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POIs Josh Smith's implementation of the RelayCommand flawed?
 

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  1. POIs Josh Smith's implementation of the RelayCommand flawed?
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    <p>Consider the reference <a href="http://msdn.microsoft.com/en-us/magazine/dd419663.aspx" rel="noreferrer">Josh Smith' article WPF Apps With The Model-View-ViewModel Design Pattern</a>, specifically the example implementation of a <code>RelayCommand</code> (In Figure 3). (No need to read through the entire article for this question.) </p> <p>In general, I think the implementation is excellent, but I have a question about the delegation of <code>CanExecuteChanged</code> subscriptions to the <code>CommandManager</code>'s <code>RequerySuggested</code> event. The <a href="http://msdn.microsoft.com/en-us/library/system.windows.input.commandmanager.requerysuggested%28VS.100%29.aspx" rel="noreferrer">documentation for <code>RequerySuggested</code></a> states:</p> <blockquote> <p>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.</p> </blockquote> <p>Yet the sample implementation of <code>RelayCommand</code> does not maintain any such to the subscribed handler:</p> <pre><code>public event EventHandler CanExecuteChanged { add { CommandManager.RequerySuggested += value; } remove { CommandManager.RequerySuggested -= value; } } </code></pre> <ol> <li>Does this leak the weak reference up to the <code>RelayCommand</code>'s client, requiring that the user of the <code>RelayCommand</code> understand the implementation of <code>CanExecuteChanged</code> and maintain a live reference themselves?</li> <li><p>If so, does it make sense to, e.g., modify the implementation of <code>RelayCommand</code> to be something like the following to mitigate the potential premature GC of the <code>CanExecuteChanged</code> subscriber:</p> <pre><code>// 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; } } </code></pre></li> </ol>
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    1. USGennady Vanin Геннадий Ванин
    1. USGreg D
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    1. COGreat question... I also wonder htf is RequerySuggested knowing when is a requery necessary..
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      1. USAndrei Rînea
    2. CORequerySuggested is documented to fire when the CanExecute state may need to be refreshed. I believe it can also be manually fired by clients if the client knows something that WPF itself doesn't know.
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      1. USGreg D
    3. CO@Greg - And by what parameters does RequerySuggested know if CanExecute state may need to be refreshed? :)
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      1. USVitalyB
 

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