume，onPause，onStop的流程都和Activity一样，销毁阶段，包含了onDestroyView，onDestroy，onDetach方法。

下面用例子来演示fragment的生命周期：

初次启动的Activity：

MainActivity.java

 public class MainActivity extends Activity {

     private Button goFragment;
     private Button goActivity;
     private boolean first = true;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         Log.v("test", "Activity-onCreate");
         initViews();
     }

     @Override
     protected void onResume() {
         super.onResume();
         Log.v("test", "Activity-onResume");
     }

     @Override
     protected void onPause() {
         super.onPause();
         Log.v("test", "Activity-onPause");
     }

     @Override
     protected void onStop() {
         super.onStop();
         Log.v("test", "Activity-onStop");
     }

     @Override
     protected void onDestroy() {
         super.onDestroy();
         Log.v("test", "Activity-onDestroy");
     }

     private void initViews() {
         goFragment = (Button) findViewById(R.id.go_fragment);
         goActivity = (Button) findViewById(R.id.go_activity);
         goFragment.setOnClickListener(new OnClickListener() {

             @Override
             public void onClick(View v) {
                 FragmentManager fm = getFragmentManager();
                 FragmentTransaction ft = fm.beginTransaction();
                 ft.replace(R.id.container, new FragmentB());
                 ft.commit();
             }
         });

         goActivity.setOnClickListener(new OnClickListener() {

             @Override
             public void onClick(View v) {
                 startActivity(new Intent(MainActivity.this, Activity2.class));
             }
         });
     }

     @Override
     protected void onStart() {
         Log.v("test", "Activity-onStart");
         super.onStart();
         //防止重复加载fragment
         if(first) {
             first = false;
             FragmentManager fm = getFragmentManager();
             FragmentTransaction ft = fm.beginTransaction();
             ft.add(R.id.container, new FragmentA());
             ft.commit();
         }
     }
 }

布局文件：

 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:gravity="center_horizontal"
     android:orientation="vertical"
     tools:context=".MainActivity" >

     <FrameLayout
         android:id="@+id/container"
         android:layout_width="match_parent"
         android:layout_height="350dp"
         />
     <Button
         android:id="@+id/go_fragment"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="go fragment2" />
     <Button
         android:id="@+id/go_activity"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="go Activity2" />

 </LinearLayout>

界面上一个frameLayout，用来装载fragment,2个按钮，一个用来替换fragment，一个用来跳转activity

FragmentA的代码：

 public class FragmentA extends Fragment {

     @Override
     public void onAttach(Activity activity) {
         super.onAttach(activity);
         Log.v("test", "FragmentA:" + "onAttach");
     }

     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         Log.v("test", "FragmentA:" + "onCreate");
     }

     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container,
             Bundle savedInstanceState) {
         Log.v("test", "FragmentA:" + "onCreateView");

         return inflater.inflate(R.layout.fragment1, null);
     }

     @Override
     public void onActivityCreated(Bundle savedInstanceState) {
         super.onActivityCreated(savedInstanceState);
         Log.v("test", "FragmentA:" + "onActivityCreated");
     }

     @Override
     public void onStart() {
         super.onStart();
         Log.v("test", "FragmentA:" + "onStart");
     }

     @Override
     public void onResume() {
         super.onResume();
         Log.v("test", "FragmentA:" + "onResume");
     }

     @Override
     public void onPause() {
         super.onPause();
         Log.v("test", "FragmentA:" + "onPause");
     }

     @Override
     public void onStop() {
         super.onStop();
         Log.v("test", "FragmentA:" + "onStop");
     }

     @Override
     public void onDestroyView() {
         super.onDestroyView();
         Log.v("test", "FragmentA:" + "onDestroyView");
     }

     @Override
     public void onDestroy() {
         super.onDestroy();
         Log.v("test", "FragmentA:" + "onDestroy");
     }

     @Override
     public void onDetach() {
         super.onDetach();
         Log.v("test", "FragmentA:" + "onDetach");
     }
 }

Activity和fragment的回调中都进行了日志打印。

在Acitivity的onStart方法中，调用了如下代码：

 FragmentManager fm = getFragmentManager();
 FragmentTransaction ft = fm.beginTransaction();
 ft.add(R.id.container, new FragmentA());
 ft.commit();

作用是在Acitivity启动后在界面上加载一个fragment。

启动程序，观察控制台输出:

 10-27 18:20:23.423: V/test(3128): Activity-onCreate
 10-27 18:20:23.423: V/test(3128): Activity-onStart
 10-27 18:20:23.423: V/test(3128): FragmentA:onAttach
 10-27 18:20:23.423: V/test(3128): FragmentA:onCreate
 10-27 18:20:23.423: V/test(3128): FragmentA:onCreateView
 10-27 18:20:23.423: V/test(3128): FragmentA:onActivityCreated
 10-27 18:20:23.423: V/test(3128): FragmentA:onStart
 10-27 18:20:23.423: V/test(3128): Activity-onResume
 10-27 18:20:23.423: V/test(3128): FragmentA:onResume

解释：MainActivity onCreate执行后，接着执行onStart，onStart中添加了fragment到framelayout中，这时fragment的onAttach回调被调用。接着是onCreate,onCreateView,onActivityCreated依次被回调，现在点击gofragment2按钮，移除fragment1，加入fragment2，观察logcat：

 10-27 18:44:23.513: V/test(3128): FragmentA:onPause
 10-27 18:44:23.513: V/test(3128): FragmentA:onStop
 10-27 18:44:23.513: V/test(3128): FragmentA:onDestroyView
 10-27 18:44:23.513: V/test(3128): FragmentA:onDestroy
 10-27 18:44:23.513: V/test(3128): FragmentA:onDetach

正是第一张图片所示的从前台到销毁的过程。

现在重启程序，试着点击第二个按钮go Activity2，观察logcat输出：

 10-27 19:08:34.253: V/test(3128): FragmentA:onPause
 10-27 19:08:34.253: V/test(3128): Activity-onPause
 10-27 19:08:34.573: V/test(3128): FragmentA:onStop
 10-27 19:08:34.573: V/test(3128): Activity-onStop

可以看到，当前Activity被覆盖后，fragment的onPause,onStop和Activity的onPause,onStop同时被调用。

这时，按物理返回键，退出第二个Activity，回到第一个Activity，观察logcat输出：

 10-27 19:29:07.063: V/test(9162): Activity-onStart
 10-27 19:29:07.063: V/test(9162): FragmentA:onStart
 10-27 19:29:07.063: V/test(9162): Activity-onResume
 10-27 19:29:07.063: V/test(9162): FragmentA:onResume

可见，在activity重新进入前台，执行onStart和onResume时，fragment也执行一样的操作

现在修改一下跳转activity的按钮的代码，让当前activity finish

 goActivity.setOnClickListener(new OnClickListener() {

     @Override
     public void onClick(View v) {
         startActivity(new Intent(MainActivity.this, Activity2.class));
         finish();
     }
 });

重启程序，点击跳转activity的按钮，观察输出：

 10-27 19:33:54.893: V/test(9810): FragmentA:onPause
 10-27 19:33:54.893: V/test(9810): Activity-onPause
 10-27 19:33:55.213: V/test(9810): FragmentA:onStop
 10-27 19:33:55.213: V/test(9810): Activity-onStop
 10-27 19:33:55.213: V/test(9810): FragmentA:onDestroyView
 10-27 19:33:55.213: V/test(9810): FragmentA:onDestroy
 10-27 19:33:55.213: V/test(9810): FragmentA:onDetach
 10-27 19:33:55.213: V/test(9810): Activity-onDestroy