Question
Is Josh Smith's implementation of the RelayCommand flawed?
Consider the reference Josh Smith' article WPF Apps With The Model-View-ViewModel Design Pattern, specifically the example implementation of a RelayCommand
(In Figure 3). (No need to read through the entire article for this question.)
In general, I think the implementation is excellent, but I have a question about the delegation of CanExecuteChanged
subscriptions to the CommandManager
's RequerySuggested
event. The documentation for RequerySuggested
states:
Since this event is static, it will only hold onto the handler as a weak reference. Objects that listen for this event should keep a strong reference to their event handler to avoid it being garbage collected. This can be accomplished by having a private field and assigning the handler as the value before or after attaching to this event.
Yet the sample implementation of RelayCommand
does not maintain any such to the subscribed handler:
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
- Does this leak the weak reference up to the
RelayCommand
's client, requiring that the user of theRelayCommand
understand the implementation ofCanExecuteChanged
and maintain a live reference themselves? If so, does it make sense to, e.g., modify the implementation of
RelayCommand
to be something like the following to mitigate the potential premature GC of theCanExecuteChanged
subscriber:// This event never actually fires. It's purely lifetime mgm't. private event EventHandler canExecChangedRef; public event EventHandler CanExecuteChanged { add { CommandManager.RequerySuggested += value; this.canExecChangedRef += value; } remove { this.canExecChangedRef -= value; CommandManager.RequerySuggested -= value; } }