让Fragment监听返回键

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。

 1 public abstract class BackHandledFragment extends Fragment {
 3     protected BackHandledInterface mBackHandledInterface;
 5     /**
 6      * 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
 7      * 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
 8      * 如果没有Fragment消息时FragmentActivity自己才会消费该事件
 9      */
10     protected abstract boolean onBackPressed();
11
12     @Override
13     public void onCreate(Bundle savedInstanceState) {
14         super.onCreate(savedInstanceState);
15         // 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
16         if(!(getActivity() instanceof BackHandledInterface)){
17             throw new ClassCastException("Hosting Activity must implement BackHandledInterface");
18         }else{
19             this.mBackHandledInterface = (BackHandledInterface)getActivity();
20         }
21     }
22
23     @Override
24     public void onStart() {
25         super.onStart();
26         //告诉FragmentActivity，当前Fragment在栈顶
27         mBackHandledInterface.setSelectedFragment(this);
28     }
29
30 }

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。

1 public interface BackHandledInterface {
2     public abstract void setSelectedFragment(BackHandledFragment selectedFragment);
3 }

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。

 1 public class MainActivity extends FragmentActivity implements BackHandledInterface{
 3     private BackHandledFragment mBackHandedFragment;
 4     private boolean hadIntercept;
 6     private Button mBtn;
 7
 8     @Override
 9     protected void onCreate(Bundle savedInstanceState) {
10         super.onCreate(savedInstanceState);
11         setContentView(R.layout.activity_main);
12         this.mBtn = (Button)findViewById(R.id.btn);
13         this.mBtn.setOnClickListener(new OnClickListener() {
14
15             @Override
16             public void onClick(View v) {
17                 MyFragment f = new MyFragment();
18                 FragmentManager fm = getSupportFragmentManager();
19                 FragmentTransaction ft = fm.beginTransaction();
20                 ft.replace(R.id.container, f);
21                 ft.addToBackStack("tag");
22                 ft.commit();
23             }
24         });
25     }
26
27     @Override
28     public void setSelectedFragment(BackHandledFragment selectedFragment) {
29         this.mBackHandedFragment = selectedFragment;
30     }
31
32     @Override
33     public void onBackPressed() {
34         if(mBackHandedFragment == null || !mBackHandedFragment.onBackPressed()){
35             if(getSupportFragmentManager().getBackStackEntryCount() == 0){
36                 super.onBackPressed();
37             }else{
38                 getSupportFragmentManager().popBackStack();
39             }
40         }
41     }
42 }

在Fragment中调用使用。

 1 public class MyFragment extends BackHandledFragment {
 3     private boolean hadIntercept;
 5     @Override
 6     public View onCreateView(LayoutInflater inflater,
 7             @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
 8         return getLayoutInflater(savedInstanceState).inflate(R.layout.fragment_a, null);
 9     }
10
11     @Override
12     protected boolean onBackPressed() {
13         if(hadIntercept){
14             return false;
15         }else{
16             Toast.makeText(getActivity(), "Click MyFragment", Toast.LENGTH_SHORT).show();
17             hadIntercept = true;
18             return true;
19         }
20     }
21
22 }

Demo：https://github.com/mxy1228/FragmentHandleBackDemo/