Shift- (or Ctrl-, or ... ) Clicks and commands

[LaurentInSeattle]

Hi,

I could not find a simple and easy way to handle shilft-ed or control-led clicks on command using Avalonia API's so I created this class:

`
public sealed class Keyboard
{
private Window? window;

public KeyModifiers Modifiers { get; private set; }

public bool IsStarted { get; private set; }

public void Start (Window window)
{
    if (this.IsStarted)
    {
        Debug.WriteLine("Keyboard monitor already started"); 
        return;
    }

    this.Modifiers = KeyModifiers.None;
    this.window = window;
    this.window.AddHandler(
        InputElement.KeyDownEvent, this.OnKey, RoutingStrategies.Tunnel, handledEventsToo: true);
    this.window.AddHandler(
        InputElement.KeyUpEvent, this.OnKey, RoutingStrategies.Tunnel, handledEventsToo: true);
    this.IsStarted = true;
}

public void Stop()
{
    if (!this.IsStarted || this.window is null)
    {
        Debug.WriteLine("Keyboard monitor has no window or is not started, cannot be stopped.");
        return;
    }

    this.Modifiers = KeyModifiers.None;
    this.window.RemoveHandler(InputElement.KeyDownEvent, this.OnKey);
    this.window.RemoveHandler(InputElement.KeyUpEvent, this.OnKey);
    this.IsStarted = false;
    this.window = null;
}

// NOT handled or else cant type anything :( 
// DONT => args.Handled = true;
private void OnKey(object? _, KeyEventArgs args) => this.Modifiers = args.KeyModifiers;

}

`
I create a single instance of it with the MSFT IoC/Di system,start it and then use it anywhere this way:

`
var keyboard = App.GetRequiredService();
bool isShifted = keyboard.Modifiers.HasFlag(KeyModifiers.Shift);

`
Now the questions...
Is there a better and simple way to achieve the same functionality ?
Have I missed something ?
Thoughts, comments ?

Thanks and Happy New Year.
L.

[timunie]

I'd suggest you just listen for PointerPressed event. The event args should provide the needed modifiers. Call the command from the ViewModel then (DataContext is your VM)

[msneijders]

It is WPF's static System.Windows.Input.Keyboard.Modifier you want right? In WPF I am quite sure its implemented by asking the underlying OS (i.e. calling win32 GetKeyState). Linux and macos probably have similar ways to request keyboard state. So an alternative way could be to use pinvoke for each platform you want to support.

(only the OS knows the key state reliably -- if you start pressing CONTROL with focus in another app, and then click a button in your app, your solution will not know the Control state because it never got the KeyEvent, right?)

[LaurentInSeattle]

Yes, this is the WPF feature that I want to 'replicate'.

Since Avalonia provides the modifier keys within the pointer event args, there should be a spot in the Avalonia codebase where the keyboard state is read in a portable manner, but.. so far I have not been able to find it. If anyone knows, please tell.

For now, my "weak" solution works for the most simple cases and the special one mentioned above by msneijders is somewhat uncommon...

[timunie]

There is no such option for you available. The reason is due to not all platforms can provide such an API.

[wieslawsoltes]

You can use attached behaviors

https://github.com/wieslawsoltes/Avalonia.Xaml.Behaviors/blob/9abeb15b43aaf634b2c5180789672ecbb53ada11/samples/BehaviorsTestApplication/Views/Pages/ButtonClickEventTriggerBehaviorView.axaml#L30-L32

https://github.com/wieslawsoltes/Avalonia.Xaml.Behaviors/blob/master/src/Avalonia.Xaml.Interactions.Custom/Button/ButtonClickEventTriggerBehavior.cs

<UserControl x:Class="BehaviorsTestApplication.Views.Pages.ButtonClickEventTriggerBehaviorView"
             xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:vm="clr-namespace:BehaviorsTestApplication.ViewModels"
             mc:Ignorable="d" d:DesignWidth="600" d:DesignHeight="450"
             x:DataType="vm:MainWindowViewModel">
  <Grid RowDefinitions="*,Auto" ColumnDefinitions="30*,5,30*,5,30*">
    <Canvas Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="5" 
            Margin="5" 
            Background="{DynamicResource GrayBrush}">
      <Ellipse Canvas.Left="{Binding Position, Mode=TwoWay}" 
               Canvas.Top="200" 
               Fill="{DynamicResource RedBrush}"
               Stroke="{DynamicResource YellowBrush}" 
               StrokeThickness="5" 
               Height="100" Width="100" />
    </Canvas>
    <Button Content="Left (Hold Control), Reset (Hold Shift), Right (Hold Control+Shift)" 
            Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="5" 
            Margin="5,0,0,5">
      <Interaction.Behaviors>
        <ButtonClickEventTriggerBehavior KeyModifiers="Control">
          <InvokeCommandAction Command="{Binding MoveLeftCommand}" />
        </ButtonClickEventTriggerBehavior>
        <ButtonClickEventTriggerBehavior KeyModifiers="Shift">
          <InvokeCommandAction Command="{Binding ResetMoveCommand}" />
        </ButtonClickEventTriggerBehavior>
        <ButtonClickEventTriggerBehavior KeyModifiers="Control,Shift">
          <InvokeCommandAction Command="{Binding MoveRightCommand}" />
        </ButtonClickEventTriggerBehavior>
      </Interaction.Behaviors>
    </Button>
  </Grid>
</UserControl>