HOW TO：使用接續鏈結多個工作
在工作平行程式庫中，已叫用 ContinueWith 方法的工作稱為前項工作，而定義於 ContinueWith 方法中的工作則稱為接續。 這個範例示範如何使用 Task 和 Task<TResult> 類別的 ContinueWith 和 ContinueWith 方法，指定會在前項工作完成後啟動的工作。
此範例也說明如何指定只有在前項已取消時才會執行的接續。
這些範例將示範如何從單一工作繼續作業。 您也可以建立會在任何或所有工作群組完成或取消後執行的接續。 如需詳細資訊，請參閱 TaskContinueWhenAll() 和 TaskContinueWhenAny()。
範例
DoSimpleContinuation 方法會顯示 ContinueWith 的基本語法。 請注意，在 ContinueWith 方法中，會提供前項工作做為 Lambda 運算式的輸入參數。 這可讓您在前項工作執行接續中的任何工作前，評估前項工作的狀態。 如果您無須將某項工作的任何狀態傳遞至另一項工作，請使用 ContinueWith 的這個簡單多載。
DoSimpleContinuationWithState 方法會顯示如何使用 ContinueWith 將前項工作的結果傳遞至接續工作。
Imports System.IO
Imports System.Threading
Imports System.Threading.Tasks
Module ContinueWith
Sub Main()
DoSimpleContinuation()
Console.WriteLine("Press any key to exit")
Console.ReadKey()
End Sub
Sub DoSimpleContinuation()
Dim path As String = "C:\users\public\TPLTestFolder\"
Try
Dim firstTask = New Task(Sub() CopyDataIntoTempFolder(path))
Dim secondTask = firstTask.ContinueWith(Sub(t) CreateSummaryFile(path))
firstTask.Start()
Catch e As AggregateException
Console.WriteLine(e.Message)
End Try
End Sub
' A toy function to simulate a workload
Sub CopyDataIntoTempFolder(ByVal path__1 As String)
System.IO.Directory.CreateDirectory(path__1)
Dim rand As New Random()
For x As Integer = 0 To 49
Dim bytes As Byte() = New Byte(999) {}
rand.NextBytes(bytes)
Dim filename As String = Path.GetRandomFileName()
Dim filepath As String = Path.Combine(path__1, filename)
System.IO.File.WriteAllBytes(filepath, bytes)
Next
End Sub
Sub CreateSummaryFile(ByVal path__1 As String)
Dim files As String() = System.IO.Directory.GetFiles(path__1)
Parallel.ForEach(files, Sub(file)
Thread.SpinWait(5000)
End Sub)
System.IO.File.WriteAllText(Path.Combine(path__1, "__SummaryFile.txt"), "did my work")
Console.WriteLine("Done with task2")
End Sub
Sub DoSimpleContinuationWithState()
Dim nums As Integer() = {19, 17, 21, 4, 13, 8, _
12, 7, 3, 5}
Dim f0 = New Task(Of Double)(Function() nums.Average())
Dim f1 = f0.ContinueWith(Function(t) GetStandardDeviation(nums, t.Result))
f0.Start()
Console.WriteLine("the standard deviation is {0}", f1)
End Sub
Function GetStandardDeviation(ByVal values As Integer(), ByVal mean As Double) As Double
Dim d As Double = 0.0R
For Each n In values
d += Math.Pow(mean - n, 2)
Next
Return Math.Sqrt(d / (values.Length - 1))
End Function
End Module
using System;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
namespace ContinueWith
{
class Continuations
{
static void Main()
{
SimpleContinuation();
Console.WriteLine("Press any key to exit");
Console.ReadKey();
}
static void SimpleContinuation()
{
string path = @"C:\users\public\TPLTestFolder\";
try
{
var firstTask = new Task(() => CopyDataIntoTempFolder(path));
var secondTask = firstTask.ContinueWith((t) => CreateSummaryFile(path));
firstTask.Start();
}
catch (AggregateException e)
{
Console.WriteLine(e.Message);
}
}
// A toy function to simulate a workload
static void CopyDataIntoTempFolder(string path)
{
System.IO.Directory.CreateDirectory(path);
Random rand = new Random();
for (int x = 0; x < 50; x++)
{
byte[] bytes = new byte[1000];
rand.NextBytes(bytes);
string filename = Path.GetRandomFileName();
string filepath = Path.Combine(path, filename);
System.IO.File.WriteAllBytes(filepath, bytes);
}
}
static void CreateSummaryFile(string path)
{
string[] files = System.IO.Directory.GetFiles(path);
Parallel.ForEach(files, (file) =>
{
Thread.SpinWait(5000);
});
System.IO.File.WriteAllText(Path.Combine(path, "__SummaryFile.txt"), "did my work");
Console.WriteLine("Done with task2");
}
static void SimpleContinuationWithState()
{
int[] nums = { 19, 17, 21, 4, 13, 8, 12, 7, 3, 5 };
var f0 = new Task<double>(() => nums.Average());
var f1 = f0.ContinueWith( t => GetStandardDeviation(nums, t.Result));
f0.Start();
Console.WriteLine("the standard deviation is {0}", f1.Result);
}
private static double GetStandardDeviation(int[] values, double mean)
{
double d = 0.0;
foreach (var n in values)
{
d += Math.Pow(mean - n, 2);
}
return Math.Sqrt(d / (values.Length - 1));
}
}
}
Task<TResult> 的型別參數將決定委派的傳回型別。 傳回值會傳遞至接續工作。 透過此方式可鏈結任意數目的工作。