ActionBar back button finishing activity from inner Fragment

Question

I have an AppCompatActivity with an ActionBar incorporating a back button. I am starting a inner Fragment class which opens a View on top of the Activity view. The hardware back button closes the Fragment and the Activity view is visible. If however I press the back button on the ActionBar whilst in the Fragment view, the app goes back to MainActivity, the Activities parent. I want to either hide the ActionBar, or ideally give it the same behaviour as the hardware button, as this is obviously annoying behaviour for the user. I have tried creating an instance of the ActionBar in my Activity, like this

ActionBar actionBar = getActionBar();
actionBar.setHomeAsUpEnabled(true);

This gives me a NullPointerException in runtime. I have searched and am stumped.

Also tried this, where mFragmentManager has .addToBackStack() before it begins the transaction with my Fragment class.

@Override
public void onBackPressed() {

    int count = mFragmentManager.getBackStackEntryCount();

    if (count == 0) {
        super.onBackPressed();
        //additional code
    } else {
        mFragmentManager.popBackStack();
    }
    Log.i(LOG_TAG, "onBackPressed. Count = " + count);
}

~No errors but the ActionBar back button behaviour is no changed. No Log message is recorded~

My mistake, this method did change behaviour but for the hardware back button. It caused a NullPointerException when trying to getBackStackCount() on mFragmentManager .

Answer 1

The action bar "back" button is actually the Up button, and according to Google it is supposed to behave differently from the hardware back button: https://developer.android.com/training/design-navigation/ancestral-temporal.html

That being said, you can intercept touches on this button and do whatever you want with them.

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    if (item.getItemId() == android.R.id.home) {
        // your code here
    }

    return super.onOptionsItemSelected(item);
}

Answer 2

尝试改用getSupportActionBar() 。