This is a tricky edge case that many developers hit when migrating to the top resumed activity pattern for exclusive resources like cameras. Let’s break down why it’s happening and how to fix it:

Root Causes #

• Callback vs. Camera Service Sync Delay: The onTopResumedActivityChanged(true) callback tells you your activity is now the top resumed one, but the system’s camera service doesn’t instantly update priority assignments. The previous camera-using app might still be in the process of releasing the camera resource (which can take a few hundred milliseconds) when you try to connect.
• Hidden High-Priority Clients: Sometimes, system-level components (like face unlock services, background barcode scanners, or even system camera previews) hold camera access with higher priority than your app. These aren’t always visible to the user, but they can block your connection attempt.
• Split-Screen State Transition Race: In split-screen mode, the system’s state transitions between activities are more complex. There’s a small window where the camera service hasn’t fully processed the priority shift before your app tries to open the camera.

Fixes & Workarounds #

• Add a Retry Mechanism with Backoff: When you get the RuntimeException connecting to the camera, wait a short interval (e.g., 200ms) and retry 2-3 times. This gives the previous client time to release the camera. Example code snippet:

  private void attemptOpenCamera() {
      try {
          mCamera = Camera.open(0);
          // Initialize preview and camera callbacks
      } catch (RuntimeException e) {
          // Only retry if we're still the top resumed activity
          if (isTopResumedActivity) {
              new Handler(Looper.getMainLooper()).postDelayed(this::attemptOpenCamera, 200);
          }
      }
  }
  

• Use CameraManager.AvailabilityCallback: Register this callback to listen for when the camera becomes available. Instead of opening the camera immediately in onTopResumedActivityChanged, wait until the callback notifies you that the camera is free.

  CameraManager cameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);
  cameraManager.registerAvailabilityCallback(new CameraManager.AvailabilityCallback() {
      @Override
      public void onCameraAvailable(String cameraId) {
          if (isTopResumedActivity && "0".equals(cameraId)) {
              attemptOpenCamera();
          }
      }
  }, null);
  

• Ensure Clean Camera Release: Double-check that when onTopResumedActivityChanged(false) is called, you’re fully releasing the camera: stop the preview, set all camera callbacks to null, and call mCamera.release() immediately. Any leftover references can delay the camera becoming available to other apps.
• Graceful Error Handling: When retries fail, show a user-friendly message (e.g., "Camera is in use by another app") and offer a retry button. Don’t let the crash go unhandled or silent.

Key Takeaway #

The onTopResumedActivityChanged callback is a signal that your app should try to acquire exclusive resources, but it doesn’t guarantee instant availability. You need to account for race conditions and service delays with retries and camera availability listening.

内容的提问来源于stack exchange，提问作者Grodriguez