[英]How to use the CancellationToken property?
與前面的RulyCanceler 類代碼相比,我想使用CancellationTokenSource
運行代碼。
我如何像Cancellation Tokens 中提到的那樣使用它,即不拋出/捕獲異常? 我可以使用IsCancellationRequested
屬性嗎?
我試圖像這樣使用它:
cancelToken.ThrowIfCancellationRequested();
和
try
{
new Thread(() => Work(cancelSource.Token)).Start();
}
catch (OperationCanceledException)
{
Console.WriteLine("Canceled!");
}
但這在cancelToken.ThrowIfCancellationRequested();
上給出了一個運行時錯誤cancelToken.ThrowIfCancellationRequested();
在方法Work(CancellationToken cancelToken)
:
System.OperationCanceledException was unhandled
Message=The operation was canceled.
Source=mscorlib
StackTrace:
at System.Threading.CancellationToken.ThrowIfCancellationRequested()
at _7CancellationTokens.Token.Work(CancellationToken cancelToken) in C:\xxx\Token.cs:line 33
at _7CancellationTokens.Token.<>c__DisplayClass1.<Main>b__0() in C:\xxx\Token.cs:line 22
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
InnerException:
我成功運行的代碼在新線程中捕獲了 OperationCanceledException:
using System;
using System.Threading;
namespace _7CancellationTokens
{
internal class Token
{
private static void Main()
{
var cancelSource = new CancellationTokenSource();
new Thread(() =>
{
try
{
Work(cancelSource.Token); //).Start();
}
catch (OperationCanceledException)
{
Console.WriteLine("Canceled!");
}
}).Start();
Thread.Sleep(1000);
cancelSource.Cancel(); // Safely cancel worker.
Console.ReadLine();
}
private static void Work(CancellationToken cancelToken)
{
while (true)
{
Console.Write("345");
cancelToken.ThrowIfCancellationRequested();
}
}
}
}
您可以按如下方式實現您的工作方法:
private static void Work(CancellationToken cancelToken)
{
while (true)
{
if(cancelToken.IsCancellationRequested)
{
return;
}
Console.Write("345");
}
}
而已。 你總是需要自己處理取消——在適當的時候退出方法退出(這樣你的工作和數據處於一致的狀態)
更新:我不喜歡寫while (!cancelToken.IsCancellationRequested)
因為通常很少有退出點可以在循環體中安全地停止執行,並且循環通常有一些邏輯條件可以退出(迭代集合中的所有項目等)。 所以我認為最好不要混合這些條件,因為它們有不同的意圖。
關於避免CancellationToken.ThrowIfCancellationRequested()
注意事項:
...替換
ThrowIfCancellationRequested
用一堆檢查IsCancellationRequested
優雅退出,正如這個答案所說。 但這不僅僅是一個實現細節; 影響可觀察行為:任務將不再以取消狀態結束,而是以RanToCompletion
。 這不僅會影響顯式狀態檢查,而且會更巧妙地影響任務鏈接,例如ContinueWith
,具體取決於所使用的TaskContinuationOptions
。 我會說避免ThrowIfCancellationRequested
是危險的建議。
與前面的RulyCanceler類代碼相比,我想使用CancellationTokenSource
運行代碼。
如取消令牌中所述,如何使用它,即不引發/捕捉異常? 我可以使用IsCancellationRequested
屬性嗎?
我試圖這樣使用它:
cancelToken.ThrowIfCancellationRequested();
和
try
{
new Thread(() => Work(cancelSource.Token)).Start();
}
catch (OperationCanceledException)
{
Console.WriteLine("Canceled!");
}
但這給了cancelToken.ThrowIfCancellationRequested();
一個運行時錯誤cancelToken.ThrowIfCancellationRequested();
在方法Work(CancellationToken cancelToken)
:
System.OperationCanceledException was unhandled
Message=The operation was canceled.
Source=mscorlib
StackTrace:
at System.Threading.CancellationToken.ThrowIfCancellationRequested()
at _7CancellationTokens.Token.Work(CancellationToken cancelToken) in C:\xxx\Token.cs:line 33
at _7CancellationTokens.Token.<>c__DisplayClass1.<Main>b__0() in C:\xxx\Token.cs:line 22
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
InnerException:
我成功運行的代碼在新線程中捕獲了OperationCanceledException:
using System;
using System.Threading;
namespace _7CancellationTokens
{
internal class Token
{
private static void Main()
{
var cancelSource = new CancellationTokenSource();
new Thread(() =>
{
try
{
Work(cancelSource.Token); //).Start();
}
catch (OperationCanceledException)
{
Console.WriteLine("Canceled!");
}
}).Start();
Thread.Sleep(1000);
cancelSource.Cancel(); // Safely cancel worker.
Console.ReadLine();
}
private static void Work(CancellationToken cancelToken)
{
while (true)
{
Console.Write("345");
cancelToken.ThrowIfCancellationRequested();
}
}
}
}
您可以使用取消令牌創建任務,當您應用轉到后台時,您可以取消此令牌。
您可以在 PCL https://developer.xamarin.com/guides/xamarin-forms/application-fundamentals/app-lifecycle 中執行此操作
var cancelToken = new CancellationTokenSource();
Task.Factory.StartNew(async () => {
await Task.Delay(10000);
// call web API
}, cancelToken.Token);
//this stops the Task:
cancelToken.Cancel(false);
花葯解決方案是 Xamarin.Forms 中的用戶計時器,當應用程序轉到后台時停止計時器https://xamarinhelp.com/xamarin-forms-timer/
您可以使用ThrowIfCancellationRequested
而不處理異常!
ThrowIfCancellationRequested
的使用意味着在Task
(而不是Thread
)中使用。 在Task
,您不必自己處理異常(並獲得未處理的異常錯誤)。 這將導致離開Task
,並且Task.IsCancelled
屬性將為 True。 不需要異常處理。
在您的特定情況下,將Thread
更改為Task
。
Task t = null;
try
{
t = Task.Run(() => Work(cancelSource.Token), cancelSource.Token);
}
if (t.IsCancelled)
{
Console.WriteLine("Canceled!");
}
您必須將CancellationToken
傳遞給 Task,它會定期監視令牌以查看是否請求取消。
// CancellationTokenSource provides the token and have authority to cancel the token
CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
CancellationToken token = cancellationTokenSource.Token;
// Task need to be cancelled with CancellationToken
Task task = Task.Run(async () => {
while(!token.IsCancellationRequested) {
Console.Write("*");
await Task.Delay(1000);
}
}, token);
Console.WriteLine("Press enter to stop the task");
Console.ReadLine();
cancellationTokenSource.Cancel();
在這種情況下,操作將在請求取消時結束, Task
將具有RanToCompletion
狀態。 如果您想確認您的任務已被取消,您必須使用ThrowIfCancellationRequested
拋出OperationCanceledException
異常。
Task task = Task.Run(async () =>
{
while (!token.IsCancellationRequested) {
Console.Write("*");
await Task.Delay(1000);
}
token.ThrowIfCancellationRequested();
}, token)
.ContinueWith(t =>
{
t.Exception?.Handle(e => true);
Console.WriteLine("You have canceled the task");
},TaskContinuationOptions.OnlyOnCanceled);
Console.WriteLine("Press enter to stop the task");
Console.ReadLine();
cancellationTokenSource.Cancel();
task.Wait();
希望這有助於更好地理解。
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