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// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
using System;
using System.Threading;
using System.Threading.Tasks;
namespace Microsoft.Extensions.Hosting
{
/// <summary>
/// Base class for implementing a long running <see cref="IHostedService"/>.
/// </summary>
public abstract class BackgroundService : IHostedService, IDisposable
{
private Task _executingTask;
private CancellationTokenSource _stoppingCts;
/// <summary>
/// This method is called when the <see cref="IHostedService"/> starts. The implementation should return a task that represents
/// the lifetime of the long running operation(s) being performed.
/// </summary>
/// <param name="stoppingToken">Triggered when <see cref="IHostedService.StopAsync(CancellationToken)"/> is called.</param>
/// <returns>A <see cref="Task"/> that represents the long running operations.</returns>
protected abstract Task ExecuteAsync(CancellationToken stoppingToken);
/// <summary>
/// Triggered when the application host is ready to start the service.
/// </summary>
/// <param name="cancellationToken">Indicates that the start process has been aborted.</param>
public virtual Task StartAsync(CancellationToken cancellationToken)
{
// Create linked token to allow cancelling executing task from provided token
_stoppingCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
// Store the task we're executing
_executingTask = ExecuteAsync(_stoppingCts.Token);
// If the task is completed then return it, this will bubble cancellation and failure to the caller
if (_executingTask.IsCompleted)
{
return _executingTask;
}
// Otherwise it's running
return Task.CompletedTask;
}
/// <summary>
/// Triggered when the application host is performing a graceful shutdown.
/// </summary>
/// <param name="cancellationToken">Indicates that the shutdown process should no longer be graceful.</param>
public virtual async Task StopAsync(CancellationToken cancellationToken)
{
// Stop called without start
if (_executingTask == null)
{
return;
}
try
{
// Signal cancellation to the executing method
_stoppingCts.Cancel();
}
finally
{
// Wait until the task completes or the stop token triggers
await Task.WhenAny(_executingTask, Task.Delay(Timeout.Infinite, cancellationToken));
}
}
public virtual void Dispose()
{
_stoppingCts?.Cancel();
}
}
}
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