CommandManager Executed Events don't fire for custom ICommands - c#

The WPF CommandManager allows you to do the following (pseudo-ish-code):
<Button Name="SomeButton" Command="{Binding Path=ViewModelCommand}"/>
And in the code-behind:
private void InitCommandEvents()
{
CommandManager.AddExecutedEventHandler(this.SomeButton, SomeEventHandler);
}
The SomeEventHandler never gets called.
To me this didn't seem like something very wrong to try and do, but if you consider what happens inside CommandManager.AddExecutedEventHandler, it makes sense why it doesn't. Add to that the fact that the documentation clearly states that the CommandManager only works with RoutedCommands.
Nonetheless, this had me very frustrated for a while and led me to this question:
What would you suggest is the best workaround for the fact that the CommandManager does not support custom ICommands? Especially if you want to add behavior around the execution of a command?
For now, I fire the command manually in code behind from the button click event.

I generally just subclass RoutedCommand and re-use its functionality instead of implementing an ICommand from scratch. Then it works well with CommandManager, etc, and everyone is happy.
On the other hand, if you do implement an ICommand from scratch, it seems to me that there's no need of CommandManager.AddExecutedEventHandler: Your custom ICommand can easily expose its own way of registering for notifications when the command executes. In fact, most custom ICommand implementations do this as the primary way of handling command execution.
Either way it doesn't seem you would ever need both custom ICommand functionality and CommandManager support at the same time.

Related

c# xamarin forms - custom event on custom control with a property

Okay, im positive this has an answer somewhere but I have been banging my head against a wall FOREVER trying to get this to work, an working around it for days, and im losing my mind here. I cannot find a single example that works or does what I want... at least not that I understand how its written.
Im writing a custom control, basically a content view with a calculator in it. One of the controls in this is an entry.
What i want is VERY simple... when you create an entry in XAML you can do
<Entry TextChanged="FunctionToRun">
and then whenever the text is changed, an event is fired and that function is run.
In my case i want to add a custom event to my calculator class so that when i create one on a page:
<local:myCalculator CountUpdated="FunctionToRun">
that function gets run.
Everything I look at online talks about using an ICommand and all this - but literally every single example I have tried leads me to either:
A) Not be able to link my function in XAML (errors)
B) Only calls something inside the calculator class.... but doesnt trigger any events, and i cannot force it to.
I think i completely do not understand ICommand, and no matter how many examples I ahve looked at I cannot get what im after.
Anyone able to help? im sure its stupidly simple...
Turns out I was being completely blind and didnt realize i had implemented my event on the item with:
public EventHandler<EventArgs> EventName = {get;set;}
Which is absolutely not how you do it - I wont rewrite the reasoning when I can just find an existing answer:
event EventHandler vs EventHandler
Why do we need the "event" keyword while defining events?
Anyhow - it was a blank-minded mistake while coding a lot at once. This should be
public event EventHandler<EventArgs> EventName;
for many reasons - one of the most minor being it allows you to bind properly from XAML.

Difference between Command (ICommand) and Click event

When should I use the Command and when to use the Click event?
F.e. if I have a Button in my UWP app what should I use?
When should I use the Command and when to use the Click event?
Yours is a broad question and I would simply answer with: "It depends".
Because:
The Command implements the ICommand interface and this means more code to add to your application but usually this won't change. Instead, the event handler doesn't require any interface implementation.
For every command you want, you have to provide the code that will handle the click and the CanExecute logic, to say when the command can execute. This is not requested in a simple event handler (like MyButton_Click). This means that, using a Command, you will have more control over the elements of your UI (the button won't execute anything if CanExecute is false).
When you want to add a Command, you will bind it to your DataContext (the ViewModel, if you implement the MVVM pattern). Instead, when you add a simple event handler (like MyButton_Click), the code will be placed in your code-behind that is the logic behind your main window. This means that implementing a Command, according to me, you'll have everything you need to modify in just one place (the ViewModel) instead of logic scattered everywhere in your project.
Of course, you can use whatever you want and my points are there just to give you an insight about these different implementations and you have to consider which solution is suitable for you, considering also the requirements you have been given (like: "Don't use event handlers" or "The Command is too advanced, let's just use something simple", etc.) and/or other constraints in your project.

Event handling in MVVM

In MVVM, Model will usually have the data model, view is UI (XAML) which is further binded to the properties VM, ViewModel which typically inherits INotifyPropertyChanged.
When it comes to event handling, are there any specific pattern to handle all events on UI?
For Ex: Lets say if we have save/new/close button or some other button... and desired goal is when user does some operation and clicks on any of the button, control should go to code behind and should perform desired operation... how should I make sure that I have done the event handling in proper manner? and which interface I should use / when and how?
like we have ICommand interface/Relay command/Delegate command... I am not clear with this..
Thanks in advance for your response to my query...
Amit, if you are planning to hook up buttons, the accepted way is to use an implementation of ICommand (my personal preference is RoutedCommand). If you are aiming of to raise and handle events, have a look at Event Aggregators which is based on Publisher/Subscriber pattern.
In this, you will register a method (message handler) to ‘listen’ to a message (event) that matches a pattern. Once you done that, you can raise/publish messages (events) and when a match is found, the correct handler will gat raised
PRISM framework by Microsoft has done a good job of implementing event aggregate pattern
http://msdn.microsoft.com/en-us/library/ff921122(v=pandp.20).aspx
Hope this is useful

Why is ICommand better than code behind calling the VM?

I have a co-worker that asked me why he has to use the ICommand pattern.
He wants to add a button and then make an event for it in the code behind. Then from the event he wants to call a method on the ViewModel.
I gave him the obvious answer: This adds coupling between the View and the ViewModel. But he argued that the View and the ViewModel are already coupled. (We set our view's DataContext to the ViewModel in the View's code behind: DataContext = new MyViewModel();
Yes, I told him that his way adds "more coupling", but it sounded a bit lame even to me.
So, I know that ICommand is the clean way, and I do it that way. But what else does ICommand buy you besides not using an already existing coupling?
It's not about decoupling, but how deep you can penetrate inside your ModelView hierarchy: not event pumping, but event routing, built-in in the framework.
It's about UI managent: Command has state (CanExecute), if assign the command to the control, if command's state becomes false, control becomes disabled. It gives you powerful UI state management way, avoiding a lot of spaghetti coding, for complicated UI especially.
I have a co-worker that asked me why he has to use the ICommand
pattern.
It seems implied this is a standard at your company (whether explicitly stated or unspoken). That should be answer enough to his question.
If all company code is supposed to use that pattern, it can cause co-developer confusion and frustration when someone else has to debug his code.
Also, in my opinion, using ICommand is faster to develop / mock up because you don't NEED to have the ICommand property on the context to run your program. It lets your UI designers (if you are lucky enough to have them) completely finish their tasks even if you are behind in your coding.
ICommand can also give you a place for handling wether or not a specific button can be used right then. this would be handled through the canexecute method.
You can bind the CanExecute method of the command to the properties of a control, also a Command encapsulates an action in a nice way. In my opinion / experience this approach makes a lot of sense because you have both the condition and the execute action in a single abstraction, which makes it easier to understand and test.
If in the future you find that this action is repeated you can abstract it easily in your own custom ICommand and use it in several places.
One thing that I don't see in the previous answers is that using the ICommand promotes code reuse by allowing the same action to be used by different GUI components. For example, if I had a command that should result in the opening of a window and that command could be invoked in three or for different screens in the application, an ICommand implementation lets me define that logic in a single place. With the code-behind event handlers, I have to copy and paste redundant code, in violation of DRY (or else, I'd have to roll my own implementation by abstracting out to a class, at which point, I might as well use ICommand).

Event Handler located in different class than MainWindow

So I followed the guide on the following site to restrict the characters a textbox can accept.
http://www.rhyous.com/2010/06/18/how-to-limit-or-prevent-characters-in-a-textbox-in-csharp/
My problem is I can't figure out how to make the event handler trigger in the secondary class. Basically how do I tell VS to look for the event handler code in that class instead of MainWindow? I tried searching, but apparently don't know the correct terms to use. The xaml reference I used was
xmlns:DigitBox="clr-namespace:System.Windows.Controls;assembly=PresentationFramework"
Any ideas?
Simplest way I've found to do it is assign the event in your constructor.
public MainWindow()
{
InitializeComponent();
TextBoxCurrency.GotFocus += expandedTextBoxEvents.TextBoxCurrencyGotFocus;
TextBoxCurrency.LostFocus += expandedTextBoxEvents.TextBoxCurrencyLostFocus;
}
I've searched a way to do it in XAML and I did not found an easy and clean way to do it.
You are much better off using commands and command bindings. I'm not sure what the specific command that would would bind to for a text box for your desired functionality, but one of the goals for WPF was to lessen the use of Event Handlers in code behind.
Check out this article for an overview of commands and this article for a way to hook up commands with events. WPF commanding is one of the coolest features to enable true separation of concerns between UI and business logic.
As a worst case scenario solution, you could create your own text box that inherits from the text box control and hook up the events in that class. Your control would then be reusable.

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