为何statusBarBackground始终为null？如何获取StatusBar视图？DrawerLayout是否影响？

Hey there! Let's break down why you're getting null for the status bar view, and whether using DrawerLayout as your root layout plays a role here.

Why Are You Getting Null? #

The most common culprit is trying to fetch the view too early. If you call getWindow().getDecorView().findViewById(android.R.id.statusBarBackground) in onCreate(), onStart(), or even early in onResume(), the DecorView hasn't finished initializing yet—so the statusBarBackground view hasn't been added to the view tree, hence the null result.

Another thing to check: if your Activity is in fullscreen mode or has the StatusBar hidden entirely, this view won't exist at all, so returning null is expected behavior.

Correct Ways to Fetch StatusBarBackground #

You need to wait until the view hierarchy is fully drawn. Here are two reliable approaches:

1. Use ViewTreeObserver to Listen for Layout Completion #

This lets you trigger the fetch right after the view tree is laid out:

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

    View decorView = getWindow().getDecorView();
    decorView.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
        @Override
        public void onGlobalLayout() {
            // Remove the listener to avoid repeated calls
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
                decorView.getViewTreeObserver().removeOnGlobalLayoutListener(this);
            } else {
                // For older SDKs
                decorView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
            }
            
            View statusBarBackground = decorView.findViewById(android.R.id.statusBarBackground);
            if (statusBarBackground != null) {
                // Now you can use this view for shared element transitions!
            }
        }
    });
}

2. Fetch in onWindowFocusChanged() #

When the Activity gains window focus, the entire view hierarchy is guaranteed to be ready:

@Override
public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);
    if (hasFocus) {
        View statusBarBackground = getWindow().getDecorView().findViewById(android.R.id.statusBarBackground);
        if (statusBarBackground != null) {
            // Handle your shared element logic here
        }
    }
}

Does DrawerLayout as Root Layout Affect This? #

No, not directly. The statusBarBackground view is part of the Window's DecorView, which sits above your Activity's root layout (whether it's DrawerLayout, ConstraintLayout, etc.). A few things to note though:

• If you've set fitsSystemWindows="true" on your DrawerLayout, it will adjust its own padding to account for the StatusBar, but this doesn't affect the existence of the statusBarBackground view.
• As long as the StatusBar is visible and you fetch the view at the right time, DrawerLayout won't cause it to be null.

Quick Additional Checks #

• Make sure your targetSdkVersion is at least 21—android.R.id.statusBarBackground was introduced in API 21, so it won't exist on older versions.
• Verify your app's theme isn't setting the StatusBar to be fully transparent or hidden (e.g., with android:statusBarColor="@android:color/transparent" without proper window flags—this might make the view invisible or gone).

内容的提问来源于stack exchange，提问作者bojan