Fragemt监听back事件

Activity可以很容易的得到物理返回键的监听事件，而Fragment却不能。所以使用到了以下的方法。

 首先创建一个抽象类BackHandledFragment，该类有一个抽象方法onBackPressed()，
 所有BackHandledFragment的子类在onBackPressed方法中处理各自对Back事件的消费逻辑。
 onBackPressed返回布尔值，宿主FragmentActivity将会根据该方法的返回值判断子Fragment
 是否有消费Back事件。此外，宿主FragmentActivity还会保持一份当前Fragment的引用，当用户
 按下Back键时，宿主Activity会判断当前Fragment是否需要消费该事件，如果没有Fragment消费
 才会自己消费。

 public abstract class BackHandledFragment extends Fragment { 
      protected BackHandledInterface mBackHandledInterface; 
      /**
       * 所有继承BackHandledFragment的子类都将在这个方法中实现物理Back键按下后的逻辑
       * FragmentActivity捕捉到物理返回键点击事件后会首先询问Fragment是否消费该事件
       * 如果没有Fragment消息时FragmentActivity自己才会消费该事件
       */
     protected abstract boolean onBackPressed();

     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         // instanceof 运算符是用来在运行时指出对象是否是特定类的一个实例
         if(!(getActivity() instanceof BackHandledInterface)){
             throw new ClassCastException("Hosting Activity must implement BackHandledInterface");
         }else{
             this.mBackHandledInterface = (BackHandledInterface)getActivity();
         }
     }

     @Override
     public void onStart() {
         super.onStart();
         //告诉FragmentActivity，当前Fragment在栈顶
         mBackHandledInterface.setSelectedFragment(this);
     }

 }

宿主FragmentActivity需要继承BackHandledIntegerface，子Fragment会通过该接口告诉宿主FragmentActivity自己是当前屏幕可见的Fragment。

 public interface BackHandledInterface {
     public abstract void setSelectedFragment(BackHandledFragment selectedFragment);
 }

所以在Fragment的onCreate中会判断宿主FragmentActivity是否已继承了该接口。在Fragment的onStart()方法中就会调用该接口告诉宿主FragmentActivity自己是当前屏幕可见的Fragment。
宿主FragmentActivity就可以在onBackPressed()方法中对Back事件进行判断处理了。

  public class MainActivity extends FragmentActivity implements BackHandledInterface{
      private BackHandledFragment mBackHandedFragment;
      private boolean hadIntercept;  
      private Button mBtn;

      @Override
      protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         this.mBtn = (Button)findViewById(R.id.btn);
         this.mBtn.setOnClickListener(new OnClickListener() {

             @Override
             public void onClick(View v) {
                 MyFragment f = new MyFragment();
                 FragmentManager fm = getSupportFragmentManager();
                 FragmentTransaction ft = fm.beginTransaction();
                 ft.replace(R.id.container, f);
                 ft.addToBackStack("tag");
                 ft.commit();
             }
         });
     }

     @Override
     public void setSelectedFragment(BackHandledFragment selectedFragment) {
         this.mBackHandedFragment = selectedFragment;
     }

     @Override
     public void onBackPressed() {
         if(mBackHandedFragment == null || !mBackHandedFragment.onBackPressed()){
             if(getSupportFragmentManager().getBackStackEntryCount() == 0){
                 super.onBackPressed();
             }else{
                 getSupportFragmentManager().popBackStack();
             }
         }
     }
 }

在Fragment中调用使用。

  public class MyFragment extends BackHandledFragment {
      private boolean hadIntercept;
      @Override
      public View onCreateView(LayoutInflater inflater,
              @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
          return getLayoutInflater(savedInstanceState).inflate(R.layout.fragment_a, null);
      }

     @Override
     protected boolean onBackPressed() {
         if(hadIntercept){
             return false;
         }else{
             Toast.makeText(getActivity(), "Click MyFragment", Toast.LENGTH_SHORT).show();
             hadIntercept = true;
             return true;
         }
     }
 }