Fragment: which callback invoked when press back b

问题:

I have a fragment:

public class MyFragment extends Fragment{
     ...
     @Override
     public View onCreateView(...){...}    
     ...
}

I instantiate it:

MyFragment myFragment = new MyFragment();

I use the above fragment to replace the current fragment:

FragmentManager fragmentManager = activity.getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

// replace fragment
fragmentTransaction.replace(R.id.fragment_placeholder, myFragment, "myTag");

// NOTE: I did not add to back stack

Now, myFragment is showing on the screen. NOTE: I did not add myFragment to back stack.

My two questions:

1. If now, I press mobile phone back button, which fragment's life cycle callback will be invoked??

2. How can I customize the back button click listener in MyFragment class? (please do not suggest me to do myFragment.getView().setOnclickListener, but do it in MyFragment class)

回答1:

Question 1: See http://developer.android.com/reference/android/app/Fragment.html#Lifecycle:

"As a fragment is no longer being used, it goes through a reverse series of callbacks:

onPause() - fragment is no longer interacting with the user either because its activity is being paused or a fragment operation is modifying it in the activity.

onStop() - fragment is no longer visible to the user either because its activity is being stopped or a fragment operation is modifying it in the activity.

onDestroyView() - allows the fragment to clean up resources associated with its View.

onDestroy() - called to do final cleanup of the fragment's state.

onDetach() - called immediately prior to the fragment no longer being associated with its activity."

Question 2: If you must know that it was the back button specifically that is triggering the callbacks, You can capture the back button press in your Fragment's Activity and use your own method to handle it:

public class MyActivity extends Activity
{
    //...
    //Defined in Activity class, so override
    @Override
    public void onBackPressed()
    {
        super.onBackPressed();
        myFragment.onBackPressed();
    }
}

public class MyFragment extends Fragment
{
    //Your created method
    public void onBackPressed()
    {
        //Handle any cleanup you don't always want done in the normal lifecycle
    }
}

回答2:

androidx.activity 1.0.0-alpha01 is released and introduces ComponentActivity, a new base class of the existing FragmentActivity and AppCompatActivity.

You can now register an OnBackPressedCallback via addOnBackPressedCallback to receive onBackPressed() callbacks without needing to override the method in your activity.