这篇简单讲asp.net core 中的后台任务
用到的包：
Microsoft.AspNetCore.App metapackage
或者加入
一. Timed background tasks（定时后台任务）
使用到System.Threading.Timer类。定时器触发任务的DoWork方法。定时器在StopAsync上停止，并且释放是在Dispose上
internal class TimedHostedService : IHostedService, IDisposable
{
private readonly ILogger _logger;
private Timer _timer;
public TimedHostedService(ILogger<TimedHostedService> logger)
{
_logger = logger;
}
public Task StartAsync(CancellationToken cancellationToken)
{
_logger.LogInformation("Timed Background Service is starting.");
_timer = new Timer(DoWork, null, TimeSpan.Zero,
TimeSpan.FromSeconds(5));
return Task.CompletedTask;
}
private void DoWork(object state)
{
_logger.LogInformation("Timed Background Service is working.");
}
public Task StopAsync(CancellationToken cancellationToken)
{
_logger.LogInformation("Timed Background Service is stopping.");
_timer?.Change(Timeout.Infinite, 0);
return Task.CompletedTask;
}
public void Dispose()
{
_timer?.Dispose();
}
}
服务是在Startup.ConfigureServices上使用AddHostedService扩展方法注册：
services.AddHostedService<TimedHostedService>();
二. Consuming a scoped service in a background task 在后台任务中运行scoped service
使用IHostService中的scoped services, 创建一个scope. 对于一个hosted service默认没有scope被创建。
这个scoped 后台任务服务包含后台任务逻辑。下面的例子中，一个ILogger被注入到了service中：
internal interface IScopedProcessingService
{
void DoWork();
}
internal class ScopedProcessingService : IScopedProcessingService
{
private readonly ILogger _logger;
public ScopedProcessingService(ILogger<ScopedProcessingService> logger)
{
_logger = logger;
}
public void DoWork()
{
_logger.LogInformation("Scoped Processing Service is working.");
}
}
这个hosted service 创建了一个scope解析了scoped后台任务服务来调用它的DoWork方法：
internal class ConsumeScopedServiceHostedService : IHostedService
{
private readonly ILogger _logger;
public ConsumeScopedServiceHostedService(IServiceProvider services,
ILogger<ConsumeScopedServiceHostedService> logger)
{
Services = services;
_logger = logger;
}
public IServiceProvider Services { get; }
public Task StartAsync(CancellationToken cancellationToken)
{
_logger.LogInformation(
"Consume Scoped Service Hosted Service is starting.");
DoWork();
return Task.CompletedTask;
}
private void DoWork()
{
_logger.LogInformation(
"Consume Scoped Service Hosted Service is working.");
using (var scope = Services.CreateScope())
{
var scopedProcessingService =
scope.ServiceProvider
.GetRequiredService<IScopedProcessingService>();
scopedProcessingService.DoWork();
}
}
public Task StopAsync(CancellationToken cancellationToken)
{
_logger.LogInformation(
"Consume Scoped Service Hosted Service is stopping.");
return Task.CompletedTask;
}
}
服务注册在Startup.ConfigureServices中。IHostedService的实现用AddHostedService扩展方法注册：
services.AddHostedService<ConsumeScopedServiceHostedService>(); services.AddScoped<IScopedProcessingService, ScopedProcessingService>();
三. Queued background tasks 排队的后台任务
public interface IBackgroundTaskQueue
{
void QueueBackgroundWorkItem(Func<CancellationToken, Task> workItem);
Task<Func<CancellationToken, Task>> DequeueAsync(
CancellationToken cancellationToken);
}
public class BackgroundTaskQueue : IBackgroundTaskQueue
{
private ConcurrentQueue<Func<CancellationToken, Task>> _workItems =
new ConcurrentQueue<Func<CancellationToken, Task>>();
private SemaphoreSlim _signal = new SemaphoreSlim(0);
public void QueueBackgroundWorkItem(
Func<CancellationToken, Task> workItem)
{
if (workItem == null)
{
throw new ArgumentNullException(nameof(workItem));
}
_workItems.Enqueue(workItem);
_signal.Release();
}
public async Task<Func<CancellationToken, Task>> DequeueAsync(
CancellationToken cancellationToken)
{
await _signal.WaitAsync(cancellationToken);
_workItems.TryDequeue(out var workItem);
return workItem;
}
}
在 QueueHostedService中，队列中的后台任务出队列并且作为BackroundService执行。BackgroundService是一个实现了IHostedService接口的类。
public class QueuedHostedService : BackgroundService
{
private readonly ILogger _logger;
public QueuedHostedService(IBackgroundTaskQueue taskQueue,
ILoggerFactory loggerFactory)
{
TaskQueue = taskQueue;
_logger = loggerFactory.CreateLogger<QueuedHostedService>();
}
public IBackgroundTaskQueue TaskQueue { get; }
protected async override Task ExecuteAsync(
CancellationToken cancellationToken)
{
_logger.LogInformation("Queued Hosted Service is starting.");
while (!cancellationToken.IsCancellationRequested)
{
var workItem = await TaskQueue.DequeueAsync(cancellationToken);
try
{
await workItem(cancellationToken);
}
catch (Exception ex)
{
_logger.LogError(ex,
$"Error occurred executing {nameof(workItem)}.");
}
}
_logger.LogInformation("Queued Hosted Service is stopping.");
}
}
服务注册在Startup.ConfigureService方法中。IHostedService的实现用AddHostedService扩展方法注册：
services.AddHostedService<QueuedHostedService>(); services.AddSingleton<IBackgroundTaskQueue, BackgroundTaskQueue>();
在Index page model类中：
- IBackgroundTaskQueue被注入到构造函数并且指定给Queue
- 一个 IServiceScopeFactory被注入并且指定给_serviceScopeFactory. 这个工厂用来创建IServiceScope实例， IServiceScope实例是用来在scope内创建 services的。一个scope被创建时为了用应用的AppDbContext（a scoped service）来写数据库记录在 IBackgroundTaskQueue 中（a singleton service）.
public class IndexModel : PageModel
{
private readonly AppDbContext _db;
private readonly ILogger _logger;
private readonly IServiceScopeFactory _serviceScopeFactory;
public IndexModel(AppDbContext db, IBackgroundTaskQueue queue,
ILogger<IndexModel> logger, IServiceScopeFactory serviceScopeFactory)
{
_db = db;
_logger = logger;
Queue = queue;
_serviceScopeFactory = serviceScopeFactory;
}
public IBackgroundTaskQueue Queue { get; }
当 Index page 上的Add Task按钮被选中时，OnPostAddTask方法被执行。QueueBackgroundWorkItem被调用来使work item入队。
public IActionResult OnPostAddTaskAsync()
{
Queue.QueueBackgroundWorkItem(async token =>
{
var guid = Guid.NewGuid().ToString();
using (var scope = _serviceScopeFactory.CreateScope())
{
var scopedServices = scope.ServiceProvider;
var db = scopedServices.GetRequiredService<AppDbContext>();
for (int delayLoop = 1; delayLoop < 4; delayLoop++)
{
try
{
db.Messages.Add(
new Message()
{
Text = $"Queued Background Task {guid} has " +
$"written a step. {delayLoop}/3"
});
await db.SaveChangesAsync();
}
catch (Exception ex)
{
_logger.LogError(ex,
"An error occurred writing to the " +
$"database. Error: {ex.Message}");
}
await Task.Delay(TimeSpan.FromSeconds(5), token);
}
}
_logger.LogInformation(
$"Queued Background Task {guid} is complete. 3/3");
});
return RedirectToPage();
}
四. 总结
注意上面的方法都有一个共同点：即直接或间接实现 IHostedService 方法
IHostedService interface
Hosted servcies实现IHostService接口. 这个接口定义了两个方法，为被主机管理的对象：
- StartAsync - StartAsync包含启动后台任务的逻辑。
- StopAsync - 当host 执行关闭时触发。StopAsync包含终止后台任务的逻辑。实现IDisposable 和finalizers 来释放任意unmanaged resources.
你可以把这种用法的后台任务加到任意应用，例如web api , mvc , 控制台等，因为后台服务在应用启动时，就被加载了。它是被以服务的方式加到了管道上了
参考网址：