What is the best way to trigger an animation while using the MVVM pattern? More specifically, I have a form which has several fields. When a user click's the save Button, I would like to show an animation. I have been able to achieve this by exposing a property ShowMessage and setting it to True and a DataTrigger picks up this value and starts an animation. However, while the animation is still active, I would like to reset the form to a clean state. So far I've done the reset by subscribing to the Complete event on the Storyboard and resetting the view model's state on that event.
Ideally I would like to be able to somehow trigger an animation from the view model (because the save would be an asynchronous operation) and let the animation run to completion. The way I have it now wouldn't work because once I change the value of the ShowMessage property (on the reset), the animation stops and doesn't run to completion.
Does any one have a better solution?
Thanks!
Two options comes into my mind. The first is VSM, and the second is attached behaviors. You may find useful the following post from Marlon Grech: Animations and MVVM.
Hope this helps.
Related
I have several textboxes on my view. One of them needs to get focus. Which one exactly is defined in the data model. Currently I call the Focus method right after binding is done. I get false as a result which means the focus cannot be set. Question is
what is right time for doing this?
how do I know this moment being in the view model without knowing about the view anything?
The right answer is to schedule setting the focus via Dispatcher.BeginInvoke. It means the view model has to have a reference to the Dispatcher object which is somewhat tricky to do but worth the effort.
I have an audio recording app in Windows Phone 7.
The app allows a user to play the recorded sounds.
I try to stick to MVVM guidelines where it is possible.
I have a play/stop button in a list of all recordings. Each recording has its own ViewModel, which, besides all, also controls the look of the corresponding play/stop button.
The button has a custom visual state defined in its' style.
The Visual State is bound to the ViewModel's property using the approach, shown here:
http://tdanemar.wordpress.com/2009/11/15/using-the-visualstatemanager-with-the-model-view-viewmodel-pattern-in-wpf-or-silverlight/
Having implemented this approach, whenever I want to change the look of the play/stop button, I need to set the public string property (named "PlayStopVisualState") in my ViewModel to either "PlayingState" or "Normal", and that will assign an appropriate visual state to my button.
The problem is that when user presses the play button, a SoundEffectInstance is created in a background thread, which plays the sound. The thread then waits for the playing to end. When the recording playing is over (I have to track it in the same background thread, or create another for just tracking SoundEffectInstance.State) I set the PlayStopVisualState property back to "Normal", but I get a cross-thread reference exception. Isn't MVVM specifically designed to allow developers to manipulate logical variables in a view model, and not having to worry about how the changes to them are reflected in a View?
I know that I need to do the adjustment of the PlayStopVisualState property in a Dispatcher thread in order for the problem to disappear, but this is just no right. It, from my point of view, defeats the whole purpose of MVVM, leaving only the organizational advantage.
Or am I doing something wrong? Thanks.
UPDATE:
I have worked around the problem by using
Deployment.Current.Dispatcher
but it seems to me as a very "ugly" solution, given that I almost all over have MVVM pattern followed.
Using the Dispatcher to reflect a UI-bound value is the correct way to do it, yes.
What you're forgetting is that your ViewModel is created on the UI thread. So any change to the ViewModel from a background thread, would a cross-thread operation.
You should consider if a background thread is really needed. , or if you could just schedule your action on the UI thread directly.
I need an elegant solution (I am working on silverlight 4.0) to solve this simple problem(?) using the MVVM pattern:
My mainpage xaml has my two custom user controls like this (say):
<uc:MyCustomUC1>
<uc:MyCustomUC2>
Each one has its own view model and both these user controls are independent of each other.
When an asynchronous operation in MyCustomUC1 has completed, I want an ICommand in MyCustomUC2's viewmodel to be invoked thus refreshing data in MyCustomUC2. I want this done by the parent page and all in xaml.
Exposing dependency properties, event handlers etc in the user controls...anything is ok since I own the user control ...whatever makes sense.
Any ideas ?
Use Mvvm Lights messenger, you can register a listener in MyCustomUC2's viewmodel to refresh. Then in MyCustomUC1's async call back, send the message to refresh.
You could use a PropertyObserver, which I believe you can find info on here:
Property Observer.
It'll allow you to check when something has changed in one ViewModel and then take the appropriate action in another. I've used this quite a bit recently in a project and it has worked pretty well.
Apologies if I've picked up the question incorrectly.
Like the title says I am trying to architecture into my application a way to distinguish the source of a variable change, either from UI or code-behind.
My problem is that I need to trigger some action after a property changed its value, but I only need to do this when the change comes from the UI because otherwise I don-t want to perform that action. I am having some trouble because, for example when a checkbox(two way binding), changes state, my binded property gets updated and then I use the checked and uncheked events to trigger that action.The problem is that when I change the property in codebehind it also triggers those events and I do not want that. Right now, i am using a flag that enables, or not, the actions at the event handlers but I do not feel that this is a good idea.
Any sugestions or ideas?
I am considering using only one-way binding and control everything my self, using commands.
It looks like you have some confusion between your model and your controller. There shouldn't be any cases where it matters if the a change to the model comes from the user or not. If you want to have something like a confirm message it the user makes the change, then don't bind the view control directly to the model, but have the controller respond to the event.
That way, if the control is changed to be the same as the model, then the change is internal, and no confirm is required, but if the control is changed by the user, then the control state differs from the model, and a confirm can be shown.
In our Silverlight 2 project we have created an attached property to perform on-the-fly translation to text properties of various user controls. To achieve this, we hook the Loaded event of the FrameworkElement when the property is set. When the event fires, we take the existing text property value and perform some simple string substitutions on it, before replacing the property value with the translated text. However, this results in the control being rendered with the untranslated text, then the text is quickly replaced with the translated version.
Is there an alternate event we can hook that would fire before the control is rendered?
I've changed my code so that it now performs the translation as soon as the setter for the attached property is called. There's no need to wait for the FrameworkElement to have finished loading, as I can change the Text property long before the element is rendered.
My initial thoughts on using the Loaded event were to reduce the startup time of the application by only translating the controls that were visible on the screen. As it turns out, I'm duplicating some of the work performed by the runtime, as the runtime won't call the property setter until it needs to anyway.
I'm not totally sure about this, but can you use the LayoutUpdated event. It will fire when the control is resized and such (you could take measures to ensure your code only executes once.)
I know it doesn't seem like the "right" event for this but unfortunately Silverlight kinda leaves you standing there holding it when it comes to events.