android - 如何在 kotlin 协程上进行指数退避重试

Question

我正在使用 kotlin 协程进行网络请求，使用扩展方法调用改造中的类，像这样

public suspend fun <T : Any> Call<T>.await(): T {

  return suspendCancellableCoroutine { continuation -> 

    enqueue(object : Callback<T> {

        override fun onResponse(call: Call<T>?, response: Response<T?>) {
            if (response.isSuccessful) {
                val body = response.body()
                if (body == null) {
                    continuation.resumeWithException(
                            NullPointerException("Response body is null")
                    )
                } else {
                    continuation.resume(body)
                }
            } else {
                continuation.resumeWithException(HttpException(response))
            }
        }

        override fun onFailure(call: Call<T>, t: Throwable) {
            // Don't bother with resuming the continuation if it is already cancelled.
            if (continuation.isCancelled) return
            continuation.resumeWithException(t)
        }
    })

      registerOnCompletion(continuation)
  }
}

然后从调用方我使用上面这样的方法

private fun getArticles()  = launch(UI) {

    loading.value = true
    try {
        val networkResult = api.getArticle().await()
        articles.value =  networkResult

    }catch (e: Throwable){
        e.printStackTrace()
        message.value = e.message

    }finally {
        loading.value = false
    }

}

我想在某些情况下以指数方式重试这个 api 调用，即(IOException)我怎样才能实现它？？

Answer 1

我建议写一个助手 higher-order function为您的重试逻辑。您可以使用以下实现作为开始:

suspend fun <T> retryIO(
    times: Int = Int.MAX_VALUE,
    initialDelay: Long = 100, // 0.1 second
    maxDelay: Long = 1000,    // 1 second
    factor: Double = 2.0,
    block: suspend () -> T): T
{
    var currentDelay = initialDelay
    repeat(times - 1) {
        try {
            return block()
        } catch (e: IOException) {
            // you can log an error here and/or make a more finer-grained
            // analysis of the cause to see if retry is needed
        }
        delay(currentDelay)
        currentDelay = (currentDelay * factor).toLong().coerceAtMost(maxDelay)
    }
    return block() // last attempt
}

使用这个函数很简单:

val networkResult = retryIO { api.getArticle().await() }

您可以根据具体情况更改重试参数，例如:

val networkResult = retryIO(times = 3) { api.doSomething().await() }

您还可以完全更改 retryIO 的实现以适应您的应用程序的需要。例如，您可以硬编码所有重试参数，摆脱重试次数限制，更改默认值等。