Task類學習教程—ContinueWith

Task類學習教程—ContinueWith

一、簡介

通過任務，可以指定在任務完成之后，應開始運行之后另一個特定任務。ContinueWith是Task根據其自身狀況，決定后續應該作何操作。也就是說，在運行完task后，會執行task.continuewith(XX)中的XX語句，但是是否執行、如何執行等需要看task的運行情況。例如：一個使用前一個任務的結果的新任務，如果前一個任務失敗了，這個任務就應執行一些清理工作。任務處理程序都不帶參數或者帶一個對象參數，而任務的連續處理方法都有一個Task類型的參數。

二、案例

案例一：

代碼：

       static int TaskMethod(string name, int seconds)
        {
            Console.WriteLine("Frist Task Method : Task {0} is running on a thread id: {1}. Is thread pool thread: {2}",
            name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            return 60 * seconds;
        }
        static void Main(string[] args)
        {

            var FirstTask = new Task<int>(() => TaskMethod("Frist Task", 3));
            FirstTask.ContinueWith(t => Console.WriteLine("Frist Task Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion);//線程池線程
            FirstTask.Start();
           
            Console.ReadKey();
        }

結果：

Start()和ContinueWith()的先后順序沒有關系，ContinueWith()會等待直到firstTask運行狀態達到 IsCompleted，因為TaskContinuationOptions中的OnlyOnRanToCompletion.必須指出的是，ContinueWith()中的參數是需要以Task為參數的，也就是firstTask作為參數被傳入，而且ContinueWith()運行在線程池中的線程中。我覺得比較重要的一點是：把ContinueWith()中的語句當做一塊新的語句塊，他們獨立於主線程。無論如何，他們都要被判斷，如果狀態（status）不滿足，那么他們不執行；當指定了多個狀態，則使用合理的對應狀態。

案例二：

代碼：

 class Program
    {
        static int TaskMethod(string name, int seconds)
        {
            Console.WriteLine("Task Method : Task {0} is running on a thread id: {1}. Is thread pool thread: {2}",
            name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            return 60 * seconds;
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Main Thread id {0}, Is in Thred Pool: {1}",
            Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);

            var firstTask = new Task<int>(() => TaskMethod("frist task", 3));
            var secondTask = new Task<int>(() => TaskMethod("second task", 2));

            firstTask.ContinueWith(t => Console.WriteLine("Result：Frist Thread Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion);

            firstTask.Start();
            secondTask.Start();

            Thread.Sleep(TimeSpan.FromSeconds(4)); //給予足夠時間，讓firstTask、secondTask及其后續操作執行完畢。

            Task continuation = secondTask.ContinueWith(t => Console.WriteLine("Result：Second Thread Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion | TaskContinuationOptions.ExecuteSynchronously);
            Console.ReadLine();
            Console.ReadKey();
        }
    }

結果：

這里主線程休眠了足足4秒鍾，足以讓firstTask和secondTask兩個任務完成運行，而后，由於secondTask的后續除了接受OnlyOnRanToCompletion外，還接受ExecuteSynchronously。因此，后續運行中，由於主線程還沒有結束，因此 ExecuteSynchronously得到認可，故secondTask的后續是在主線程上運行。

案例三：

代碼：

   class Program
    {
        static int TaskMethod(string name, int seconds)
        {
            Console.WriteLine("Task Method : Task {0} is running on a thread id: {1}. Is thread pool thread: {2}",
            name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            return 60 * seconds;
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Main Thread id {0}, Is in Thred Pool: {1}",
            Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);

            var firstTask = new Task<int>(() => TaskMethod("frist task", 3));
            var secondTask = new Task<int>(() => TaskMethod("second task", 2));

            firstTask.ContinueWith(t => Console.WriteLine("Result：Frist Thread Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion);

            firstTask.Start();
            secondTask.Start();

           //Thread.Sleep(TimeSpan.FromSeconds(4)); //給予足夠時間，讓firstTask、secondTask及其后續操作執行完畢。

            Task continuation = secondTask.ContinueWith(t => Console.WriteLine("Result：Second Thread Result: {0}. Thread id: {1}, Is in Thred Pool: {2}",
            t.Result, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread),
            TaskContinuationOptions.OnlyOnRanToCompletion | TaskContinuationOptions.ExecuteSynchronously);
            Console.ReadLine();
            Console.ReadKey();
        }
    }

結果：

然而，如果把4秒鍾的休眠注釋掉，那么由於主線程很早就結束了，因此secondTask只能接受到OnlyOnRanToCompletion，因此還是運行在線程池中。