Compose中的startActivityForResult的正确姿势

之前在 Jetpack Compose中的导航路由里简单的提到了从 Compose 导航到其他 Activity 页面的方式，对于不带返回结果的则就是跟以前一样简单的启动Activity的代码，而如果是startActivityForResult方式的，需要使用带回调的方式去启动，那么在以前，我们要么是使用三方库，要么是自己封装一个简单的库来使用 (至于背后原理也不是什么新鲜事了) 。

后来研究了一下，在Compose 中startActivityForResult也有了新的姿势，要理解Compose 中startActivityForResult的姿势，这还得从androidx的startActivityForResult的姿势说起，因为Compose 就是在androidx的基础上利用其API简单封装了一下而已。

倒也不是说以前的不能用了，毕竟有系统自带的，谁还去用第三方的呀，用第三方的还得导入一个依赖库不是，能少依赖三方的就尽量少依赖吧。

androidx之后如何正确的startActivityForResult

如何使用

如果是在Activity或Fragment内部使用的话，直接调用registerForActivityResult方法即可。

例如，选择文件：

val launcher = registerForActivityResult(ActivityResultContracts.GetContent()) { uri ->
    uri?.apply { showToast(uri.toString()) }
}

launcher.launch("image/*")

ActivityResultContracts.GetContent()的方式获取文件launch时需要通过指定 mime type来过滤文件类型, 例如 image/* ，这会打开系统自带的一个文件选择器供你选择文件。

录制视频：

val outVideoFile = File(externalCacheDir, "/${System.currentTimeMillis()}.mp4")
val videoUri = FileProvider.getUriForFile(this, "${packageName}.provider", outVideoFile)
val launcher = registerForActivityResult(ActivityResultContracts.CaptureVideo()) { isSuccess ->
    if (isSuccess) {
        showToast(outVideoFile.path)
    }
}

launcher.launch(videoUri)

拍照：

val outPictureFile = File(externalCacheDir, "/${System.currentTimeMillis()}.jpeg")
val pictureUri = FileProvider.getUriForFile(this, "${packageName}.provider", outPictureFile)
val launcher = registerForActivityResult(ActivityResultContracts.TakePicture()) { isSuccess ->
    if (isSuccess) {
        showToast(outPictureFile.path)
    }
}

launcher.launch(pictureUri)}

这两种方式需要指定Uri, 这个Uri获取有点费劲，需要先进行FileProvider配置，不过配置好就很方便了。（如果你的minSdk配置的是29，那么需要另外配置，可自行查阅相关资料，不过国内基本不会兼容这么高的版本，一般minSdk配置的会是21）

自己的内部业务Activity之间的跳转：

// MainActivity.kt 
val target = Intent(this, OtherActivity::class.java).apply {
    putExtra("name", "张三")
    putExtra("uid", 123)
}
val launcher = registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { activityResult ->
    activityResult.data?.apply {
        val name = getStringExtra("name")
        name?.let { showToast(it) }
    }
}

launcher.launch(target)

// OtherActivity.kt
class OtherActivity: ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val name = intent.getStringExtra("name") 
        val uid = intent.getIntExtra("uid", -1)
        setContent {
            MyComposeApplicationTheme {
                Surface(modifier = Modifier.fillMaxSize()) {
                    Column {
                        Text("name: $name fromCompose: $fromCompose uid: $uid", fontSize = 20.sp)
                        Button(onClick = {
                            val data = Intent().apply { putExtra("name", "小明") }
                            setResult(RESULT_OK, data)
                            finish()
                        }) { Text("back with result") }
                    }
                }
            }
        }
    }
}

在registerForActivityResult的回调接口lambda中，仍然可以像以前那样使用activityResult.resultCode == RESULT_OK 来判断是正常返回了，还是取消操作。

在Activity/Fragment以外的类中使用：

class MyLifecycleObserver(private val registry : ActivityResultRegistry) : DefaultLifecycleObserver {
    lateinit var getContent : ActivityResultLauncher<String>

    override fun onCreate(owner: LifecycleOwner) {
        getContent = registry.register("key", owner, ActivityResultContracts.GetContent()) { uri ->
            // Handle the returned Uri
        }
    }

    fun selectImage() {
        getContent.launch("image/*")
    }
}

很简单，将代码移到一个LifecycleObserver的实现类中即可，然后在Activity/Fragment中将这个LifecycleObserver的实现类的观察者对象添加到其本身的lifecycle中即可。

class MyFragment : Fragment() {
    lateinit var observer : MyLifecycleObserver

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // ...
        observer = MyLifecycleObserver(requireActivity().activityResultRegistry)
        lifecycle.addObserver(observer)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        val selectButton = view.findViewById<Button>(R.id.select_button)

        selectButton.setOnClickListener {
            // Open the activity to select an image
            observer.selectImage()
        }
    }
}

使用总结：

整体来说，终于支持callback回调方式回传结果了，
而且启动时也不用带requestCode了，
并且还支持在Activity/Fragment以外的类中使用。

源码追踪

看一下ComponentActivity中registerForActivityResult 方法的实现：

// androidx.activity.ComponentActivity.java 1.6.1
 @NonNull
 @Override
 public final <I, O> ActivityResultLauncher<I> registerForActivityResult(
         @NonNull final ActivityResultContract<I, O> contract,
         @NonNull final ActivityResultRegistry registry,
         @NonNull final ActivityResultCallback<O> callback) {
     return registry.register(
             "activity_rq#" + mNextLocalRequestCode.getAndIncrement(), this, contract, callback);
 }

 @NonNull
 @Override
 public final <I, O> ActivityResultLauncher<I> registerForActivityResult(
         @NonNull ActivityResultContract<I, O> contract,
         @NonNull ActivityResultCallback<O> callback) {
     return registerForActivityResult(contract, mActivityResultRegistry, callback);
 }

这里2个重载方法，下面的方法其实是调用了上面的方法，所以本质就是上面的三个参数的方法，也就是 registry.register() 方法，而在Fragment中也有两个类似的方法：

   // androidx.fragment.app.Fragment.java 1.5.4
@MainThread
   @NonNull
   @Override
   public final <I, O> ActivityResultLauncher<I> registerForActivityResult(
           @NonNull final ActivityResultContract<I, O> contract,
           @NonNull final ActivityResultCallback<O> callback) {
       return prepareCallInternal(contract, new Function<Void, ActivityResultRegistry>() {
           @Override
           public ActivityResultRegistry apply(Void input) {
               if (mHost instanceof ActivityResultRegistryOwner) {
                   return ((ActivityResultRegistryOwner) mHost).getActivityResultRegistry();
               }
               return requireActivity().getActivityResultRegistry();
           }
       }, callback);
   }

   @MainThread
   @NonNull
   @Override
   public final <I, O> ActivityResultLauncher<I> registerForActivityResult(
           @NonNull final ActivityResultContract<I, O> contract,
           @NonNull final ActivityResultRegistry registry,
           @NonNull final ActivityResultCallback<O> callback) {
       return prepareCallInternal(contract, new Function<Void, ActivityResultRegistry>() {
           @Override
           public ActivityResultRegistry apply(Void input) {
               return registry;
           }
       }, callback);
   }

如果继续跟踪prepareCallInternal方法会发现，Fragment中最终还是依托mHost宿主的ActivityResultRegistry对象来执行registry.register() 方法的。

也就是说其实在androidx以后，任意Activity对象中调用getActivityResultRegistry() 方法，它都会返回一个ActivityResultRegistry 对象，然后调用该对象的register方法来注册回调接口，而在Fragment中自然能获取到宿主Activity对象也能拿到这个ActivityResultRegistry 对象。

着重看一下registry.register() 方法：

 public final <I, O> ActivityResultLauncher<I> register(
         @NonNull final String key,
         @NonNull final LifecycleOwner lifecycleOwner,
         @NonNull final ActivityResultContract<I, O> contract,
         @NonNull final ActivityResultCallback<O> callback) {
         ......
}

可以看出register方法它需要四个参数：

key: 唯一标识字符串，代替以前的requestCode，其实内部还是使用的整型的requestCode，只不过是根据传入的key生成随机数然后存入一个Map中，用的时候再从Map中获取。
lifecycleOwner：lifecycle持有者，我们知道在androidx以后Activity和Fragment默认就是LifecycleOwner接口的实现者
contract：ActivityResultContract<I, O> 类型，契约类，它有两个泛型，一个输入类型，一个输出类型
callback：回调接口，代替以前的onActivityResult方法，内部会将前面的key参数和callback保存到一个Map中，以便后面需要的时候方便获取

register方法的返回值是一个 ActivityResultLauncher<I>类型的对象，它就是最终真正用来执行启动动作的对象，通过执行它的launch方法进行启动。

register方法还有一个重载方法：

 public final <I, O> ActivityResultLauncher<I> register(
         @NonNull final String key,
         @NonNull final ActivityResultContract<I, O> contract,
         @NonNull final ActivityResultCallback<O> callback) {
 	......
}

这个方法与上面那个方法的区别就是少了一个lifecycleOwner参数，但是少了这个参数也就意味着你需要自己的onDestroy方法中执行对应的 ActivityResultLauncher.unregister() 方法，不如上面那个方便了。因为带lifecycleOwner参数的register方法内部其实处理好了：

public final <I, O> ActivityResultLauncher<I> register(
            @NonNull final String key,
            @NonNull final LifecycleOwner lifecycleOwner,
            @NonNull final ActivityResultContract<I, O> contract,
            @NonNull final ActivityResultCallback<O> callback) {

        Lifecycle lifecycle = lifecycleOwner.getLifecycle();

        if (lifecycle.getCurrentState().isAtLeast(Lifecycle.State.STARTED)) {
            throw new IllegalStateException("LifecycleOwner " + lifecycleOwner + " is "
                    + "attempting to register while current state is "
                    + lifecycle.getCurrentState() + ". LifecycleOwners must call register before "
                    + "they are STARTED.");
        }

        registerKey(key); // 注册
        ...
        LifecycleEventObserver observer = new LifecycleEventObserver() {
            @Override
            @SuppressWarnings("deprecation")
            public void onStateChanged(
                    @NonNull LifecycleOwner lifecycleOwner,
                    @NonNull Lifecycle.Event event) {
                if (Lifecycle.Event.ON_START.equals(event)) {
                	// 将用户传入的callback封装保存到一个Map中
                    mKeyToCallback.put(key, new CallbackAndContract<>(callback, contract));
                    if (mParsedPendingResults.containsKey(key)) {
                        @SuppressWarnings("unchecked")
                        final O parsedPendingResult = (O) mParsedPendingResults.get(key);
                        mParsedPendingResults.remove(key);
                        callback.onActivityResult(parsedPendingResult);
                    }
                    final ActivityResult pendingResult = mPendingResults.getParcelable(key);
                    if (pendingResult != null) {
                        mPendingResults.remove(key);
                        callback.onActivityResult(contract.parseResult(
                                pendingResult.getResultCode(),
                                pendingResult.getData()));
                    }
                } else if (Lifecycle.Event.ON_STOP.equals(event)) {
                    mKeyToCallback.remove(key);
                } else if (Lifecycle.Event.ON_DESTROY.equals(event)) {
                    unregister(key); // 反注册
                }
            }
        };

这里借助生命周期感知那一套，确保了register方法在调用的时候必须在STARTED之前注册，而在ON_DESTROY状态之后则会自动帮你反注册。这里的mKeyToCallback是一个Map<String, CallbackAndContract<?>> 类型的对象，以<key, 用户传入的Callback>形式保存回调。

看一下unregister(key)方法：

final void unregister(@NonNull String key) {
    if (!mLaunchedKeys.contains(key)) {
        // Only remove the key -> requestCode mapping if there isn't a launch in flight
        Integer rc = mKeyToRc.remove(key);
        if (rc != null) {
            mRcToKey.remove(rc);
        }
    }
    mKeyToCallback.remove(key); // 从map中移除callback对象
    ......
    LifecycleContainer lifecycleContainer = mKeyToLifecycleContainers.get(key);
    if (lifecycleContainer != null) {
        lifecycleContainer.clearObservers();
        mKeyToLifecycleContainers.remove(key);
    }
}

这里一个关键的地方就是LifecycleEventObserver在感知页面销毁时，就在unregister(key)方法中从mKeyToCallback这个map中删除用户传入的callback回调对象，从而避免了Activity销毁之后匿名内部类还会持有Activity对象的引用导致内存泄漏的问题。

对于contract参数，在ActivityResultContracts中提供了常用的内置类型，看名字也可以猜出是干嘛的，比如拍视频、选图片等，如果有需要，开发者可以之前从其中选择，如果是普通的Activity之间的跳转就选择StartActivityForResult这个类型即可。

在这里插入图片描述
不过这些类型你也可以统统不用，自己创建也可以，只需实现ActivityResultContract类满足它的输入输出类型约束即可。

对于ActivityResultLauncher.launch方法也没有什么神秘的，内部调用了ActivityResultRegistry 的onLaunch方法，而ActivityResultRegistry 是一个抽象类，其实例对象是在ComponentActivity的构造方法（源码一直在变，今天看又直接放到了成员变量中）创建的：

public abstract class ActivityResultRegistry {
	...
	@MainThread
    public abstract <I, O> void onLaunch(
            int requestCode,
            @NonNull ActivityResultContract<I, O> contract,
            @SuppressLint("UnknownNullness") I input,
            @Nullable ActivityOptionsCompat options);
	....
	@NonNull
    public final <I, O> ActivityResultLauncher<I> register(
            @NonNull final String key,
            @NonNull final LifecycleOwner lifecycleOwner,
            @NonNull final ActivityResultContract<I, O> contract,
            @NonNull final ActivityResultCallback<O> callback) {
    	.......
    	return new ActivityResultLauncher<I>() {
            @Override
            public void launch(I input, @Nullable ActivityOptionsCompat options) {
                Integer innerCode = mKeyToRc.get(key);
                if (innerCode == null) {
                    throw new IllegalStateException("Attempting to launch an unregistered "
                            + "ActivityResultLauncher with contract " + contract + " and input "
                            + input + ". You must ensure the ActivityResultLauncher is registered "
                            + "before calling launch().");
                }
                mLaunchedKeys.add(key);
                try {
                    onLaunch(innerCode, contract, input, options);
                } catch (Exception e) {
                    mLaunchedKeys.remove(key);
                    throw e;
                }
            }
            @Override
            public void unregister() {
                ActivityResultRegistry.this.unregister(key);
            }
            @NonNull
            @Override
            public ActivityResultContract<I, ?> getContract() {
                return contract;
            }
        };
    }
}

public ComponentActivity() {
		this.mActivityResultRegistry = new ActivityResultRegistry() {
            public <I, O> void onLaunch(final int requestCode, @NonNull ActivityResultContract<I, O> contract, I input, @Nullable ActivityOptionsCompat options) {
                ComponentActivity activity = ComponentActivity.this;
                	... 
                    ActivityCompat.startActivityForResult(activity, intent, requestCode, optionsBundle);
                }
            }
        };
}

这里最终还是执行的Activity的startActivityForResult方法，而在onActivityResult方法中又将结果转发回mActivityResultRegistry对象中：

public class ComponentActivity {
	...
    @Deprecated
    @CallSuper
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        if (!this.mActivityResultRegistry.dispatchResult(requestCode, resultCode, data)) {
            super.onActivityResult(requestCode, resultCode, data);
        }
    } 
}

public abstract class ActivityResultRegistry {
	...
    @MainThread
    public final boolean dispatchResult(int requestCode, int resultCode, @Nullable Intent data) {
        String key = mRcToKey.get(requestCode);
        if (key == null) {
            return false;
        }
        doDispatch(key, resultCode, data, mKeyToCallback.get(key));
        return true;
    }
    
	private <O> void doDispatch(String key, int resultCode, @Nullable Intent data,
            @Nullable CallbackAndContract<O> callbackAndContract) {
        if (callbackAndContract != null && callbackAndContract.mCallback != null
                && mLaunchedKeys.contains(key)) {
            ActivityResultCallback<O> callback = callbackAndContract.mCallback;
            ActivityResultContract<?, O> contract = callbackAndContract.mContract;
            callback.onActivityResult(contract.parseResult(resultCode, data));
            mLaunchedKeys.remove(key);
        } else {
            // Remove any parsed pending result
            mParsedPendingResults.remove(key);
            // And add these pending results in their place
            mPendingResults.putParcelable(key, new ActivityResult(resultCode, data));
        }
    }
}

很明显了，这里根据resultCode得到的key就是当初我们在register方法中传入的key，而传入的key又和传入的callback建立了Map映射关系，因此这里可以拿到我们在register方法中传入的callback进行结果回调。

总而言之一句话，只要官方想玩，源码在手，随时可以改，加个回调也是分分钟的事，就是这件事做的有点晚了，在androidx以前开发者对于startActivityForResult操作还是十分痛苦的（明明一整套的姿势，你非得给我拆开）。但是迟来总比没有好，相信很多公司目前仍有很多老项目有大量的代码甚至迁移到androidx都比较困难。

Jetpack Compose中如何正确的startActivityForResult

简单了解了androidx中如何正确的startActivityForResult后，在Jetpack Compose中使用就非常简单了，在Composable中使用主要通过一个rememberLauncherForActivityResult函数，可以看一下其实现源码：

@Composable
public fun <I, O> rememberLauncherForActivityResult(
    contract: ActivityResultContract<I, O>,
    onResult: (O) -> Unit
): ManagedActivityResultLauncher<I, O> {
    // Keep track of the current contract and onResult listener
    val currentContract = rememberUpdatedState(contract)
    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 = rememberSaveable { UUID.randomUUID().toString() }

    val activityResultRegistry = checkNotNull(LocalActivityResultRegistryOwner.current) {
        "No ActivityResultRegistryOwner was provided via LocalActivityResultRegistryOwner"
    }.activityResultRegistry
    val realLauncher = remember { ActivityResultLauncherHolder<I>() }
    val returnedLauncher = remember {
        ManagedActivityResultLauncher(realLauncher, currentContract)
    }

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

可以看到，它就是在DisposableEffect这个副作用Api中调用了以前的activityResultRegistry.register而已，返回的是一个ManagedActivityResultLauncher<I, O>类型的对象，这个对象中包装了以前的realLauncher和contract对象。所以说使用跟之前其实是差不多类似的。

选择文件示例：

@Composable
fun PickImage() {
    var uri by remember { mutableStateOf<Uri?>(null) }
    val launcher = rememberLauncherForActivityResult(ActivityResultContracts.GetContent()) {
        uri = it
    }
    Column(horizontalAlignment = Alignment.CenterHorizontally) {
        Button(onClick = { launcher.launch("image/*") }) {
            Text(text = "Pick a picture")
        }
        if (uri != null) {
            Image(
                painter = rememberAsyncImagePainter(uri) , // 使用coil加载图片
                modifier = Modifier.fillMaxWidth(),
                contentDescription = null
            )
        }
    }
}

拍照示例：

@Composable
fun TakePhotoForResult() {
    var resultBitmap by remember { mutableStateOf<Bitmap?>(null) }
    val scope = rememberCoroutineScope()

    val outPictureFile = LocalContext.current.getCacheImgFile()
    val pictureUri = LocalContext.current.getOutUri(outPictureFile)
    val launcher = rememberLauncherForActivityResult(ActivityResultContracts.TakePicture()) { isSuccess ->
        if (isSuccess) {
            scope.launch { resultBitmap = BitmapFactory.decodeFile(outPictureFile.path) }
        }
    }

    Column(horizontalAlignment = Alignment.CenterHorizontally) {
        Button(onClick = { launcher.launch(pictureUri) }) {
            Text(text = "Take a picture")
        }
        resultBitmap?.let { image ->
            Image(image.asImageBitmap(), null, modifier = Modifier.fillMaxWidth())
        }
    }
}

private fun Context.getCacheImgFile() : File {
    return File(externalCacheDir, "/${System.currentTimeMillis()}.jpeg")
}

private fun Context.getOutUri(file: File) : Uri {
    return FileProvider.getUriForFile(this, "${packageName}.provider", file)
}

如果只是想获得一张拍照预览图，可以使用下面的方式：

@Composable
fun TakePhotoForResult2() {
    var resultBitmap by remember { mutableStateOf<Bitmap?>(null) }
    val launcher = rememberLauncherForActivityResult(ActivityResultContracts.TakePicturePreview()) { bitmap ->
        resultBitmap = bitmap
    }

    Column(horizontalAlignment = Alignment.CenterHorizontally) {
        Button(onClick = { launcher.launch() }) {
            Text(text = "Take a picture")
        }
        resultBitmap?.let { image ->
            Image(image.asImageBitmap(), null, modifier = Modifier.fillMaxWidth())
        }
    }
}

这种方式更加简单，但是这种方式得到的是一个略缩图的尺寸大小，比较模糊。

启动其他的业务Activity：

@Composable
fun StartActivityForResultExample() {
    var result by remember { mutableStateOf("") } 
    val target = Intent(LocalContext.current, OtherActivity::class.java).apply {
        putExtra("name", "张三")
        putExtra("uid", 123)
    }
    val launcher = rememberLauncherForActivityResult(ActivityResultContracts.StartActivityForResult()) { activityResult ->
        activityResult.data?.apply {
            result = getStringExtra("name").toString()
        }
    }
    Column(horizontalAlignment = Alignment.CenterHorizontally) {
        Button(onClick = { launcher.launch(target) }) {
            Text(text = "StartActivityForResult")
        }
        Text(text = "result: $result", fontSize = 22.sp)
    }
}

可见使用方式跟之前几乎是一致的。

注：本文示例代码所有涉及到使用权限的地方需要自己提前申请好。

参考：
获取 activity 的结果