通过ASP.NET Core中的控制器操作运行后台任务

问题描述

我正在使用带有ASP.NET Core 2.0的C#的REST API开发Web应用程序.

I am developing a web application with a REST API using C# with ASP.NET Core 2.0.

我想要实现的是，当客户端向端点发送请求时，我将运行一个与客户端请求上下文分离的后台任务，如果任务成功启动，该任务将结束.

What I want to achieve is when the client send a request to an endpoint I will run a background task separated from the client request context which will be ended if the task started successfully.

我知道有 HostedService ，但是问题是 HostedService 在服务器启动时启动，据我所知没有办法启动 HostedService .

I know there is HostedService but the problem is that the HostedService starts when the server starts, and as far as I know there is no way to start the HostedService manually from a controller.

这是一个演示问题的简单代码.

Here is a simple code that demonstrates the question.

[Authorize(AuthenticationSchemes = "UsersScheme")]
public class UsersController : Controller
{
    [HttpPost]
    public async Task<JsonResult> StartJob([FromForm] string UserId, [FromServices] IBackgroundJobService backgroundService)
    {
        // check user account
        (bool isStarted, string data) result = backgroundService.Start();

        return JsonResult(result);
    }
}

推荐答案

您仍然可以结合使用 IHostedService 和 BlockingCollection 作为后台任务的基础.

You still can use IHostedService as base for background tasks in combination with BlockingCollection.

为 BlockingCollection 创建包装器，以便将其作为单例注入.

Create wrapper for BlockingCollection so you can inject it as singleton.

public class TasksToRun
{
    private readonly BlockingCollection<TaskSettings> _tasks;

    public TasksToRun() => _tasks = new BlockingCollection<TaskSettings>();

    public void Enqueue(TaskSettings settings) => _tasks.Add(settings);

    public TaskSettings Dequeue(CancellationToken token) => _tasks.Take(token);
}

然后在实现 IHostedService 的过程中监听"任务，并在任务到达"时执行它.
如果集合为空，则 BlockingCollection 将停止执行-这样，您的 while 循环就不会占用处理器时间.
.Take 方法接受 cancellationToken 作为参数.使用令牌，您可以在应用程序停止时取消等待"下一个任务.

Then in implementation of IHostedService "listen" for tasks and when tasks "arrive" execute it.
BlockingCollection will stop execution if collection is empty - so your while loop will not consume processor time.
.Take method accept cancellationToken as argument. With token you can cancel "waiting" for next task when application stops.

public class BackgroundService : IHostedService
{
    private readonly TasksToRun _tasks;

    private CancellationTokenSource _tokenSource;

    private Task _currentTask;

    public BackgroundService(TasksToRun tasks) => _tasks = tasks;

    public async Task StartAsync(CancellationToken cancellationToken)
    {
        _tokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        while (cancellationToken.IsCancellationRequested == false)
        {
            try
            {
                var taskToRun = _tasks.Dequeue(_tokenSource.Token);

                // We need to save executable task,
                // so we can gratefully wait for it's completion in Stop method
                _currentTask = ExecuteTask(taskToRun);
                await _currentTask;
            }
            catch (OperationCanceledException)
            {
                // execution cancelled
            }
        }
    }

    public async Task StopAsync(CancellationToken cancellationToken)
    {
        _tokenSource.Cancel(); // cancel "waiting" for task in blocking collection

        if (_currentTask == null) return;

        // wait when _currentTask is complete
        await Task.WhenAny(_currentTask, Task.Delay(-1, cancellationToken));
    }
}

在控制器中，您只需将要运行的任务添加到我们的集合中

And in the controller you simply add task you want to run to our collection

public class JobController : Controller
{
    private readonly TasksToRun _tasks;

    public JobController(TasksToRun tasks) => _tasks = tasks;

    public IActionResult PostJob()
    {
        var settings = CreateTaskSettings();

        _tasks.Enqueue(settings);

        return Ok();
    }
}

用于阻止收集的包装器应注册为单例进行依赖项注入

Wrapper for blocking collection should be registered for dependency injection as singleton

services.AddSingleton<TasksToRun, TasksToRun>();

注册后台服务

services.AddHostedService<BackgroundService>();

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