In that situation I use a converter to convert the boolean (from the
CanExecute) into an appropriate visibility value.
Converters rock.
From: [email protected] [mailto:[email protected]]
On Behalf Of Miguel Madero <[email protected]>
Sent: Tuesday, 9 November 2010 2:46 PM
To: ozWPF <[email protected]>
Subject: Re: Events to command binding
For this I usually have a CanExecute property (which you usually have
any way for Delegate or RelayCommands). That way the UI binds the
IsEnable to the CanSave property. Seems like an extra step in Xaml, but
that's one step less in the VM, plus it keeps it clean as well. I don't
like invalidating commands when all I want to say is CanSave=true. I
know there're implementations or RelayCommand that will Invalidate the
command on property changed.
Also this works fine for buttons and cases where you want to disable
something, but what if you want to make it invisible instead? You use a
different type of CommandBinder and then things start to get messy when
you could simple bind Visibility to the same CanSave property.
On Tue, Nov 9, 2010 at 4:15 PM, <[email protected]> wrote:
I agree. MVVM is a pattern, and patterns can be implemented in any
number of ways.
That said, ICommand is extremely useful if you drive it's CanExecute in
your ViewModel. I mostly use ICommand (via a delegate command) on
buttons and menus which allows me to automatically disable these
elements from data driven events on the ViewModel without the ViewModel
knowing anything about the UI.
Carl.
From: [email protected] [mailto:[email protected]]
On Behalf Of Miguel Madero <[email protected]>
Sent: Tuesday, 9 November 2010 1:36 PM
To: ozWPF <[email protected]>
Subject: Re: Events to command binding
I personally don't think using a command would make it more or less
MVVM. I don't think commands are adding any value. Some would say they
make it explicit that something is a UI Command, for me a public method
is just as explicit. In terms of testability and separation of concerns,
I think a command actually does just the opposite than helping. In terms
of line of code (a metric we probably shouldn't be concern about
anyway), it's pretty much the same, although IMO the Codebehind approach
is clearer.
The only part where I would probably agree with the purists is that we
run the risk to accidentally end up with too much logic in the UI.
On Tue, Nov 9, 2010 at 2:34 PM, Winston Pang <[email protected]>
wrote:
Sometimes I am tempted to write code like that, but I'm afraid some MVVM
purist or some pedantic bastard will give me a purist talk.
On Tue, Nov 9, 2010 at 2:33 PM, Miguel Madero <[email protected]>
wrote:
>From codebehind yes.
Let's assume you have a button in your listBox's datatemplate.
public OnDeleteButtonClick(o...e..){
var orderToDelete = ((Button)o).DataContext;
var viewModel = (MyViewModel)DataContext;
viewModel.DeleteOrder(orderToDelete);
}
I tend to have a property that gives me the ViewModel just to remove the
cast from my code.
On Tue, Nov 9, 2010 at 8:47 AM, Winston Pang <[email protected]>
wrote:
Oh right, as in, casting DataContext to the respective VM and invoking
it with respective params?
On Tue, Nov 9, 2010 at 9:42 AM, Miguel Madero <[email protected]>
wrote:
I rarely have to pass parameters. The CallMethodAction doesn't have this
option. I prefer parameterless methods and databind to properties to
give my VMs the extra context. In cases where this isn't possible I do
it from code behind. I guess it would be really simple to write a
CallMethodAction that takes a parameter or parameter list if you need
this functionality. Since this isn't a common task for me, it has been
simpler to write an event handler and directly call the method.
On Mon, Nov 8, 2010 at 7:45 PM, Winston Pang <[email protected]>
wrote:
Nice, that's interesting, never seen that one, I hate having to declare
ICommand's it's such a hassle just to wire something up.
How do you pass args/params with CallMethodAction
On Sun, Nov 7, 2010 at 8:34 PM, Miguel Madero <[email protected]>
wrote:
BTW I love the CallMethodAction in Blend so I don't have to define
Command in my ViewModels. It feels more POCOish. Just a public method.
If I need the IsEnabled I just add a property and bind it to either
IsEnabled or Visibility.
<rant>
Commands are just too messy and feels like leaking view concerns into my
VMs. Just for the same reason I don't expose properties of type
Visibility, I don't like exposing ICommand.
Setting purity aside, it's just too much work, create another property
that delegates on your method, initialise it in the constructor, then
find ways to actually be able to call that from code... naaa. That's
just messy.
</rant>
On Sun, Nov 7, 2010 at 7:26 PM, Miguel Madero <[email protected]>
wrote:
+1 for Blend Triggers and Actions
+1 for Code Behind for uncommon scenarios as a second option.
On Tue, Nov 2, 2010 at 2:36 PM, Paul Stovell <[email protected]>
wrote:
There is an Expression Blend trigger action that can invoke a
command IIRC. That's probably the easiest way.
Caliburn also has a nice approach of mapping an event to a
ViewModel method via an attached property - something like <Button
cal:Message.Attach='[Event MouseOver] = DoSomething()' />
Personally, if there's no command out of the box and it's not a
common scenario, I'm quite happy to write a plain old event handler and
invoke the method on the VM manually. There's nothing wrong with a
little code behind.
Paul
From: [email protected]
[mailto:[email protected]] On Behalf Of Greg Keogh
Sent: Tuesday, 2 November 2010 2:31 PM
To: 'ozWPF'
Subject: Events to command binding
Some have scoffed when I expressed dismay at the artifice that
creates binding. I've just discovered that I'm wasting hours of time
converting events into commands. For example, iIt took me an hour to
find a piece of sample code that converted KeyDown on TreeView nodes to
a binding (found HERE
<http://stackoverflow.com/questions/612966/keyboard-events-in-a-wpf-mvvm
-application> ), but it needed delicate merging with existing ICommand
processing classes I use.
I estimate that 50% of the time I spend writing WPF apps is
wasted trying to follow MVVM and bind the control XAML to my controller
class. It seems that every other man and his dog who is trying to follow
the MVVM pattern as well has created untold amounts of confusing and
conflicting code for the purpose. I'm sick of searching for and finding
jumbles of code that I have to tidy up and include in my projects.
I feel compelled to write a small infrastructure that allows
event-to-command binding in a generalised way, but I'll bet it's been
done already (multiple times and in multiple ways). Has anyone got any
suggestions or comments on this? Surely I'm not the first person in the
world to have stumbled across these hurdles.
MVVM would be wonderful if all of the wiring was just built-in.
Greg
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