Jetpack Compose：从 Composable 函数启动 ActivityResultContract 请求

Question

从androidx.activity:activity-ktx的1.2.0-beta01 androidx.activity:activity-ktx ，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，并在此处的Google 问题中看到。

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的实例向下传递链，然后轻松启动请求。

例如，可以通过以下方式解决新行为：在 Activity 的onCreate函数之外实例化后，将注册活动结果的类传递到链中，然后在Composable启动请求。 但是，无法通过这种方式注册完成后要执行的回调。

可以通过创建自定义ActivityResultContract来解决这个问题，该自定义ActivityResultContract在启动时接受回调。 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; 博士

应用程序如何从@Composable函数启动ActivityResultsContract请求？

Answer 1

从androidx.activity:activity-compose:1.3.0-alpha06 ， registerForActivityResult() API 已重命名为rememberLauncherForActivityResult()以更好地指示返回的ActivityResultLauncher是代表您记住的托管对象。

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 。 这才是真正的底层工作。
• ComponentActivity和Fragment实现的ActivityResultCaller中的便捷接口，将 Activity Result 请求与 Activity 或 Fragment 的生命周期联系起来

Composable 具有与 Activity 或 Fragment 不同的生命周期（例如，如果您从层次结构中删除 Composable，它应该在其自身之后进行清理），因此使用ActivityResultCaller API（例如registerForActivityResult()永远不是正确的做法。

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，以创建一个可与 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
}

然后可以通过代码在您自己的 Composable 中使用它，例如：

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及更高版本开始，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())
    }
}

（来自此处给出的示例）

Answer 4

对于那些没有通过@ianhanniballake 提供的要点返回结果的人，在我的情况下， returnedLauncher实际上捕获了realLauncher的已经处置的值。

因此，虽然删除间接层应该可以解决问题，但这绝对不是执行此操作的最佳方法。

这是更新的版本，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

添加以防万一有人开始新的外部意图。 就我而言，我想在单击 jetpack compose 中的按钮时启动谷歌登录提示。

宣布你的意图发射

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