private async Task<TResult> DoSomeStuffAsync(..) { .. await .. .. }l 关键词 async 本身不具备什么意义,只是装饰,当方法冠以 async 关键词,方法体内允许使用 await
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非阻止操作 |
阻止操作 |
备注 |
获取任务返回 | await task | task.Wait / task.Result |
非阻塞:线程遇到 await 时会返回上一层调用者继续执行,如果没有上一级调用者,则释放该线程; 阻塞:线程在等待期间不能执行其他任务,也不释放线程,硬等
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任一任务完成 |
await Task.WhenAny |
Task.WaitAny |
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所有任务完成 |
await Task.WhenAll |
Task.WaitAll |
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等待一段时间 |
await Task.Delay |
Thread.Sleep |
l 异步方法返回的值总是 Task 的实例,可以是 Task 类型或 Task<TResult>,其中 TResult 是执行的方法的返回类型
public static async Task Main(string[] args) { logMessage("Main <<----"); var task = GetUrlContentLengthAsync(); logMessage("Main ---->>"); await task; logMessage("ALL COMPLETED"); } static async Task<int> GetUrlContentLengthAsync() { logMessage("GetUrlContentLengthAsync start "); using var client = new HttpClient(); Task<string> getStringTask = client.GetStringAsync("https://learn.microsoft.com/dotnet"); // Do some independent work.. // 堆代码 duidaima.com var contents = await getStringTask; logMessage("GetUrlContentLengthAsync end "); return contents.Length; } static void logMessage(string msg) { Console.WriteLine($"{DateTime.Now.ToString("MM-dd HH:mm:ss.fff")} [{Thread.CurrentThread.ManagedThreadId}] {msg}"); }打印结果:
11-18 18:34:34.563 [1] Main <<---- 11-18 18:34:34.632 [1] GetUrlContentLengthAsync start 11-18 18:34:34.810 [1] Main ---->> 11-18 18:34:37.283 [7] GetUrlContentLengthAsync end 11-18 18:34:37.286 [7] ALL COMPLETED先分析一下 GetUrlContentLengthAsync 这个异步方法,简单归纳会存在以下步骤:
public static async Task Main(string[] args) { logMessage("Main <<----"); // fault example: if do not wait the result then we will do not know what happened on it GetSomeStuff(); logMessage("Main ---->>"); }这个异步任务已经在执行,但是却没有后续处理,不知道是成功或是失败,又或者一直在执行没有办法停止,这是危险的。
public async Task<ResultResponse> ValidateReceiptAsync(List<Guid> customerIds, ReceiptRequest request, CancellationToken cancellationToken) { var tokenTask = _medicalCheckServiceHelper.GetServiceToken(cancellationToken); var medicalProvidersTask = _medicalProviderRepo.GetMedicalProvidersNames(); var customersTask = _customerService.GetByIdsAsync(customerIds); var checkResults = await Task.WhenAll(request.UuidList.Select(uuid => _medicalCheckServiceHelper.GetResultAsync(uuid, tokenTask.Result, cancellationToken))); return await ConsolidateReceiptsResult(medicalProvidersTask.Result, checkResults, await customersTask); }思考一下
public async Task<ResultResponse> ValidateReceiptAsync(List<string> customerIds, ReceiptRequest request, CancellationToken cancellationToken) { var tokenTask = _medicalCheckServiceHelper.GetServiceToken(cancellationToken); var medicalProvidersTask = _medicalProviderRepo.GetMedicalProvidersNames(); var token = await tokenTask; var checkResults = await Task.WhenAll(request.UuidList.Select(uuid => _medicalCheckServiceHelper.GetResultAsync(uuid, token, cancellationToken))); var medicalProviders = await medicalProvidersTask; var customers = await _customerService.GetByIdsAsync(customerIds); return await ConsolidateReceiptsResult(medicalProviders, checkResults, customers); }i. taskA 取自于配置中心,也即它会走 HTTP 请求
public async Task<ResultResponse> ValidateReceiptAsync(List<string> customerIds, ReceiptRequest request, CancellationToken cancellationToken) { var tokenTask = _medicalCheckServiceHelper.GetServiceToken(cancellationToken); var medicalProvidersTask = _medicalProviderRepo.GetMedicalProvidersNames(); var customersTask = _customerService.GetByIdsAsync(customerIds); var token = await tokenTask; var checkResultsTask = Task.WhenAll(request.UuidList.Select(uuid => _medicalCheckServiceHelper.GetResultAsync(uuid, token, cancellationToken))); var medicalProviders = await medicalProvidersTask; var customers = await customersTask; var checkResults = await checkResultsTask; return await ConsolidateReceiptsResult(medicalProviders, checkResults, customers); }但其实并不尽完善,因为这种写法的可读性并没有那么好,看起来更追求资源优化。