关于Fragment返回按键监听

①先定义接口BackHandleInterface

public interface BackHandleInterface {

    void onSelectedFragment(BackHandleFragment backHandleFragment);

}

②定义公用的Fragment

public abstract class BackHandleFragment extends Fragment {

    private BackHandleInterface backHandleInterface;

    /**
     * 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
     */
    public abstract boolean onBackPressed();

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if(getActivity() instanceof BackHandleInterface){
            this.backHandleInterface = (BackHandleInterface)getActivity();
        }else{
            throw new ClassCastException(getActivity().getClass().getName()+" must implement BackHandledInterface");
        }
    }

    @Override
    public void onStart() {
        super.onStart();
        backHandleInterface.onSelectedFragment(this);
    }
}

③需要实现监听的Fragment的Activity实现接口 主要的是onSelectedFragment()和onBackPressed()其他方法可以忽略

public class EdittextActivity extends AppCompatActivity implements BackHandleInterface {

    private BackHandleFragment backHandleFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_layout);
        addFragment(R.id.fragmentContainer, new EdittextFragment());
    }

    public void addFragment(int containerViewId, Fragment fragment){
        final FragmentTransaction transaction = this.getSupportFragmentManager().beginTransaction();
        transaction.add(containerViewId, fragment);
        transaction.commit();
    }

    @Override
    public void onSelectedFragment(BackHandleFragment backHandleFragment) {
        this.backHandleFragment = backHandleFragment;
    }

    @Override
    public void onBackPressed() {
        //if判断里面就调用了来自Fragment的onBackPressed()
        //一样！！，如果onBackPressed是返回false，就会进入条件内进行默认的操作
        if(backHandleFragment == null || !backHandleFragment.onBackPressed()){
            if(getSupportFragmentManager().getBackStackEntryCount() == 0){
                super.onBackPressed();
            }else{
                getSupportFragmentManager().popBackStack();
            }
        }
    }
}

④需要监听的Fragment

public class EdittextFragment extends BackHandleFragment {

    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_edittext, container, false);
        return view;
    }

    @Override
    public boolean onBackPressed() {
        Toast.makeText(getActivity(), "按了返回键", Toast.LENGTH_SHORT).show();
        return true;//因为这里return true 所以不会返回上一个页面，方便我截图
    }
}