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。

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;
        }
    }

}

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