Xamarin Forms MvvmCross binding a button command error

Question

I'm working on a Xamarin Forms MvvmCross project. I'm binding a common button command on the view with a IMvxAsyncCommand on the view model, as follows:

View

 <views:MvxContentPage xmlns="http://xamarin.com/schemas/2014/forms"
                   xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                   xmlns:views="clr- namespace:MvvmCross.Forms.Views;assembly=MvvmCross.Forms"
                   x:Class="TestProject.Pages.TestPage">
 <ContentView>
     <StackLayout>
         <Button Text="Test the command!" Command="{Binding TestAsyncCommand}"/>
     </StackLayout>
 </ContentView>

</views:MvxContentPage>

View model

 namespace TestProject.ViewModels
 {
     public class TestViewModel : MvxNavigationViewModel
     {
         public TestViewModel(IMvxLogProvider logProvider, IMvxNavigationService navigationService)
             : base(logProvider, navigation)
         {
         }

         public IMvxAsyncCommand TestAsyncCommand => new MvxAsyncCommand(async () => await TestAsyncCommandMethod());

         private async Task TestAsyncCommandMethod()
         {
             //await some stuff
         }
     }
 }

Anythings seems to work fine but rarely sometimes after a pression of the button it becames disable. What could happen? It does not happen if I use Command in place of MvxAsyncCommand, like here:

public Command TestAsyncCommand => new Command(async () => await TestAsyncCommandMethod());

Note: TestAsyncCommandMethod is entirely encased in a try-catch block to avoid possible exceptions.

var ret = await Xamarin.Forms.Application.Current.MainPage.DisplayAlert(...); if (!ret) return; ret = await BackgroundBleCommunication(); if (!ret) await Xamarin.Forms.Application.Current.MainPage.DisplayAlert(...); — AndreaGobs
Command is from native Xamarin.Forms,but IMvxAsyncCommand is from MvvmCross.By the way, Suggest that using public IMvxAsyncCommand TestAsyncCommand { get; private set; }. — Junior Jiang

FreakyAli FreakyAli · Accepted Answer · 2019-03-08T18:37:17

I think what you are doing wrong here is that instead of creating a Property to Bind the View and the ViewModel you are using a field can you try this:

public IMvxAsyncCommand TestAsyncCommand {get; set;}

And then in your constructor do this:

public TestViewModel(IMvxLogProvider logProvider, IMvxNavigationService navigationService)
        : base(logProvider, navigation)
    {
      TestAsyncCommand = new MvxAsyncCommand(async () => await TestAsyncCommandMethod());
    }

UPDATE

This seems to be a bug with MvvmCross! Issue can be found in the added link

https://github.com/MvvmCross/MvvmCross/issues/1589

Xamarin Forms MvvmCross binding a button command error

1 Answers

UPDATE