What do I need to do to migrate Xamarin-based EventToCommandBehavior command binding to work for .NET MAUI?

dg2k 1,396

I have Xamarin-based reusable controls based on ContentView. One use case is for password input and encryption key input built around a flexible custom control that can be configured for user screen lock password enable/change/disable as well as data encryption key enable/change/disable. This works fine with Xamarin without any issues.

When I attempted to port the same code to .NET MAUI there is one notable issue although a workaround exists. The EventToCommandBehavior command binding to Mouse Click event returns null on MAUI-WinUI while expected value is returned for Xamarin-UWP. If however the Click Event is routed to code-behind (rather than binding to ViewModel with EventToCommandBehavior ) it works as expected. This is the workaround referred to.

Surely, something needs to be tweaked for EventToCommandBehavior to work on MAUI-WinUI as I see a clear difference between Xamarin and .NET MAUI.

<controls:LoginControl  
    ButtonCancelLabel="Cancel"  
    ButtonOkLabel="OK"  
    IsErrorMessageVisible="{Binding IsPwdErrorMessageVisible}"  
    OnPasswordEnter="PwdControl_OnPasswordEnter"  
    PwdValidationMode="{Binding LoginControlValidationMode, Mode=TwoWay}">  
    <controls:LoginControl.Behaviors>  
        <b:EventToCommandBehavior Command="{Binding OnLoginControlOkButtonClickedCommand}" EventName="OnPasswordEnter" />  
    </controls:LoginControl.Behaviors>  
</controls:LoginControl>

LoginControl is a reusable custom control based on ContentView.
OnPasswordEnter is public event EventHandler OnPasswordEnter; in the custom control which is triggered by OnPasswordEnter?.Invoke(this, <event-info>).
The preferred way to capture the OnPasswordEnter mouse click event is with command binding to ViewModel (line 7 to 9) but this returns null for MAUI-WinUI. Capturing the event in code-behind (line 5) works but not ideal.

Can you suggest what changes I need to make for EventToCommandBehavior to work on MAUI-WinUI just like for Xamarin?

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 30,666 Reputation points Microsoft Vendor

2022-07-29T08:47:40.753+00:00
Hi anonymous user-4673, I'm not clear about your b:EventToCommandBehavior and your ViewModel. I copied this EventToCommandBehavior class, added it to my MAUI sample, and created a new ViewModel

public class LoginControlViewModel : INotifyPropertyChanged { public LoginControlViewModel() { OnLoginControlOkButtonClickedCommand = new Command(execute: () => { this.Name = "OnLoginControlOkButtonClickedCommand";// Add <Label Text="{Binding Name}"...... in MainPage and display the Name for testing }); } public ICommand OnLoginControlOkButtonClickedCommand { private set; get; } ..... }

Then I added the control to MainPage, set BindingContext of the page to LoginControlViewModel, and triggered OnPasswordEnter event in the control. But I can't reproduce your issue. Would you mind sharing your MAUI version, your machine info and some more code snippets about your CommandBehavior and ViewModel? As noted in the Q&A FAQ: How to write a quality question
dg2k 1,396 Reputation points

2022-07-29T10:13:46.453+00:00

Hi @Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.)

For the most common cases, I can use the EventToCommandBehavior command binding for Maui without a problem.

The issue arises when I try to use with a Custom Control. May be I didn't make this clear enough.

What I suspect is that the way event binding is done for a Custom Control, to be further relayed to view-model binding from a host ContentPage, may be different for Maui. May be events are forwarded with a subtle Command binding for Maui??.

No problem with event detection without EventArgs object but I'd like to pass on event info as event parameter. This is what shows up as null for Maui.

I looked around for examples and no luck. As I said, the binding from Custom Control works without issues for Xamarin.
Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 30,666 Reputation points Microsoft Vendor

2022-08-01T07:15:41.883+00:00

I tested by adding a custom control, it works.

What I suspect is that the way event binding is done for a Custom Control, to be further relayed to view-model binding from a host ContentPage, may be different for Maui.

For further investigate the issue, could you provide more details about how you set the ViewModel and data-binding. And would you mind sharing a basic minimal reproducible sample with me?
dg2k 1,396 Reputation points

2022-08-02T20:20:12.583+00:00
Hi @Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.)
I have uploaded a demo and a zipped project can be downloaded here from my OneDrive Public Folder.

Quick Guide

User control is in Controls project folder.

The demo page is DemoPage.xaml. A button is provided on the start-up MainPage to get to DemoPage.

A view-model for DemoPage.xaml appears under ViewModels project folder.

A User Control appears as a password Entry with Cancel and OK buttons as shown with a screenshot below. It would suffice to tap on the OK button for this demo. The code is redacted just to demonstrate the issue described in this Question. You may also find some redundant code which should be ignored.

When you tap on the OK button, an event object with a value is expected in the view model (bound to EventToCommandBehavior Command) but shows up as null.

If you, instead, capture in code-behind the event is non-null (what is expected in view-model!). There is the code-behind event handler that you can uncomment and verify.

I have inserted System.Diagnostics.Debugger.Break(); at an appropriate point with explanatory comments there.

Accepted answer

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 30,666 Reputation points Microsoft Vendor

2022-08-03T09:06:56.32+00:00
Hello anonymous user-4673 ,

Please try to copy this EventToCommandBehavior class and BehaviorBase class to your project.

namespace QuestionAndAnswer.Demo.Behaviors { public class EventToCommandBehavior : BehaviorBase<VisualElement> {...} }

Then you can use it on your DemoPage, it works on my side, the EventArgs object won't be null in OnButtonTapped method of DemoPageViewModel.

<ContentPage ... xmlns:behavior ="clr-namespace:QuestionAndAnswer.Demo.Behaviors" ...> <StackLayout VerticalOptions="CenterAndExpand"> ... <demo:DemoUserControl ...> <demo:DemoUserControl.Behaviors> <behavior:EventToCommandBehavior Command="{Binding OnButtonTappedCommand}" EventName="OnPasswordButtonTapped" /> </demo:DemoUserControl.Behaviors> </demo:DemoUserControl> ... </ContentPage>

I checked your sample, you are using CommunityToolkit, it is not supported on Q&A, you can report this at https://github.com/CommunityToolkit/Maui.

Best Regards,
Wenyan Zhang

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dg2k 1,396 Reputation points

2022-08-03T13:04:20.737+00:00

Thank you @Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.)

This is in effect going back to the Xamarin code, and as I've indicated, everything was working in Xamarin for me. I was persuaded to use the CommunityToolkit based on what was touted about Maui Behaviors.

Does it mean that CommunityToolkit is yet not ready with the likes of Behaviors as described??

Interestingly, all my event-to-command bindings are based on CommunityToolkit's EventToCommandBehavior and only failed me when I used it in conjunction with a User Control as shown in the demo I've provided.

Honestly, the experience thus far gives me no confidence in .NET MAUI which was supposed to be released as far back as November 2021. There are a lot of critically missing functionalities detailed in my recent Questions here, notably the issues with CollectionView and MSAL sign-in, which work fine in Xamarin but not with .NET MAUI.

Let's say going back to Xamarin code provided the workaround while CommunityToolkit wrt Behaviors is still work in progress.
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What do I need to do to migrate Xamarin-based EventToCommandBehavior command binding to work for .NET MAUI?

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