Fragemt監聽back事件

Activity可以很容易的得到物理返回鍵的監聽事件，而Fragment卻不能。所以使用到了以下的方法。

 首先創建一個抽象類BackHandledFragment，該類有一個抽象方法onBackPressed()，
 所有BackHandledFragment的子類在onBackPressed方法中處理各自對Back事件的消費邏輯。
 onBackPressed返回布爾值，宿主FragmentActivity將會根據該方法的返回值判斷子Fragment
 是否有消費Back事件。此外，宿主FragmentActivity還會保持一份當前Fragment的引用，當用戶
 按下Back鍵時，宿主Activity會判斷當前Fragment是否需要消費該事件，如果沒有Fragment消費
 纔會自己消費。

 public abstract class BackHandledFragment extends Fragment { 
      protected BackHandledInterface mBackHandledInterface; 
      /**
       * 所有繼承BackHandledFragment的子類都將在這個方法中實現物理Back鍵按下後的邏輯
       * FragmentActivity捕捉到物理返回鍵點擊事件後會首先詢問Fragment是否消費該事件
       * 如果沒有Fragment消息時FragmentActivity自己纔會消費該事件
       */
     protected abstract boolean onBackPressed();

     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         // instanceof 運算符是用來在運行時指出對象是否是特定類的一個實例
         if(!(getActivity() instanceof BackHandledInterface)){
             throw new ClassCastException("Hosting Activity must implement BackHandledInterface");
         }else{
             this.mBackHandledInterface = (BackHandledInterface)getActivity();
         }
     }

     @Override
     public void onStart() {
         super.onStart();
         //告訴FragmentActivity，當前Fragment在棧頂
         mBackHandledInterface.setSelectedFragment(this);
     }

 }

宿主FragmentActivity需要繼承BackHandledIntegerface，子Fragment會通過該接口告訴宿主FragmentActivity自己是當前屏幕可見的Fragment。

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

所以在Fragment的onCreate中會判斷宿主FragmentActivity是否已繼承了該接口。在Fragment的onStart()方法中就會調用該接口告訴宿主FragmentActivity自己是當前屏幕可見的Fragment。
宿主FragmentActivity就可以在onBackPressed()方法中對Back事件進行判斷處理了。

  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中調用使用。

  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;
         }
     }
 }