1. Background

So far, we have already completed:

1. Permission checking and dynamic permission requests
2. Real-time CameraX preview
3. Compose state-driven UI

Behind all of these features, there is one core mechanism: callbacks + asynchronous logic.

Understanding callbacks is essential for understanding modern Android asynchronous programming and Compose state-driven UI.

2. What is a callback?

A callback is essentially this:

I write a function, hand it to the system or a library, and when a certain event finishes, that function gets called automatically.

Analogy

• You place a food delivery order, leave your phone number, and the delivery person calls you when the order arrives.
• That “phone call” is the callback.

3. Callback examples from Phase 0

1. Permission request callback

cameraPermissionLauncher = registerForActivityResult(
    ActivityResultContracts.RequestPermission()
) { granted ->
    hasCameraPermission.value = granted
}

• registerForActivityResult registers a permission request
• The lambda { granted -> ... } is the callback
• After the user chooses “allow” or “deny,” the callback fires
• Updating hasCameraPermission.value automatically triggers Compose recomposition

Tap button -> system dialog -> user choice -> callback -> UI update

2. CameraX initialization callback

val cameraProviderFuture = ProcessCameraProvider.getInstance(ctx)

cameraProviderFuture.addListener({
    val cameraProvider = cameraProviderFuture.get()

    val preview = Preview.Builder().build().also {
        it.setSurfaceProvider(previewView.surfaceProvider)
    }

    val cameraSelector = CameraSelector.DEFAULT_BACK_CAMERA

    cameraProvider.unbindAll()
    cameraProvider.bindToLifecycle(activity, cameraSelector, preview)

}, ContextCompat.getMainExecutor(ctx))

• addListener({ ... }, executor): register a callback
• After CameraProvider is ready asynchronously, the callback runs
• Inside the callback, Preview is bound and rendered
• The callback runs on the main thread with ContextCompat.getMainExecutor(ctx) so UI operations stay safe

CameraX initialization -> async completion -> callback binds Preview -> preview appears

4. Asynchronous vs synchronous

• Synchronous: code runs in order, and slow operations block the main thread
• Asynchronous: slow work runs in the background, and the main thread gets notified later through a callback

In Phase 0:

1. The permission dialog is asynchronous
2. CameraProvider initialization is asynchronous
3. Callbacks + Compose state make the UI update automatically

The benefit is that the UI stays responsive and avoids lag or ANR.

5. Compose + async + state

if (hasCameraPermission.value) {
    CameraPreview(activity = this)
} else {
    NoPermissionView(onRequestPermission = {
        cameraPermissionLauncher.launch(Manifest.permission.CAMERA)
    })
}

• No permission -> show a button
• User taps the button -> request permission asynchronously -> callback fires
• Update hasCameraPermission.value -> UI automatically switches to CameraPreview

Core idea: asynchronous events + state-driven UI = reactive Compose programming

6. Key takeaways

1. Callbacks are the core of asynchronous logic
2. Async operations decouple UI from slow tasks
3. Compose state + callbacks = reactive UI

User action -> asynchronous task -> callback -> state update -> automatic UI refresh

贡献者: Junyuan He