对 Kotlin 协程的现有 3 函数回调

Question

我有一个带有具体示例的一般性问题：我想在 Android 拍照时使用 Kotlin 协程魔术而不是回调地狱。

manager.openCamera(cameraId, object : CameraDevice.StateCallback() {
    override fun onOpened(openedCameraDevice: CameraDevice) {
        println("Camera onOpened")
        // even more callbacks with openedCameraDevice.createCaptureRequest()....
    }

    override fun onDisconnected(cameraDevice: CameraDevice) {
        println("Camera onDisconnected")
        cameraDevice.close()
    }
    ...

我如何将其转换为不那么丑陋的东西？ 是否可以采用三个左右函数的平均回调，并通过将主要流程指定为承诺-结果路径将其转变为承诺链？ 如果是这样，should/do 我使用协程使其异步？

我喜欢带有 async 和 .await 的东西，它会导致

manager.open(cameraId).await().createCaptureRequest()

我正在尝试通过类似以下的方法来实现，但我认为我使用的 CompletableDeferred 不对！

suspend fun CameraManager.open(cameraId:String): CameraDevice {
    val response = CompletableDeferred<CameraDevice>()
    this.openCamera(cameraId, object : CameraDevice.StateCallback() {
        override fun onOpened(cameraDevice: CameraDevice) {
            println("camera onOpened $cameraDevice")
            response.complete(cameraDevice)
        }

        override fun onDisconnected(cameraDevice: CameraDevice) {
            response.completeExceptionally(Exception("Camera onDisconnected $cameraDevice"))
            cameraDevice.close()
        }

        override fun onError(cameraDevice: CameraDevice, error: Int) {
            response.completeExceptionally(Exception("Camera onError $cameraDevice $error"))
            cameraDevice.close()
        }
    }, Handler())
    return response.await()
}

Answer 1

对于这类事情，我使用了 2 种解决方案。

1：将接口包装在扩展中

CameraDevice.openCamera(cameraId: Integer, 
                onOpenedCallback: (CameraDevice) -> (), 
          onDisconnectedCallback: (CameraDevice) ->()) {

    manager.openCamera(cameraId, object : CameraDevice.StateCallback() {
        override fun onOpened(openedCameraDevice: CameraDevice) {
            onOpenedCallback(openedCameraDevice)
        }

        override fun onDisconnected(cameraDevice: CameraDevice) {
            onDisconnectedCallback(cameraDevice)
        }
   })
}

2: 制作一个简单的容器 class 具有更实用的界面:

class StateCallbackWrapper(val onOpened: (CameraDevice) -> (), val onClosed: (CameraDevice) ->()): CameraDevice.StateCallback() {
    override fun onOpened(openedCameraDevice: CameraDevice) {
        onOpened(openedCameraDevice)
    }

    override fun onDisconnected(cameraDevice: CameraDevice) {
        onClosed(cameraDevice)
    }
}

就我个人而言，我会从这些东西开始，然后在此基础上构建任何线程差异。

Answer 2

在这种特殊情况下，您可以使用通用方法通过 suspendCoroutine 函数将基于回调的 API 转换为挂起函数：

suspend fun CameraManager.openCamera(cameraId: String): CameraDevice? =
    suspendCoroutine { cont ->
        val callback = object : CameraDevice.StateCallback() {
            override fun onOpened(camera: CameraDevice) {
                cont.resume(camera)
            }

            override fun onDisconnected(camera: CameraDevice) {
                cont.resume(null)
            }

            override fun onError(camera: CameraDevice, error: Int) {
                // assuming that we don't care about the error in this example
                cont.resume(null) 
            }
        }
        openCamera(cameraId, callback, null)
    }

现在，在您的应用程序代码中，您只需执行 manager.openCamera(cameraId) 并获取对 CameraDevice 的引用（如果打开成功）或 null（如果未成功打开）。

Answer 3

使用 suspendCancellableCoroutine 而不是具有适当异常处理的 suspendCoroutine

suspend fun CameraManager.openCamera(cameraId: String): CameraDevice? =
        suspendCancellableCoroutine { cont ->
             val callback = object : CameraDevice.StateCallback() {
                 override fun onOpened(camera: CameraDevice) {
                   cont.resume(camera)
               }

               override fun onDisconnected(camera: CameraDevice) {
                   cont.resume(null)
               }

               override fun onError(camera: CameraDevice, error: Int) {
                   // Resume the coroutine by throwing an exception or resume with null
                   cont.resumeWithException(/* Insert a custom exception */) 
               }
        }
        openCamera(cameraId, callback, null)
    }

最好始终选择 suspendCancellableCoroutine 来处理协程范围的取消，或者从底层传播取消 API。 Source with other great examples

对 Kotlin 协程的现有 3 函数回调

Existing 3-function callback to Kotlin Coroutines

android

kotlin

kotlin-coroutines