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APM(Asynchronous Programming Model)，异步编程模型。 APM的核心(IAsyncResult) IAsyncResult接口是APM机制的核心： public interface IAsyncResult { 当任何一个BeginXXX方法被调用的时候，都会返回一个继承于该接口的对象。该对象可以用来监视异步动作的执行，例如，当调用BeginXXX后，我们可以通过该对象的属性判断异步动作是否完成，同时如果异步执行的时候有异常，那么就会将异常信息传给EndXXX方法，EndXXX方法等待异步动作执行完成后，将异常抛出。WaitHandle AsyncWaitHandle { get; } // For Wait-Until-Done technique Boolean IsCompleted { get; } // For Polling technique Object AsyncState { get; } // For Callback technique Boolean CompletedSynchronously { get; } // Almost never used } AsyncResultNoResult类继承于IAsyncResult接口，该类用于无特殊返回值的异步动作(执行成功或失败)，例如stream的BeginWrite、EndWrite。 public override void EndWrite( AsyncResultNoResult类：  internal class AsyncResultNoResult : IAsyncResult { // Fields set at construction which never change while // operation is pending private readonly AsyncCallback m_AsyncCallback; private readonly Object m_AsyncState; // Fields set at construction which do change after // operation completes private const Int32 c_StatePending = 0; private const Int32 c_StateCompletedSynchronously = 1; private const Int32 c_StateCompletedAsynchronously = 2; private Int32 m_CompletedState = c_StatePending; // Field that may or may not get set depending on usage private ManualResetEvent m_AsyncWaitHandle; // Fields set when operation completes private Exception m_exception; public AsyncResultNoResult(AsyncCallback asyncCallback, Object state) { m_AsyncCallback = asyncCallback; m_AsyncState = state; } public void SetAsCompleted( Exception exception, Boolean completedSynchronously) { // Passing null for exception means no error occurred. // This is the common case m_exception = exception; // The m_CompletedState field MUST be set prior calling the callback Int32 prevState = Interlocked.Exchange(ref m_CompletedState, completedSynchronously ? c_StateCompletedSynchronously : c_StateCompletedAsynchronously); if (prevState != c_StatePending) throw new InvalidOperationException( "You can set a result only once"); // If the event exists, set it if (m_AsyncWaitHandle != null) m_AsyncWaitHandle.Set(); // If a callback method was set, call it if (m_AsyncCallback != null) m_AsyncCallback(this); } public void EndInvoke() { // This method assumes that only 1 thread calls EndInvoke // for this object if (!IsCompleted) { // If the operation isn't done, wait for it AsyncWaitHandle.WaitOne(); AsyncWaitHandle.Close(); m_AsyncWaitHandle = null; // Allow early GC } // Operation is done: if an exception occured, throw it if (m_exception != null) throw m_exception; } #region Implementation of IAsyncResult public Object AsyncState { get { return m_AsyncState; } } public Boolean CompletedSynchronously { get { return Thread.VolatileRead(ref m_CompletedState) == c_StateCompletedSynchronously; } } public WaitHandle AsyncWaitHandle { get { if (m_AsyncWaitHandle == null) { Boolean done = IsCompleted; ManualResetEvent mre = new ManualResetEvent(done); if (Interlocked.CompareExchange(ref m_AsyncWaitHandle, mre, null) != null) { // Another thread created this object's event; dispose // the event we just created mre.Close(); } else { if (!done && IsCompleted) { // If the operation wasn't done when we created // the event but now it is done, set the event m_AsyncWaitHandle.Set(); } } } return m_AsyncWaitHandle; } } public Boolean IsCompleted { get { return Thread.VolatileRead(ref m_CompletedState) != c_StatePending; } } #endregion } 还有一个重要的属性(m_AsyncWaitHandle)，我们在EndInvoke方法可以看到m_AsyncWaitHandle.WaitOne()这行代码，执行到这里的时候，m_AsyncWaitHandle如果没有被释放的话，程序将一直等待，释放的程序在SetAsCompleted方法中m_AsyncWaitHandle.Set()。 AsyncResult类： internal class AsyncResult<TResult> : AsyncResultNoResult { // Field set when operation completes private TResult m_result = default(TResult); public AsyncResult(AsyncCallback asyncCallback, Object state) : base(asyncCallback, state) { } public void SetAsCompleted(TResult result, Boolean completedSynchronously) { // Save the asynchronous operation's result m_result = result; // Tell the base class that the operation completed // sucessfully (no exception) base.SetAsCompleted(null, completedSynchronously); } new public TResult EndInvoke() { base.EndInvoke(); // Wait until operation has completed return m_result; // Return the result (if above didn't throw) } } 利用自定义的IAsyncResult实现APM 现在我们已经知道如何定义IAsyncResult，那么就开始展示如何使用AsyncResult<TResult>和AsyncResultNoResult。 定义一个类：LongTask，该类对一些很耗时的方法进行异步处理，并且当执行完成的时候返回一个时间数据。 internal sealed class LongTask { private Int32 m_ms; // Milliseconds; public LongTask(Int32 seconds) { m_ms = seconds * 1000; } // Synchronous version of time-consuming method public DateTime DoTask() { Thread.Sleep(m_ms); // Simulate time-consuming task return DateTime.Now; // Indicate when task completed } // Asynchronous version of time-consuming method (Begin part) public IAsyncResult BeginDoTask(AsyncCallback callback, Object state) { // Create IAsyncResult object identifying the // asynchronous operation AsyncResult<DateTime> ar = new AsyncResult<DateTime>( callback, state); // Use a thread pool thread to perform the operation ThreadPool.QueueUserWorkItem(DoTaskHelper, ar); return ar; // Return the IAsyncResult to the caller } // Asynchronous version of time-consuming method (End part) public DateTime EndDoTask(IAsyncResult asyncResult) { // We know that the IAsyncResult is really an // AsyncResult<DateTime> object AsyncResult<DateTime> ar = (AsyncResult<DateTime>)asyncResult; // Wait for operation to complete, then return result or // throw exception return ar.EndInvoke(); } // Asynchronous version of time-consuming method (private part // to set completion result/exception) private void DoTaskHelper(Object asyncResult) { // We know that it's really an AsyncResult<DateTime> object AsyncResult<DateTime> ar = (AsyncResult<DateTime>)asyncResult; try { // Perform the operation; if sucessful set the result DateTime dt = DoTask(); ar.SetAsCompleted(dt, false); } catch (Exception e) { // If operation fails, set the exception ar.SetAsCompleted(e, false); } } } 测试和性能 1、测试LongTask private static void FunctionalTest() { IAsyncResult ar; LongTask lt = new LongTask(5); // Prove that the Wait-until-done technique works ar = lt.BeginDoTask(null, null); Console.WriteLine("Task completed at: {0}", lt.EndDoTask(ar)); // Prove that the Polling technique works ar = lt.BeginDoTask(null, null); while (!ar.IsCompleted) { Console.WriteLine("Not completed yet."); Thread.Sleep(1000); } Console.WriteLine("Task completed at: {0}", lt.EndDoTask(ar)); // Prove that the Callback technique works lt.BeginDoTask(TaskCompleted, lt); Console.ReadLine(); } private static void TaskCompleted(IAsyncResult ar) { LongTask lt = (LongTask)ar.AsyncState; Console.WriteLine("Task completed at: {0}", lt.EndDoTask(ar)); Console.WriteLine("All done, hit Enter to exit app."); }  2、比较自定义APM和delegate之间的性能 private const Int32 c_iterations = 100 * 1000; // 100 thousand private static Int32 s_numDone; private delegate DateTime DoTaskDelegate(); private static void PerformanceTest() { AutoResetEvent are = new AutoResetEvent(false); LongTask lt = new LongTask(0); Stopwatch sw; s_numDone = 0; sw = Stopwatch.StartNew(); for (Int32 n = 0; n < c_iterations; n++) { lt.BeginDoTask(delegate(IAsyncResult ar) { if (Interlocked.Increment(ref s_numDone) == c_iterations) are.Set(); }, null); } are.WaitOne(); Console.WriteLine("AsyncResult Time: {0}", sw.Elapsed); s_numDone = 0; DoTaskDelegate doTaskDelegate = lt.DoTask; sw = Stopwatch.StartNew(); for (Int32 n = 0; n < c_iterations; n++) { doTaskDelegate.BeginInvoke(delegate(IAsyncResult ar) { if (Interlocked.Increment(ref s_numDone) == c_iterations) are.Set(); }, null); } are.WaitOne(); Console.WriteLine("Delegate Time: {0}", sw.Elapsed); }  从结果上看，既然自定义的APM性能要比FCL的APM好点，其实在这里大家仔细看下就可以清楚： 区别就在于ManualResetEvent，FCL的APM是在创建IAsyncResult对象的同时，也创建了ManualResetEvent。而AsyncResultNoResult中，可以看下面的代码，只有在真正需要用到ManualResetEvent的时候，才去创建，一般不实例化。有个注意的地方：ManualResetEvent是内核对象，而创建或使用一个内核对象都是很耗资源的，所以只有当真正需要的时候才去使用。 public WaitHandle AsyncWaitHandle { get { if (m_AsyncWaitHandle == null) { Boolean done = IsCompleted; ManualResetEvent mre = new ManualResetEvent(done); if (Interlocked.CompareExchange(ref m_AsyncWaitHandle, mre, null) != null) { mre.Close(); } else { if (!done && IsCompleted) { m_AsyncWaitHandle.Set(); } } } return m_AsyncWaitHandle; } |
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来自： scholes_goal > 《技术》
