Task類學習教程—組合任務.ContinueWith
一、簡介
通過任務，可以指定在任務完成之後，應開始執行之後另一個特定任務。ContinueWith是Task根據其自身狀況，決定後續應該作何操作。也就是說，在執行完task後，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的執行情況。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。任務處理程式都不帶引數或者帶一個物件引數，而任務的連續處理方法都有一個Task型別的引數。
二、案例
案例一：
程式碼：
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);//執行緒池執行緒
FirstTask.Start(); Console.ReadKey();
}
結果：
Start()和ContinueWith()的先後順序沒有關係，ContinueWith()會等待直到firstTask執行狀態達到 IsCompleted，因為TaskContinuationOptions中的OnlyOnRanToCompletion.必須指出的是，ContinueWith()中的引數是需要以Task為引數的，也就是firstTask作為引數被傳入，而且ContinueWith()執行線上程池中的執行緒中。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，他們獨立於主執行緒。無論如何，他們都要被判斷，如果狀態（status）不滿足，那麼他們不執行；當指定了多個狀態，則使用合理的對應狀態。
案例二：
程式碼：
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及其後續操作執行完畢。 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：
這裡主執行緒休眠了足足4秒鐘，足以讓firstTask和secondTask兩個任務完成執行，而後，由於secondTask的後續除了接受OnlyOnRanToCompletion外，還接受ExecuteSynchronously。因此，後續執行中，由於主執行緒還沒有結束，因此 ExecuteSynchronously得到認可，故secondTask的後續是在主執行緒上執行。
案例三：
程式碼：
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及其後續操作執行完畢。 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秒鐘的休眠註釋掉，那麼由於主執行緒很早就結束了，因此secondTask只能接受到OnlyOnRanToCompletion，因此還是執行線上程池中。