用ActivityResultContracts代替startActivityForResult

activity之间的传递不一定是单向的，有时候会需要从跳转过去的activity往前回传数据（例如，您的应用可启动相机应用并接收拍摄的照片作为结果），过去我们一般采用底层 startActivityForResult() 和 onActivityResult() API，现在谷歌推出了新的Activity Result API为我们解决这类问题。

即ActivityResultContracts

基础用法

1. 在 ComponentActivity 或 Fragment 中，使用 registerForActivityResult() API，用于注册结果回调。此方法传入两个参数，ActivityResultContract 和 ActivityResultCallback，该方法返回 ActivityResultLauncher，供您用来启动另一个 activity。

   val launcher = 
   registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { res:ActivityResult ? ->
    if (result.resultCode == BACTIVITY_RESULT_CODE){
           val data = result.data?.extras?.getString("extra_data")
           data?.let { toast(it) }
       }
   }
   

2. 用返回的launcher启动另一个activity.

   val intent = Intent(this, MainActivity::class.java)
   launcher.launch(intent)
   

   注意：步骤一registerForActivityResult要在oncreate之前调用(即作为当前activity的属性声明)；

   步骤二 launch 要在oncreate之后调用。

3. 第二个activity还是照常使用setResult回传数据。

   setResult(BACTIVITY_RESULT_CODE, Intent().putExtra("extra-data", "data"))
   

ActivityResultContract （协定）

第一步的registerForActivityResult方法传的第一个参数是ActivityResultContract，我们称之为协定。里面要约定两个东西：1. 你启动launcher时要传入的对象（本例中为intent）；2. 返回到当前页面时带回来的对象（本例中为ActivityResult）。

第二个参数就是带着你约定好的对象的回调了。

示例中用的约定是：ActivityResultContracts.StartActivityForResult()，这是系统提供的，意思是传入Intent,返回ActivityResult。

除此之外，系统还提供了很多，如ActivityResultContracts.GetContent()，传入string,返回UrI,一般是用来获取系统资源用的，像传入"image/*",然后返回图片URI{content://} 之类的。

val getContent = registerForActivityResult(GetContent()) { uri: Uri? ->
    // Handle the returned Uri
}

override fun onCreate(savedInstanceState: Bundle?) {
    // ...

    val selectButton = findViewById(R.id.select_button)

    selectButton.setOnClickListener {
        // Pass in the mime type you'd like to allow the user to select
        // as the input
        getContent.launch("image/*")
    }
}

我们用Input表示传入的约定，Output表示回传回来的约定。协定就可以抽象成：

public abstract class ActivityResultContract {
    /**
     * 创建Intent，不管你约定的输入是啥，要跳转最后还是用的Intent跳，所以在这得自己创建Intent
     **/
    public abstract Intent createIntent(Context context,I input);
   
    /**
     * 解析回调的结果
     **/
    public abstract O parseResult(int resultCode, @Nullable Intent intent);
    
    /**
     * 可选
     **/
    public SynchronousResult getSynchronousResult(Context context,I input) {
        return null;
    }
}

先来看示例中用的这个协定 - StartActivityForResult

官方提供的协定

public static final class StartActivityForResult extends ActivityResultContract {

    @NonNull
    @Override
    public Intent createIntent(@NonNull Context context, @NonNull Intent input) {
        return input;
    }

    @NonNull
    @Override
    public ActivityResult parseResult(
            int resultCode, @Nullable Intent intent) {
        return new ActivityResult(resultCode, intent);
    }
}

很简单，传入的就是intent，返回的就是resultCode + intent的封装类；

再看下刚刚提到的传图片的那个：

public static class GetContent extends ActivityResultContract {

    //CallSuper是提醒继承该类的话应该重写该方法，上面那个是final不能重写，所以没有
    @CallSuper
    @NonNull
    @Override
    public Intent createIntent(@NonNull Context context, @NonNull String input) {
        return new Intent(Intent.ACTION_GET_CONTENT)
                .addCategory(Intent.CATEGORY_OPENABLE)
                .setType(input);
    }

    @Nullable
    @Override
    public final SynchronousResult getSynchronousResult(@NonNull Context context,
            @NonNull String input) {
        return null;
    }

    @Nullable
    @Override
    public final Uri parseResult(int resultCode, @Nullable Intent intent) {
        if (intent == null || resultCode != Activity.RESULT_OK) return null;
        return intent.getData();
    }
}

传入特定的Intent，返回的结果先解析下再返回回去。

理解了这个协定后，我们也可以自己约定协定。

创建自定义协定

比如说一个时间选择的Activity，它的返回到上一页面的时候要携带当前界面选择的时间。因为进入到该界面的时候Intent是固定的，所以Input可以传入void，意思是不用传。

class TimeResultContract : ActivityResultContract(){
    override fun createIntent(context: Context, input: Unit?): Intent {
        return Intent(context,TimeSelectActivity::class.java)
    }

    override fun parseResult(resultCode: Int, intent: Intent?): String {
        if (resultCode != RESULT_OK){
            return "error"
        }
        return intent?.getStringExtra(TimeSelectActivity_EXTRA_DATE).toString()
    }

}

用的时候只需要在需要跳转的activity中声明即可。

val timeLauncher = registerForActivityResult(TimeResultContract()){ time ->
    toast(time)
    viewBinding.tvTime.text = time
}

相比以前需要把requestCode判断的逻辑写到各个Activity中的写法，这样封装起来看起来是不是干净多了？

如果你还想把registerForActivityResult这个方法也提到activity外面去，也是可以做到的。

进阶

在单独的类中接收 activity 结果

之所以能在ComponentActivity 和 Fragment 类中直接调用registerForActivityResult()方法是因为它们实现了ActivityResultCaller接口。如果你想在未实现ActivityResultCaller接口的类中获取launcher，那么就需要用到ActivityResultRegistry 类了。

例如，您可能需要实现一个 LifecycleObserver，用于处理协定的注册和启动器的启动：

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

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

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

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

    override fun onCreate(savedInstanceState: Bundle?) {
        // ...

        observer = MyLifecycleObserver(requireActivity().activityResultRegistry)
        lifecycle.addObserver(observer)
    }

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

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

源码解析

从上面的用法可以看出，实现这个功能的核心其实是传进来的registry:ActivityResultRegistry。 而在ComponentActivity和Fragment中都维护了有一个registry,以activity为例：

private final ActivityResultRegistry mActivityResultRegistry = new ActivityResultRegistry() {

    @Override
    public  void onLaunch( final int requestCode, @NonNull ActivityResultContract contract,
            I input, @Nullable ActivityOptionsCompat options) {
        ComponentActivity activity = ComponentActivity.this;

        // Immediate result path
        final ActivityResultContract.SynchronousResult synchronousResult =
                contract.getSynchronousResult(activity, input);
        if (synchronousResult != null) {
            new Handler(Looper.getMainLooper()).post(new Runnable() {
                @Override
                public void run() {
                    dispatchResult(requestCode, synchronousResult.getValue());
                }
            });
            return;
        }

        // Start activity path
        Intent intent = contract.createIntent(activity, input);
        Bundle optionsBundle = null;
        // If there are any extras, we should defensively set the classLoader
        if (intent.getExtras() != null && intent.getExtras().getClassLoader() == null) {
            intent.setExtrasClassLoader(activity.getClassLoader());
        }
        if (intent.hasExtra(EXTRA_ACTIVITY_OPTIONS_BUNDLE)) {
            optionsBundle = intent.getBundleExtra(EXTRA_ACTIVITY_OPTIONS_BUNDLE);
            intent.removeExtra(EXTRA_ACTIVITY_OPTIONS_BUNDLE);
        } else if (options != null) {
            optionsBundle = options.toBundle();
        }
        if (ACTION_REQUEST_PERMISSIONS.equals(intent.getAction())) {

            // requestPermissions path
            String[] permissions = intent.getStringArrayExtra(EXTRA_PERMISSIONS);

            if (permissions == null) {
                permissions = new String[0];
            }

            ActivityCompat.requestPermissions(activity, permissions, requestCode);
        } else if (ACTION_INTENT_SENDER_REQUEST.equals(intent.getAction())) {
            IntentSenderRequest request =
                    intent.getParcelableExtra(EXTRA_INTENT_SENDER_REQUEST);
            try {
                // startIntentSenderForResult path
                ActivityCompat.startIntentSenderForResult(activity, request.getIntentSender(),
                        requestCode, request.getFillInIntent(), request.getFlagsMask(),
                        request.getFlagsValues(), 0, optionsBundle);
            } catch (final IntentSender.SendIntentException e) {
                new Handler(Looper.getMainLooper()).post(new Runnable() {
                    @Override
                    public void run() {
                        dispatchResult(requestCode, RESULT_CANCELED,
                                new Intent().setAction(ACTION_INTENT_SENDER_REQUEST)
                                        .putExtra(EXTRA_SEND_INTENT_EXCEPTION, e));
                    }
                });
            }
        } else {
            // startActivityForResult path
            ActivityCompat.startActivityForResult(activity, intent, requestCode, optionsBundle);
        }
    }
};

然后实际调用的是registry.register()方法（这是个抽象类，该方法写在抽象类中）

public final  ActivityResultLauncher register(
        @NonNull final String key,
        @NonNull final LifecycleOwner lifecycleOwner,
        @NonNull final ActivityResultContract contract,
        @NonNull final ActivityResultCallback 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.");
    }

    final int requestCode = registerKey(key);
    LifecycleContainer lifecycleContainer = mKeyToLifecycleContainers.get(key);
    if (lifecycleContainer == null) {
        lifecycleContainer = new LifecycleContainer(lifecycle);
    }
    LifecycleEventObserver observer = new LifecycleEventObserver() {
        @Override
        public void onStateChanged(
                @NonNull LifecycleOwner lifecycleOwner,
                @NonNull Lifecycle.Event event) {
            if (Lifecycle.Event.ON_START.equals(event)) {
                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);
            }
        }
    };
    lifecycleContainer.addObserver(observer);
    mKeyToLifecycleContainers.put(key, lifecycleContainer);

    return new ActivityResultLauncher() {
        @Override
        public void launch(I input, @Nullable ActivityOptionsCompat options) {
            mLaunchedKeys.add(key);
            onLaunch(requestCode, contract, input, options);
        }

        @Override
        public void unregister() {
            ActivityResultRegistry.this.unregister(key);
        }

        @NonNull
        @Override
        public ActivityResultContract getContract() {
            return contract;
        }
    };
}

可以看到这里对生命周期所在状态进行了很多判断，可以防止声明周期外的内存泄漏的问题。

调用launch方法的时候实际走的是onLaunch()，也就是第一段代码里面的内容，最终调用的还是startActivityForResult方法，所以这个新的用法还是基于startActivityForResult的，但是却很好的将回传的数据的判断相关功能封装到了协定类中，以后就算有多个回传数据，用起来也会很清晰，代码就不会臃肿到onActivityResult中了。

示例-获取权限

request_permission.setOnClickListener {
    requestPermission.launch(permission.BLUETOOTH)
}

request_multiple_permission.setOnClickListener {
    requestMultiplePermissions.launch(
        arrayOf(
            permission.BLUETOOTH,
            permission.NFC,
            permission.ACCESS_FINE_LOCATION
        )
    )
}

// Request permission contract
private val requestPermission =
    registerForActivityResult(ActivityResultContracts.RequestPermission()) { isGranted ->
        // Do something if permission granted
        if (isGranted) toast("Permission is granted")
        else toast("Permission is denied")
    }

// Request multiple permissions contract
private val requestMultiplePermissions =
    registerForActivityResult(ActivityResultContracts.RequestMultiplePermissions()) { permissions : Map ->
        // Do something if some permissions granted or denied
        permissions.entries.forEach {
            // Do checking here
        }                                                                             
}

相较原来的获取权限的写法清晰多了。