在使用 BeginInvoke 和 EndInvoke 进行异步调用时,调用 BeginInvoke 之后,可以执行下列操作来结束异步调用:
· 进行某些操作,然后调用 EndInvoke 一直阻止到调用完成。
· 使用 IAsyncResult..::.AsyncWaitHandle 属性获取 WaitHandle,使用它的 WaitOne 方法一直阻止执行直到发出 WaitHandle 信号,然后调用 EndInvoke。
· 轮询由 BeginInvoke 返回的 IAsyncResult,确定异步调用何时完成,然后调用 EndInvoke。
· 将用于回调方法的委托传递给 BeginInvoke。异步调用完成后,将在 ThreadPool 线程上执行该方法。该回调方法将调用 EndInvoke。
异步委托
下面的代码演示异步调用同一个长时间运行的方法 TestMethod 的各种方式。TestMethod 方法会显示一条控制台消息,说明它已开始处理,休眠了几秒钟,然后结束。TestMethod 有一个 out 参数,该参数用于演示此种参数添加到 BeginInvoke 和 EndInvoke 的签名中的方式。可以按同样的方式处理 ref 参数。
TestMethod 的定义和名为 AsyncMethodCaller 的、可用来异步调用 TestMethod 的委托。若要编译任何代码示例,必须包括 TestMethod 的定义和 AsyncMethodCaller 委托。
using System; using System.Threading; namespace Examples.AdvancedProgramming.AsynchronousOperations { public class AsyncDemo { // The method to be executed asynchronously. public string TestMethod(int callDuration, out int threadId) { Console.WriteLine("Test method begins."); Thread.Sleep(callDuration); threadId = Thread.CurrentThread.ManagedThreadId; return String.Format("My call time was {0}.", callDuration.ToString()); } } // The delegate must have the same signature as the method // it will call asynchronously. public delegate string AsyncMethodCaller(int callDuration, out int threadId); }
使用 EndInvoke 等待异步调用
异步执行方法最简单的方式是通过调用委托的 BeginInvoke 方法来开始执行方法,在主线程上执行一些工作,然后调用委托的 EndInvoke 方法。EndInvoke 可能会阻止调用线程,因为它直到异步调用完成之后才返回。这种技术非常适合文件或网络操作,但是由于 EndInvoke 会阻止它,所以不要从服务于用户界面的线程中调用它。
using System; using System.Threading; namespace Examples.AdvancedProgramming.AsynchronousOperations { public class AsyncMain { public static void Main() { // The asynchronous method puts the thread id here. int threadId; // Create an instance of the test class. AsyncDemo ad = new AsyncDemo(); // Create the delegate. AsyncMethodCaller caller = new AsyncMethodCaller(ad.TestMethod); // Initiate the asychronous call. IAsyncResult result = caller.BeginInvoke(3000, out threadId, null, null); Thread.Sleep(0); Console.WriteLine("Main thread {0} does some work.", Thread.CurrentThread.ManagedThreadId); // Call EndInvoke to wait for the asynchronous call to complete, // and to retrieve the results. string returnValue = caller.EndInvoke(out threadId, result); Console.WriteLine("The call executed on thread {0}, with return value \"{1}\".", threadId, returnValue); } } }
使用 WaitHandle 等待异步调用
您可以使用 BeginInvoke 返回的 IAsyncResult 的 AsyncWaitHandle 属性来获取 WaitHandle。异步调用完成时会发出 WaitHandle 信号,而您可以通过调用 WaitOne 方法等待它。
如果您使用 WaitHandle,则在异步调用完成之前或之后,在通过调用 EndInvoke 检索结果之前,还可以执行其他处理。
using System; using System.Threading; namespace Examples.AdvancedProgramming.AsynchronousOperations { public class AsyncMain { static void Main() { // The asynchronous method puts the thread id here. int threadId; // Create an instance of the test class. AsyncDemo ad = new AsyncDemo(); // Create the delegate. AsyncMethodCaller caller = new AsyncMethodCaller(ad.TestMethod); // Initiate the asychronous call. IAsyncResult result = caller.BeginInvoke(3000, out threadId, null, null); Thread.Sleep(0); Console.WriteLine("Main thread {0} does some work.", Thread.CurrentThread.ManagedThreadId); // Wait for the WaitHandle to become signaled. result.AsyncWaitHandle.WaitOne(); // Perform additional processing here. // Call EndInvoke to retrieve the results. string returnValue = caller.EndInvoke(out threadId, result); Console.WriteLine("The call executed on thread {0}, with return value \"{1}\".", threadId, returnValue); } } }
轮询异步调用完成
您可以使用由 BeginInvoke 返回的 IAsyncResult 的 IsCompleted 属性来发现异步调用何时完成。从用户界面的服务线程中进行异步调用时可以执行此操作。轮询完成允许调用线程在异步调用在 ThreadPool 线程上执行时继续执行。
using System; using System.Threading; namespace Examples.AdvancedProgramming.AsynchronousOperations { public class AsyncMain { static void Main() { // The asynchronous method puts the thread id here. int threadId; // Create an instance of the test class. AsyncDemo ad = new AsyncDemo(); // Create the delegate. AsyncMethodCaller caller = new AsyncMethodCaller(ad.TestMethod); // Initiate the asychronous call. IAsyncResult result = caller.BeginInvoke(3000, out threadId, null, null); // Poll while simulating work. while(result.IsCompleted == false) { Thread.Sleep(10); } // Call EndInvoke to retrieve the results. string returnValue = caller.EndInvoke(out threadId, result); Console.WriteLine("The call executed on thread {0}, with return value \"{1}\".", threadId, returnValue); } }
异步调用完成时执行回调方法
如果启动异步调用的线程不需要是处理结果的线程,则可以在调用完成时执行回调方法。回调方法在 ThreadPool 线程上执行。
若要使用回调方法,必须将引用回调方法的 AsyncCallback 委托传递给 BeginInvoke。也可以传递包含回调方法将要使用的信息的对象。例如,可以传递启动调用时曾使用的委托,以便回调方法能够调用 EndInvoke。
using System; using System.Threading; namespace Examples.AdvancedProgramming.AsynchronousOperations { public class AsyncMain { // Asynchronous method puts the thread id here. private static int threadId; static void Main() { // Create an instance of the test class. AsyncDemo ad = new AsyncDemo(); // Create the delegate. AsyncMethodCaller caller = new AsyncMethodCaller(ad.TestMethod); // Initiate the asychronous call. Include an AsyncCallback // delegate representing the callback method, and the data // needed to call EndInvoke. IAsyncResult result = caller.BeginInvoke(3000, out threadId, new AsyncCallback(CallbackMethod), caller ); Console.WriteLine("Press Enter to close application."); Console.ReadLine(); } // Callback method must have the same signature as the // AsyncCallback delegate. static void CallbackMethod(IAsyncResult ar) { // Retrieve the delegate. AsyncMethodCaller caller = (AsyncMethodCaller) ar.AsyncState; // Call EndInvoke to retrieve the results. string returnValue = caller.EndInvoke(out threadId, ar); Console.WriteLine("The call executed on thread {0}, with return value \"{1}\".", threadId, returnValue); } } }
来源:https://www.cnblogs.com/luoht/archive/2010/10/01/1839804.html