为什么AppCompatActivity.onMenuOpened(int featureId，Menu menu)的菜单为空? [英] Why is menu of AppCompatActivity.onMenuOpened(int featureId, Menu menu) null?

问题描述

一个Android应用具有以下代码:

An Android app has the following code:

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

单击三点溢出菜单按钮时，菜单将正常显示，并调用此MenuOpened(int featureId，Menu menu)，但菜单为null.这正常吗?

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?

推荐答案

单击菜单图标onMenuOpened会被调用两次:第一次立即调用，然后在准备菜单时调用.显然，第一次它为空是因为它没有准备好.因此，如果您想访问此回调中的菜单-只需将其检查为null即可；否则请执行其他操作.

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.

简单的实验:

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);
    }
}

单击菜单图标时输出:

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

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