Why is menu of AppCompatActivity.onMenuOpened(int featureId, Menu menu) null?

问题

An Android app has the following code:

public class FooActivity extends AppCompatActivity{
   @Override
    public boolean onMenuOpened(int featureId, Menu menu) {
    }
}

When the 3-dot overflow menu button is clicked, the menu is displayed normally, and this MenuOpened(int featureId, Menu menu) is called, but menu is null. Is this normal?

回答1:

When you click the menu icon onMenuOpened is called twice: first time immediately, then when menu is prepared. Obviously first time it is null because it is not prepared. So if you want to access menu in this callback - just check it for null and do things if it's not.

Simple experiment:

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.activity_main_toolbar);
        setSupportActionBar(toolbar);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        menu.add("first");
        menu.add("second");
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onPrepareOptionsMenu(Menu menu) {
        Log.d("tag", "onPrepareOptionsMenu called");
        return super.onPrepareOptionsMenu(menu);
    }

    @Override
    public boolean onMenuOpened(int featureId, Menu menu) {
        Log.d("tag", "onMenuOpened called");
        if(menu!=null) {
            Log.d("tag", "menu is ready");
        } else {
            Log.d("tag", "menu is null yet");
        }
        return super.onMenuOpened(featureId, menu);
    }
}

Output when menu icon clicked:

D/tag: onMenuOpened called
D/tag: menu is null yet
D/tag: onPrepareOptionsMenu called
D/tag: onMenuOpened called
D/tag: menu is ready

来源：https://stackoverflow.com/questions/41558904/why-is-menu-of-appcompatactivity-onmenuopenedint-featureid-menu-menu-null