最近项目中统一采用Kotlin的Flow来重构了网络请求相关代码。
目前的场景是，接口在请求的时候需要一个accessToken值，因为此值会过期或者不存在，需要刷新，因此最终方案是在使用Flow请求的时候先获取accessToken值然后再进行接口请求，而获取accessToken值的方法已经封装成了一个Flow并且做了缓存，因此最后需要使用flatMapConcat操作符来连接真正需要的接口请求，如果获取的accessToken无效，又需要回头重新执行，逻辑如下：
- 判断本地是否存在accessToken并且是否过期，不存在或者已过期则请求accessToken
- 请求对应的接口
- 如果返回结果中accessToken无效，则重试
Flow提供了retry和retryWhen两种扩展方法来做重试操作：
retry源码
public fun <T> Flow<T>.retry(retries: Long = Long.MAX_VALUE,predicate: suspend (cause: Throwable) -> Boolean = { true }
): Flow<T> {require(retries > 0) { "Expected positive amount of retries, but had $retries" }return retryWhen { cause, attempt -> attempt < retries && predicate(cause) }
}
retryWhen源码
public fun <T> Flow<T>.retryWhen(predicate: suspend FlowCollector<T>.(cause: Throwable, attempt: Long) -> Boolean): Flow<T> =flow {var attempt = 0Lvar shallRetry: Booleando {shallRetry = falseval cause = catchImpl(this)if (cause != null) {if (predicate(cause, attempt)) {shallRetry = trueattempt++} else {throw cause}}} while (shallRetry)}
但是，retry和retryWhen只能通过异常来判断，如果是通过返回结果来判断，就需要借助外部变量来处理了，因此基于源码扩展了方法retry，可以接收请求结果，从而通过请求结果来判断是否需要重试。
fun <T> Flow<T>.retry(retries: Long = Long.MAX_VALUE, predicate: suspend (result: T) -> Boolean = { true }
): Flow<T> {require(retries > 0) { "Expected positive amount of retries, but had $retries" }return flow {var attempt = 0Lvar shallRetry: Booleando {shallRetry = falsetry {collect {if (attempt < retries && predicate(it)) {shallRetry = trueattempt++} else {this.emit(it)}}} catch (e: Throwable) {throw e}} while (shallRetry)}
}
最后的请求示例代码如下：
MainScope().launch {getToken().flatMapConcat {if (it is Result.Success) {sendMobileCode()} else {emptyFlow()}}.retry(1) {return@retry (it is Result.Failure) && (it.code == ErrorStatus.ACCESS_TOKEN_ERROR)}.flowOn(Dispatchers.IO).onStart {callback?.onStart()}.catch {callback?.onError(it)}.onCompletion {callback?.onComplete(it)}.collectLatest { result ->}}
感谢大家的支持，如有错误请指正，如需转载请标明原文出处！