先说下异步（Asynchronous）和多线程（Multi-threading）的区别：异步是相对与同步来说的，一般来说意味着非阻塞；异步在具体实现上是用多线程来实现的，但好处是你不用操心和管理多线程，它给你封装了一个干净的接口来调用。当然，在linux和javascript环境下，你要理解异步一般要理解事件驱动机制/Event loop: 《理解Event loop》。这里讲的主要是.NET。同步函数异步化有很多好处，但异步化是有开销的：《Understanding the Costs of Async and Await》。具体来说，下列场合适合用异步化：
- 耗时的I/O操作，例如：读文件I/O，访问网络资源网络I/O….
- 耗时的CPU的操作：建议配合多线程，例如Task.Factory启动新的线程，在线程里面进行异步访问
举例，这里有个同步方法Process（例子不太恰当，应该是耗时的I/O操作的例子比较好）
public class Test
{
public int Process(int a)
{
System.Threading.Thread.Sleep(3000);
return a + 1;
}
}
现在把这个同步方法改造为异步方法：
namespace ConsoleApplication6
{
public class Test
{
public int Process(int a)
{
System.Threading.Thread.Sleep(3000);
return a + 1;
}
public void ProcessAsync(int a, Action<int> callBackAction)
{
Func<int> func = () =>
{
System.Threading.Thread.Sleep(3000);
return a + 1;
};
func.BeginInvoke((ar) =>
{
var result = func.EndInvoke(ar);
callBackAction.Invoke(result);
},
null);
}
}
}
测试方法：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ConsoleApplication6
{
class Program
{
static void Main(string[] args)
{
var tester = new Test();
Console.WriteLine("Sync start");
var result1 = tester.Process(1);
Console.WriteLine("Sync finish: " + result1);
Console.WriteLine("Async start");
tester.ProcessAsync(2, (r) =>
{
Console.WriteLine("Async finish: " + r + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
});
Console.WriteLine("Async continue" + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
Console.Read();
}
}
}
输出结果：
Sync start (wait 3 seconds) Sync finish: 2 Async start Async continue (wait 3 seconds) Async finish: 3
如果多加几个操作，看看起了几个线程：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ConsoleApplication6
{
class Program
{
static void Main(string[] args)
{
var tester = new Test();
Console.WriteLine("Sync start");
var result1 = tester.Process(1);
Console.WriteLine("Sync finish: " + result1);
Console.WriteLine("Async start");
tester.ProcessAsync(2, (r) =>
{
Console.WriteLine("Async finish: " + r + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
});
Console.WriteLine("Async continue" + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
Console.WriteLine("Async start");
tester.ProcessAsync(2, (r) =>
{
Console.WriteLine("Async finish: " + r + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
});
Console.WriteLine("Async continue" + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
Console.WriteLine("Async start");
tester.ProcessAsync(2, (r) =>
{
Console.WriteLine("Async finish: " + r + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
});
Console.WriteLine("Async continue" + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
Console.WriteLine("Async start");
tester.ProcessAsync(2, (r) =>
{
Console.WriteLine("Async finish: " + r + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
});
Console.WriteLine("Async continue" + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
Console.Read();
}
}
}
输出：
Sync start Sync finish: 2 Async start Async continue thread id: 9 Async start Async continue thread id: 9 Async start Async continue thread id: 9 Async start Async continue thread id: 9 Async finish: 3 thread id: 13 Async finish: 3 thread id: 11 Async finish: 3 thread id: 10 Async finish: 3 thread id: 12
说明背后是用了多线程。
现在我们再用async和await关键字来改造一下：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ConsoleApplication6
{
public class Test
{
public void ProcessAsync(int a, Action<int> callBackAction)
{
Func<int> func = () =>
{
System.Threading.Thread.Sleep(3000);
return a + 1;
};
func.BeginInvoke((ar) =>
{
var result = func.EndInvoke(ar);
callBackAction.Invoke(result);
},
null);
}
public Task<int> TaskProcessAsync(Action<int> callBackAction)
{
var tcs = new TaskCompletionSource<int>();
ProcessAsync(3, (r) =>
{
callBackAction.Invoke(r);
});
return tcs.Task;
}
public async Task<int> QueryProcess(Action<int> callBackAction)
{
int i = await TaskProcessAsync(callBackAction);
return i;
}
}
public class Class1
{
public async Task<int> Test()
{
var tester = new Test();
var i = await tester.QueryProcess((r) =>
{
Console.WriteLine("await Async finish: " + r + " thread id: " +
System.Threading.Thread.CurrentThread.ManagedThreadId);
});
return i;
}
}
class Program
{
static void Main(string[] args)
{
var t = new Class1();
t.Test();
Console.Read();
}
}
}