ReactiveUI命令并发（WebClient）

Question

I'm using the latest prerelease of RxUI 8, but I think this would happen in prior versions.

I have defined this ReactiveCommand in my WPF application:

GetWebsiteCommand = ReactiveCommand.CreateFromTask(DownloadString);

private async Task<string> DownloadString()
{
    using (var client = new WebClient())
    {
        return await client.DownloadStringTaskAsync("http://www.google.es");
    }
}

When the command is executed, the following exception is thrown:

System.InvalidOperationException' at System.Reactive.Core.dll: The calling thread cannot access this object because a different thread owns it

Why is this happening? I'm not creating any new thread!

This is the Stack Trace:

System.InvalidOperationException' at System.Reactive.Core.dll: The calling thread cannot access this object because a different thread owns it

 at System.Windows.Threading.Dispatcher.VerifyAccess()
   at System.Windows.DependencyObject.GetValue(DependencyProperty dp)
   at System.Windows.Controls.Primitives.ButtonBase.get_Command()
   at System.Windows.Controls.Primitives.ButtonBase.UpdateCanExecute()
   at System.Windows.Controls.Primitives.ButtonBase.OnCanExecuteChanged(Object sender, EventArgs e)
   at System.Windows.Input.CanExecuteChangedEventManager.HandlerSink.OnCanExecuteChanged(Object sender, EventArgs e)
   at ReactiveUI.ReactiveCommand.OnCanExecuteChanged()
   at ReactiveUI.ReactiveCommand`2.<.ctor>b__9_5(Boolean _)
   at System.Reactive.AnonymousSafeObserver`1.OnNext(T value)
   at System.Reactive.Linq.ObservableImpl.RefCount`1._.OnNext(TSource value)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive(Int32 count)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive()
   at System.Reactive.Subjects.ReplaySubject`1.ReplayBase.OnNext(T value)
   at System.Reactive.Subjects.ReplaySubject`1.OnNext(T value)
   at System.Reactive.Linq.ObservableImpl.AsObservable`1._.OnNext(TSource value)
   at System.Reactive.Linq.ObservableImpl.DistinctUntilChanged`2._.OnNext(TSource value)
   at System.Reactive.Linq.ObservableImpl.CombineLatest`3._.S.OnNext(TSecond value)
   at System.Reactive.Linq.ObservableImpl.RefCount`1._.OnNext(TSource value)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive(Int32 count)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive()
   at System.Reactive.Subjects.ReplaySubject`1.ReplayBase.OnNext(T value)
   at System.Reactive.Subjects.ReplaySubject`1.OnNext(T value)
   at System.Reactive.Linq.ObservableImpl.AsObservable`1._.OnNext(TSource value)
   at System.Reactive.Linq.ObservableImpl.DistinctUntilChanged`2._.OnNext(TSource value)
   at System.Reactive.Linq.ObservableImpl.Concat`1._.OnNext(TSource value)
   at System.Reactive.Linq.ObservableImpl.Select`2._.OnNext(TSource value)
   at System.Reactive.SafeObserver`1.OnNext(TSource value)
   at System.Reactive.ScheduledObserver`1.Dispatch(ICancelable cancel)
   at System.Reactive.Concurrency.Scheduler.<>c.<ScheduleLongRunning>b__72_0(Action`1 a, ICancelable c)
   at System.Reactive.Concurrency.DefaultScheduler.LongRunning.<>c__DisplayClass1_0`1.<ScheduleLongRunning>b__0(Object arg)
   at System.Reactive.Concurrency.ConcurrencyAbstractionLayerImpl.<>c__DisplayClass7_0.<StartThread>b__0()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

Answer 1

Judging by the stack trace, you are passing a canExecute pipeline into the command. For any pipeline you provide, you are responsible for ensuring it ticks on the correct thread. If it ticks on a background thread, the commands CanExecute event will tick on that same thread and the UI will therefore attempt to update the IsEnabled property on the associated Button from the wrong thread.

So you likely need to add an ObserveOn call against your canExecute pipeline.

UPDATE: answered here .