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我有一个MainActivity包含NavigationView抽屉菜单的。的NavigationView标头包含一个TextView我想用一些用户信息填充的。问题是，有时应用程序会在onChanged方法中崩溃，NullPointerException因为textUsernameis null。我的猜测是这是因为布局还没有完全膨胀。我已经实现了null检查（如下面的代码所示），这显然textUsername在大多数情况下只会导致空白。

我的onCreate方法如下所示：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mainViewModel = ViewModelProviders.of(this).get(MainViewModel.class);

    mainDrawerLayout = (DrawerLayout) findViewById(R.id.main_drawer_layout);
    mainNavigationView = (NavigationView) findViewById(R.id.main_navigation_view);
    mainContentLayout = (CoordinatorLayout) findViewById(R.id.main_content_layout);

    // get user information
    NetworkController.getInstance().getCurrentUser(Utils.getAuthenticationToken(), new NetworkController.GetUserResponseCallback() {
        @Override
        public void onGetUser(Response<UserResponse> response) {
            if(response.isSuccessful() && response.body() != null && response.body().isSuccess()) {
                mainViewModel.setCurrentUser(response.body().getUser());
            }
        }

        @Override
        public void onRequestFailed(Throwable throwable) {

        }
    });

    mainViewModel.getCurrentUser().observe(this, new Observer<User>() {
        @Override
        public void onChanged(@Nullable User user) {
            TextView textUsername = (TextView) mainNavigationView.findViewById(R.id.drawer_header_username);
            if(textUsername != null) {
                textUsername.setText(user.getProfile().getUsername());
            }
        }
    });
}

在从回调中访问该布局之前，如何确保该布局已膨胀？谢谢。

编辑

activity_main.xml：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start"
    android:id="@+id/main_drawer_layout">

    <include
        layout="@layout/activity_main_content"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <android.support.design.widget.NavigationView
        android:id="@+id/main_navigation_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:headerLayout="@layout/drawer_menu_header"
        app:menu="@menu/main_drawer_menu">

    </android.support.design.widget.NavigationView>

</android.support.v4.widget.DrawerLayout>

activity_main_content.xml：

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/main_content_layout">

    <!-- setup the toolbar -->
    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/main_toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            android:elevation="@dimen/toolbar_elevation"
            android:popupTheme="@style/AppTheme.PopupOverlay">

        </android.support.v7.widget.Toolbar>
    </android.support.design.widget.AppBarLayout>

    <!-- include the fragment container -->
    <android.support.constraint.ConstraintLayout
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:activity="com.fishley.android.activities.MainActivity"
        android:id="@+id/main_fragment_container"
        android:paddingTop="?attr/actionBarSize">

    </android.support.constraint.ConstraintLayout>

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab_new_post"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:src="@drawable/ic_plus"
        android:tint="@android:color/white"
        android:visibility="gone"/>

</android.support.design.widget.CoordinatorLayout>

drawer_menu_header.xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="@dimen/nav_header_height"
    android:gravity="bottom"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:theme="@style/ThemeOverlay.AppCompat.Dark"
    android:background="@drawable/drawer_menu_header_background">

    <TextView
        android:id="@+id/drawer_header_username"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="@dimen/nav_header_vertical_spacing"
        android:text="Android Studio"
        android:textColor="@android:color/black"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1"/>

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@android:color/black"
        android:text="android.studio@android.com"/>

</LinearLayout>
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