I'm wondering why MS decided to call OnPropertyChangedCallback instead of firing some kind of event.
Is there any rationale behind this?
To me, raising events inside OnPropertyChangedCallback seems to add more code to write, and doesn't seem to have much performance benefits.
But I'd like to know if there are some edge cases / usage benefits that I might have missed (preferably with code examples).
Thank you : )
Generally speaking MS use a convention that events should be raised by the method called OnEventName. See this article. Thanks to that every developer knows how to raise a given event and doesn't have to write his own code to do so.
It is a pretty simple code but there is no need to copy&paste it and clutter the source code. You have to remember that a given event can be raised in many places. It is true regardless if we talk about WPF, WinForms or any other technology.
By the way, I tried to find OnPropertyChangedCallback in Reference Source and I din't find it.
Related
I researcher mvvm in wpf.
I know how to use a behavior and how to use a trigger&action, but I don't understand when!
I look for info but I find only theoretical metirial, not practical.
thank you!
The WPF Behavior is a way to encapsulate a part of complicated UI logic into reusable coded component.
http://www.wpftutorial.net/Behaviors.html.
https://msdn.microsoft.com/en-us/library/ff726531(v=expression.40).aspx
It is a bad practice when a viewmodel and a behavior are strongly-coupled with each other, this can cause a reusing problem. I think that this is a main different between these two concepts (Triggers&Action/Behavior). Trigger&Action is local solution to convert the Event to command and move the event handling logic toward the viewmodel (you can see here that wthe trigger&action approach is strongly coupled with the viewmodel).
Summary:
If you want to create piece of complicated logic, that will be a reusable concept used in more then one control, and moreovere this component won't have any access to the view model - use the behavior approach.
If you want just to convert some event to command and to move the event handling logis to viewmodel - use the trigger&action approach.
regards,
There are a lot of objects in WPF which doesn't have any triggers of actions fit to them.
for example: Rectangles.
If you want, for example, to make it possible for clicking on a rectangle and do something, use behavior.
For more information you can look here and also here.
I Mean can I do all operation in XAML with out using code behind.
The idea behind XAML was to separate presentation from business logic. Mixing those two concepts in the same file would be bad. Also writing C#/VB.NET code in a XML file could quickly turn into a nightmare. With the MVVM pattern you don't even need to setup explicitly the event handlers in the XAML. Not to mention the unit testability and maintainability of the application. Simply put: use the right gun for the right task.
Yes you can. Except if you need some UI logic. for exa if you want to do some thing particular when some event get fired from UI.
MVVM doesn't allow code behind and so event handling.
So what's the MVVM way to be notifyed that a cell been changed ?
Its a pretty legit and obvious question since there are pretty legit solutions to the problem for MVVM. I guess someone marked it down because this has probably been asked countless times before.
I know there is something built in to do this in .Net4. I am stuck at .Net3.5 so I use the CommandReference class from the WPF toolkit, to convert an event in the view to a command in a viewmodel.
Do you need to handle the CellEditEnding Event? Would it be a possibility to execute the code in the setter of the property which is bound to the cell?
"MVVM doesn't allow code behind and so event handling."
Whoops! There's the issue.
MVVM discourages code behind, it's true, but only when something can be done reasonably via Xaml. "Thou shalt not code-behind" has never been said by anyone with a clue.
Sometimes handling an event is the simplest, best way to accomplish your goal. When that's the situation, embrace it, solve your problem, and move on. Sometimes it isn't even possible to accomplish your goal in XAML. In those cases, throw the view-logic in the view's code-behind and move on.
It's far more important to maintain the architectural goals of MVVM (separation of concerns between layers) than it is to strictly abide by rules-of-thumb (such as avoiding code-behind.) If that distinction isn't made, you're missing the forest for the trees.
Since there was never an answer marked to this question; what you want is an "event to command" implementation. Basically, it captures an event of your choice and calls an ICommand implementation on your ViewModel.
Already answered here in summary (check answer by Derek Beattie).
I am in the process of learning WPF, and am puzzled by the fact that databinding exceptions do not cause a runtime/unhandled exception.
Can anyone explain the benefits of databinding working in this way? I'm assuming that there are benefits, but so far I don't see any (disclaimer: I am just getting started with databinding).
Links to resources that explain the theoretical (or practical) reasons for making this decision would work as well.
I don't know for sure, but I suspect it's because there's nowhere to handle the exception.
Suppose you have something whose properties you want to bind to, but sometimes that something is null. (For example, {Binding Name.Length}, where Name is a string property that might be null.) In this case you're happy for this to be a no-op, because you know the control will never be shown when the Name is null (due to a trigger say) or because you know this will be a transient condition while the binding source is loading its data.
Now suppose WPF propagated the NullReferenceException when trying to call Length on the null Name string. In procedural code, you'd catch this exception and swallow it because you knew it was benign. But you don't get to put an exception handler around the WPF binding code. It's called from somewhere deep inside WPF. So the exception would bubble all the way up to Application.Run, which is not a very useful place to catch it.
So rather than making you centralise your binding exception handlers all the way up in Application.Run, I think the WPF guys decided to swallow the exceptions themselves. Only a theory though...
I would argue it is because the cost of producing an Exception is prohibitive. The current implementation works even in the case there are some invalid bindings and in current form that can actually have a very significant effect on performance. Take this example, create a DataGrid that does binding and this load 1,000 records into it. Measure performance with all bindings correct and again with one of them wrong. The difference is considerable. Add on top of that standing up exception class instances and it could get out of control bad. Just my opinion.
I don't know why that is the default behavior, but there are ways around.
For example, even though it will not tell you about the binding error, the Output window will. You can kind of catch that using binding validations as well.
EDIT:
I found this article that has a very good idea to create test cases that will catch those annoying silent binding errors (and I loved the photo at the top of the article)
Here is link to a blog post where the blogger writes about a case where it argues that it does make sense that the binding errors are silent
I have been getting more involved with WPF for about a year now. A lot of things are new and sometimes it is hard to get my head wrapped around it.
At the same time I am rereading the GOF Design Patterns book.
A few times I would stop in the middle because I would realize that a certain pattern is the very one used in some WPF functionality. Whenever such a realization hits me, I feel like my understanding of the related WPF principle just took a big leap. It's kind of like an aha-effect.
I also realized that I had a much easier time understanding Prism for example because the documentation does such a great job at explaining the patterns involved.
So here is my "question" (more like an effort):
In order to help us all to understand
WPF better it would be great if anyone
who also "spotted" a design pattern in
WPF could give a short explanation.
One pretty obvious example that I found is the Routed Event:
If an event is detected by a child
control and no handler has been
specified, it passes it along to its
parent and so on until it is finally
handled or no parent is found anymore.
Lets say we have an image on a button
that is inside a StackPanel that is
inside a window. If the user clicks
the image, the event will either be
handled by it (if handling code has
been specified) or "bubble" up until
one of the controls handles it. So
each control will get a chance to
react in this order.
Image
Button
StackPanel
Window
Once a control handles it, the
bubbling will stop.
This is the short explanation, for a
more precise one consult the WPF
literature.
This kind of functionality represents
the "Chain of Responsibility
Design Pattern" which states, that if
their is a request, it gets passed
along a responsibility chain to give
each object in it a chance to handle
it. The sender of the request has no
idea who will handle it which ensures
decoupling. For a more thorough
explanation follow the link.
The purpose here is merely to show how this (seemingly old 10+ years) idea found its way into our current technology and to offer another way of looking at it.
I think this is enough for a start and hope more parallels will be collected here.
Cheers, Thorsten
I don't think it is specific for WPF but the observer design pattern seems to be the foundation on which all event handling in .Net and WPF is based.
The observer design pattern is described as "Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.". In .Net with the += operator you subscribe to such a change in state. Subsequently you unsubscribe with the -= operator.
I'd say CommandBindings are pretty important and fundamental to the way I develop.
WPF Tutorial - Command Bindings and Custom Commands
MSDN Overview