I had been merrily using WPF's built-in support for the Command Pattern for ages (see Commanding Overview Buy Cephalexin Without Prescription, , MSDN Docs, and article on implementing the command pattern in WPF, Jeff Druyt)... when suddenly it occured to me that I had no idea what triggered WPF to determine whether or not a command can be executed, buy Cephalexin online no prescription. Buy Cephalexin from canada, Let me explain by reduction to an absurd example:
Say I have a command that can only execute when
DateTime.Now.Second % 2 == 0.
I construct this command by home-brewing a static RoutedCommand instance:
public static class Commands
{
public static RoutedCommand MyCommand { get { return m_MyCommand; } }
private static RoutedCommand m_MyCommand = new RoutedCommand
(
"Execute My Command",
typeof(Commands), Cephalexin price, coupon, Cephalexin in uk,
new InputGestureCollection()
{
new KeyGesture(Key.C, ModifierKeys.Alt)
}
);
}
And then I add a Command Binding for that command to my Window, Cephalexin from international pharmacy, Buying Cephalexin online over the counter, and assign the command to a Button:
<Window x:Class="TestCommands.Window1"...>
<Window.CommandBindings>
<CommandBinding
Command="{x:Static local:Commands.MyCommand}"
Executed="MyCommandExecuted"
CanExecute="MyCommandCanExecute"
/>
</Window.CommandBindings>
...
<Button Width="200" Height="200"
Command="{x:Static local:Commands.MyCommand}"
Content="{Binding Path=IsEnabled}"
/>
.., order Cephalexin no prescription. Cephalexin paypal, </Window>
By nature of WPF's awesomeness and WPF Commanding in general, the above Button's IsEnabled property should automatically be set to true or false based on whether or not the command can or can't be executed, purchase Cephalexin online.
Speaking of which, let's set up my Command's absurd logic in the CodeBehind by implementing its Execute and CanExecute event handlers:
private void MyCommandCanExecute(object sender, CanExecuteRoutedEventArgs e)
{
DateTime now = DateTime.Now;
if (CanExecuteOutput != null)
{
CanExecuteOutput.Text = "MyCommand CanExecute determined at " +
now.ToLongTimeString() + " (and " + now.Millisecond + "ms)";
}
e.CanExecute = DateTime.Now.Second % 2 == 0;
}
private void MyCommandExecuted(object sender, buy cheap Cephalexin no rx, Over the counter Cephalexin, ExecutedRoutedEventArgs e)
{
TextOutput.Text = "MyCommand executed at " + DateTime.Now.ToLongTimeString();
}
So, my example is absurd but I bet you see my point by now: WPF is meant to automatically set the IsEnabled Property on that button to true or false, Cephalexin overseas, Cephalexin san diego, based on the results of the CanExecute method. But in this case, fast shipping Cephalexin, Where to buy Cephalexin, the results of CanExecute are a function only of time, and thus change repeatedly and independently of "obvious" application events, online buying Cephalexin hcl. Saturday delivery Cephalexin, So... how does the Commanding system know when to query CanExecute and consequently enable/disable the button once a second? Buy Cephalexin Without Prescription, In this case, without further intervention, it doesn't. It seems that when events are raised on the Window (a mouse button click, Cephalexin trusted pharmacy reviews, Free Cephalexin samples, etc.), CanExecute is re-evaluated, Cephalexin discount. Rx free Cephalexin, (I don't know the details and wish I did.) But, without further programmatic or user intervention, order Cephalexin from mexican pharmacy, Cephalexin in canada, the button will not automatically change its IsEnabled state once a second.
This led me back to the MSDN docs, where to buy Cephalexin, Cephalexin from international pharmacy, where I discovered the aptly-named InvalidateRequerySuggested event. To coerce - er, order Cephalexin no prescription, Buy Cephalexin online cod, suggest - that WPF should query CanExecute, I set up the following DispatcherTimer:
m_DispatcherTimer = new DispatcherTimer()
{
Interval = TimeSpan Buy Cephalexin Without Prescription, .FromSeconds(0.25),
IsEnabled = true
};
m_DispatcherTimer.Tick += delegate
{
CommandManager.InvalidateRequerySuggested();
};
Now, the IsEnabled property of the Button blinks on and off as the Command's ability to be executed changes with the passing seconds.
Only then did I discover there's an MSDN Docs sample called "Disable Command Source Via Dispatcher Timer Sample" which is remarkably similar, Cephalexin discount. Cephalexin in uk, There you have it. Now go forth and command WPF's Commanding, Cephalexin san diego. Buy Cephalexin without a prescription, I'm sure you can all execute on that request <g>
P.S. Code for this sample is here, Cephalexin gel, ointment, cream, pill, spray, continuous-release, extended-release. Saturday delivery Cephalexin, P.P.S. What are folks using for pasting XAML and C# code into their blogs, where can i buy Cephalexin online. Free Cephalexin samples, This entry is looking a little rough... Cephalexin in australia. Cephalexin in canada. Over the counter Cephalexin. Cephalexin from canadian pharmacy. Cephalexin overseas. Cephalexin buy. Cephalexin price, coupon. Cephalexin for sale. Cephalexin pills. Cephalexin prescriptions. Where to buy Cephalexin. Cephalexin in usa. Buy Cephalexin from canada.
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Tht’s great investigation! It was very helpfull for me. Thanks!
Me too – thanks!
thanks, very helpfull
This doesn’t work when used with a built in command like ApplicationCommands.Print, any idea why?
Kind regards,
Daniel
By default CanExecute is requeried on focus change for RoutedCommand (RoutedUICommand inherits from RoutedCommand as well).
Just what I was looking for! Thank you!
You just made my day!
Thanks a lot
:-)
InvalidateRequerySuggested() was just what I was looking for, thanks for sharing!
Pingback: kevin Mocha - WPF Commanding - When do Commands re-evaluate their CanExecute method?
Excellent!!! Thank you!
Many thanks :)
Thank god google put you on first page ;)
Saved me a lot of digging
Lifesaver.
Thanks a lot
thanks dude, exactly what I needed. Seems like the whole command architecture is really only worth it at a certain level of complexity of your application. otherwise you’d do the same thing old school with less lines of code.
Very helpfull :-)
Could not imagine how to get my button to be enabled after a search, before to user clicked in the form. The sentence “CommandManager.InvalidateRequerySuggested();” saved my day.
I normally create my own implementation of ICommand but with the following code:
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
Take a look at http://msdn.microsoft.com/en-us/magazine/dd419663.aspx#id0090030 for more details.
Oops, posted the wrong link. Should be http://joshsmithonwpf.wordpress.com/2008/06/17/allowing-commandmanager-to-query-your-icommand-objects/
Thanks a lot for this good article!
But remember that using a DispatchTimer might cause a memory leak! If you have many forms and each time the user switches to another form a new one is created, then with dispatch timer a memory leak is created.
Do not forget to deregister the timer when the form is closed.