Jetpack Compose：从 Composable 函数启动 ActivityResultContract 请求

Question

从1.2.0-beta01 或androidx.activity:activity-ktx 开始，不能再launch 使用Activity.registerForActivityResult() 创建的请求，如上面“行为更改”下的链接中突出显示的那样，并在Google issue here 中看到。

应用程序现在应该如何通过@Composable 函数启动这个请求？以前，应用程序可以通过使用Ambient 将MainActivity 的实例沿链向下传递，然后轻松启动请求。

可以通过以下方式解决新行为，例如，在 Activity 的 onCreate 函数之外实例化后，将注册活动结果的类沿链向下传递，然后在 Composable 中启动请求。但是，无法通过这种方式注册完成后执行的回调。

可以通过创建自定义ActivityResultContract 来解决此问题，该ActivityResultContract 在启动时会进行回调。但是，这意味着几乎所有内置的 ActivityResultContracts 都不能与 Jetpack Compose 一起使用。

TL;DR

应用如何从@Composable 函数启动ActivityResultsContract 请求？

Answer 1

自 androidx.activity:activity-compose:1.3.0-alpha06 起，registerForActivityResult() API 已重命名为 rememberLauncherForActivityResult()，以更好地表明返回的 ActivityResultLauncher 是代您记住的托管对象。

val result = remember { mutableStateOf<Bitmap?>(null) }
val launcher = rememberLauncherForActivityResult(ActivityResultContracts.TakePicturePreview()) {
    result.value = it
}

Button(onClick = { launcher.launch() }) {
    Text(text = "Take a picture")
}

result.value?.let { image ->
    Image(image.asImageBitmap(), null, modifier = Modifier.fillMaxWidth())
}

Answer 2

Activity Result 有两个 API 界面：

• 核心ActivityResultRegistry。这就是底层工作的实际工作。
• ActivityResultCaller 中的便利接口，ComponentActivity 和 Fragment 实现了将 Activity Result 请求与 Activity 或 Fragment 的生命周期联系起来

Composable 与 Activity 或 Fragment 具有不同的生命周期（例如，如果您从层次结构中删除 Composable，它应该自行清理），因此使用 ActivityResultCaller API，例如 registerForActivityResult() 绝不是正确的要做的事情。

相反，您应该直接使用ActivityResultRegistry API，直接调用register() 和unregister()。这最好与 rememberUpdatedState() 和 DisposableEffect 搭配使用，以创建可与 Composable 一起使用的 registerForActivityResult 版本：

@Composable
fun <I, O> registerForActivityResult(
    contract: ActivityResultContract<I, O>,
    onResult: (O) -> Unit
) : ActivityResultLauncher<I> {
    // First, find the ActivityResultRegistry by casting the Context
    // (which is actually a ComponentActivity) to ActivityResultRegistryOwner
    val owner = ContextAmbient.current as ActivityResultRegistryOwner
    val activityResultRegistry = owner.activityResultRegistry

    // Keep track of the current onResult listener
    val currentOnResult = rememberUpdatedState(onResult)

    // It doesn't really matter what the key is, just that it is unique
    // and consistent across configuration changes
    val key = rememberSavedInstanceState { UUID.randomUUID().toString() }

    // Since we don't have a reference to the real ActivityResultLauncher
    // until we register(), we build a layer of indirection so we can
    // immediately return an ActivityResultLauncher
    // (this is the same approach that Fragment.registerForActivityResult uses)
    val realLauncher = mutableStateOf<ActivityResultLauncher<I>?>(null)
    val returnedLauncher = remember {
        object : ActivityResultLauncher<I>() {
            override fun launch(input: I, options: ActivityOptionsCompat?) {
                realLauncher.value?.launch(input, options)
            }

            override fun unregister() {
                realLauncher.value?.unregister()
            }

            override fun getContract() = contract
        }
    }

    // DisposableEffect ensures that we only register once
    // and that we unregister when the composable is disposed
    DisposableEffect(activityResultRegistry, key, contract) {
        realLauncher.value = activityResultRegistry.register(key, contract) {
            currentOnResult.value(it)
        }
        onDispose {
            realLauncher.value?.unregister()
        }
    }
    return returnedLauncher
}

然后可以通过以下代码在您自己的 Composable 中使用它：

val result = remember { mutableStateOf<Bitmap?>(null) }
val launcher = registerForActivityResult(ActivityResultContracts.TakePicturePreview()) {
    // Here we just update the state, but you could imagine
    // pre-processing the result, or updating a MutableSharedFlow that
    // your composable collects
    result.value = it
}

// Now your onClick listener can call launch()
Button(onClick = { launcher.launch() } ) {
    Text(text = "Take a picture")
}

// And you can use the result once it becomes available
result.value?.let { image ->
    Image(image.asImageAsset(),
        modifier = Modifier.fillMaxWidth())
}

Answer 3

从 Activity Compose 1.3.0-alpha03 及以后开始，有一个新的实用函数 registerForActivityResult() 可以简化此过程。

@Composable
fun RegisterForActivityResult() {
    val result = remember { mutableStateOf<Bitmap?>(null) }
    val launcher = registerForActivityResult(ActivityResultContracts.TakePicturePreview()) {
        result.value = it
    }

    Button(onClick = { launcher.launch() }) {
        Text(text = "Take a picture")
    }

    result.value?.let { image ->
        Image(image.asImageBitmap(), null, modifier = Modifier.fillMaxWidth())
    }
}

（来自here给出的样本）

Answer 4

对于那些没有通过@ianhanniballake 提供的要点获得结果的人，在我的例子中，returnedLauncher 实际上捕获了realLauncher 的已处置值。

因此，虽然删除间接层应该可以解决问题，但它绝对不是这样做的最佳方式。

这是更新版本，直到找到更好的解决方案：

@Composable
fun <I, O> registerForActivityResult(
    contract: ActivityResultContract<I, O>,
    onResult: (O) -> Unit
): ActivityResultLauncher<I> {
    // First, find the ActivityResultRegistry by casting the Context
    // (which is actually a ComponentActivity) to ActivityResultRegistryOwner
    val owner = AmbientContext.current as ActivityResultRegistryOwner
    val activityResultRegistry = owner.activityResultRegistry

    // Keep track of the current onResult listener
    val currentOnResult = rememberUpdatedState(onResult)

    // It doesn't really matter what the key is, just that it is unique
    // and consistent across configuration changes
    val key = rememberSavedInstanceState { UUID.randomUUID().toString() }

    // TODO a working layer of indirection would be great
    val realLauncher = remember<ActivityResultLauncher<I>> {
        activityResultRegistry.register(key, contract) {
            currentOnResult.value(it)
        }
    }

    onDispose {
        realLauncher.unregister()
    }
    
    return realLauncher
}

Answer 5

添加以防有人开始新的外部意图。就我而言，我想在点击 jetpack compose 中的按钮时启动 google 登录提示。

声明你的意图启动

val startForResult =
    rememberLauncherForActivityResult(ActivityResultContracts.StartActivityForResult()) { result: ActivityResult ->
        if (result.resultCode == Activity.RESULT_OK) {
            val intent = result.data
            //do something here
        }
    }

启动您的新活动或任何意图。

 Button(
        onClick = {
            //important step
            startForResult.launch(googleSignInClient?.signInIntent)
        },
        modifier = Modifier
            .fillMaxWidth()
            .padding(start = 16.dp, end = 16.dp),
        shape = RoundedCornerShape(6.dp),
        colors = ButtonDefaults.buttonColors(
            backgroundColor = Color.Black,
            contentColor = Color.White
        )
    ) {
        Image(
            painter = painterResource(id = R.drawable.ic_logo_google),
            contentDescription = ""
        )
        Text(text = "Sign in with Google", modifier = Modifier.padding(6.dp))
    }

#googlesignin