Watch out for multiple subscriptions with BindCommand to event in RxUI

When using the handy binding methods in ReactiveUI, you might find yourself using the very handy overload of BindCommand() which let's you bind a command to an event:

this.BindCommand(ViewModel, vm => vm.MyCommand, v => v.TextBox, nameof(TextBox.GotFocus));

You might even have it inside a WhenActivated() in your view:

this.WhenActivated((CompositeDisposable d) =>
    this.BindCommand(ViewModel, vm => vm.MyCommand, v => v.TextBox, nameof(TextBox.GotFocus));

What you have to remember is to dispose that BindCommand() with the disposable you're given inside WhenActivated().

Unlike regular command bindings, which just binds the command to the command property of the control...

this.BindCommand(ViewModel, vm => vm.MyCommand, v => v.MyButton);

Command to event bindings adds an event listener to the event and executes the command when the event fires. If you don't dispose this binding, BindCommand() will add a new event listener when navigating back to that view. This means you'll have to event listeners, both executing the command, resulting in two executions of the command.

This will of course repeat every time you navigate back to a view, and reactivate it. The solution is to follow best practice when using WhenActivated() and dispose the binding:

this.BindCommand(ViewModel, vm => vm.MyCommand, v => v.MyButton)

Happy coding!

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