[Reactive] Enable Command cancellation

[weitzhandler]

Request

Enable better cancellation of Commands.

Implementation idea

It can be achieved by using an additional cancellation command.

For example, we add another interface ICancellableAsyncCommand (or modifying the existing), and add a Cancel property to it (of type IAsyncCommand, which when called, triggers the CancellationToken of its parent Command.

Pseudo code (pseudo):

Implementation:

public interface ICancellableAsyncCommand : IAsyncCommand, ILoadable
{
    void CancelExecution() => cancellationTokenSource.Cancel();

    IAsyncCommand Cancel { get; } // = new CancellableCommand(this); - via ctor
}

public class CancellableCommand : IAsyncCommand
{
    public CancellableCommand(ICancellatbleAsyncCommand parent);    
    
    bool CanExecute => Parent.IsExecuting;

    void Execute() => Parent.CancelExecution();
}

Model:

public async ValueTask Save(CancellationToken ct)
{
    await Task.Delay(TimeSpan.FromMinutes(1));
}

View:

<Button Text="Save" Command="{Binding Save}" />
<Button Text="Cancel save" Command="{Binding Save.Cancel}" />

The button's visibility can then be set according to each others execution state and they can be displayed overlapping each other. Even a single button that changes state, or a cancellable button.