Mvvmcross: IMvxMainThreadDispatcher needs a method where you can wait for the UI task to finish

[SimonSimCity]

In my application I have the need to execute a task on UI and wait for it to return.

I know that (at least for iOS) there is such a method ... is such a method also available for all the other platforms? If yes, I would appreciate if such a method could be provided across platforms :wink:

[Cheesebaron]

If you have a Task can't you just call Wait() on it?

[SimonSimCity]

The problem is that I don't have ...

I have to call Mvx.Resolve<IMvxMainThreadDispatcher>().RequestMainThreadAction(() => { ... }); and I give away the control. If I could wait for that task, that is run on the main-thread there - that would be great!

[slodge]

You could always just set up your own await logic using a TaskCompletionSource in your action? But really you need to be careful with blocking threads... (It's something that always bites me, any way!)

[kjeremy]

Instead of RequestMainThreadAction() can't you use the current dispatcher or synchronization context with a Task?

[SimonSimCity]

If I take a look at http://developer.xamarin.com/guides/ios/user_interface/controls/part_2_-_working_with_the_ui_thread/ I read of two different methods:

If code is executing on a background thread, in a task or a callback then it is likely NOT executing on the main UI thread. In this case you should wrap the code in a call to InvokeOnMainThread or BeginInvokeOnMainThread like this:

When you take a look at the implementation in MvvmCross (https://github.com/MvvmCross/MvvmCross/blob/e9802dfb45f0d52da3fb056b020032963142b9c3/Cirrious/Cirrious.MvvmCross.Touch/Views/MvxTouchUIThreadDispatcher.cs) it only offers that asynchronous method, where you can't wait for the action to finish before resuming.

How would you solve that with code? I just can't see a way of solving this by the given ways ...

[SimonSimCity]

You could update that interface and always use InvokeOnMainThread(), putting it into a separate task and return it, so the user can easily choose to await it. Don't know how much overload that would add when directly awaiting that task ... maybe a second method would do ...

Just my 2 cent ...

[tofutim]

I ran into this today. I had a login page that had to be called on the main thread. The code looks like

        var dispatcher = Mvx.Resolve<IMvxMainThreadDispatcher>();
        dispatcher.RequestMainThreadAction(async () =>
        {
            loginPage = new LoginPage(startUri, endUri);
            token = await loginPage.Display();
        });

        JObject tokenObj = JObject.Parse(token.Replace("%2C", ","));

of course, token never gets set because (I think) it hops straight out of the async function before token is set.

Update. In this case I am using Windows, and it calls this

    public bool RequestMainThreadAction(Action action)
    {
        if (this._uiDispatcher.HasThreadAccess)
        {
            action();
            return true;
        }

        this._uiDispatcher.RunAsync(CoreDispatcherPriority.Normal, () => ExceptionMaskedAction(action));
        return true;
    }

so... no way to wait. :P

[gabrieligbastos]

@tofutim and @SimonSimCity

why dont you just do something like:

        var myTaskCompletionSource = new TaskCompletionSource<bool>();
        var dispatcher = Mvx.Resolve<IMvxMainThreadDispatcher>();
        dispatcher.RequestMainThreadAction(async () =>
        {
            loginPage = new LoginPage(startUri, endUri);
            token = await loginPage.Display();
            myTaskCompletionSource.SetResult(true);
        });

        if(myTaskCompletionSource.Task.Result)
        {
            JObject tokenObj = JObject.Parse(token.Replace("%2C", ","));
        }

[mmierzwa]

@gabrieligbastos you just saved few hours of my life :)