silverlight dependency property call back method keeps in loop - silverlight

i have created a user control with three dependency properties
selected value
selected item
selected index
when one of them changes in the call back i am setting the other two e.g if selected value changed i am setting Selected index and selected item which in result triggering the callback of each of other properties where it sets the other two properties and keeps on in loop .how to avoid this situation

That's easy,
in each Property Setter only set the other two when the value you're setting is different from the the existing value (through the call of GetValue()). Add a If statement to check this in each of the Property setters.
You should be more careful with these type of situations, either check if you really need those 3 properties as Dependency Properties or change some of them to normal properties in the ViewModel if you're using an MVVM framework.
If you're using MVVM, many frameworks provide a method to suppress notifications like ReactiveUI (ReactiveObject.SuppressChangeNotifications()) or MVVM Light Tookit.

Related

WPF is it possible to temporarily change the Visibility Dependency Property of a DataGridColumn without destroying the original binding?

I have a WPFToolkit DataGrid with at least one column bound (via a proxy object as columns are not part of the visual tree) to a property. I wish to toggle all columns to Visible so that I can perform a calculation based on the DataGridColumnHeader (which is only created when its column is visible for the first time). Having done the calculation I want to reset the column to use the binding that was previously set.
I've attempted to get and store the Binding Expression etc, but with no joy. I have also attempted to use the DependencyObject.SetValue() method to change the property value non-destructively, but this doesn't event correctly change the value, let alone retain the original binding.
Any ideas?
You need to call SetCurrentValue() so that it won't clear the binding. SetValue destroys the old binding.
From MSDN:
This method is used by a component that programmatically sets the value of one of its own properties without disabling an application's declared use of the property. The SetCurrentValue method changes the effective value of the property, but existing triggers, data bindings, and styles will continue to work.
Given you have this
<TextBox Text="{Binding TestProperty}"/>
The SetValue you will overwrite the binding with whatever you provide. If you call SetCurrentValue, however, will ensure that the property takes on the given value, but won't destroy any bindings.
Be aware that you should not use SetCurrentValue in your dependency properties' setter/getter.
SetCurrentValue is more useful in scenarios where you need a property to take on a given value but don't want to overwrite any bindings, triggers, or styles that have been configured against your property.

WPF: How to trigger GUI behaviours in response to view-model events?

I'm developing a WPF/MVVM application and I have a listbox binding to data in a ViewModel. At various points I need the view model to cause the listbox to scroll to a given element.
How can I do this without creating a custom control and while still maintaining good separation of concerns?
I've currently got it working by creating a custom behavior class in the view layer with a dependency property VisibleIndex which the XAML code then binds to an integer in the view model:
<ListBox x:Name="myListBox"
local:ListBoxVisibilityBehavior.VisibleIndex="{Binding VisibleIndex}">
When the integer is set it triggers the dependency properties update handler which tells the listbox to scroll to the associated index.
This seems a bit hacky though because the dependency property value is never changed by the listbox and the update handler only gets called when the value changes, so the only way to ensure that the relevent item is visible is to do something like this:
// view-model code
this.VisibleIndex = -1;
this.VisibleIndex = 10;
The only reason I'm using a behaviour class at the moment is for binding my custom dependency property, is there a way to do something like this with events instead?
Attached properties are somewhat required in your case - as at some point, 'somewhere' you need to call the following method...
ListBox.ScrollIntoView(item)
or
ListBoxItem.BringIntoView();
And for that you need some sort of code behind - and attached properties/behaviors are a nice way of packaging that, w/o impacting your MVVM.
Having said that - if you just need to have your 'selected item' scrolled into view at all times (which is the case most of the time). Then you could use a different attached-property based solution (that again):
mvvm how to make a list view auto scroll to a new Item in a list view
All you have to do then is to set or bind to SelectedItem.
That's a bit 'nicer' if you wish - but the mechanism is the same.
For anyone else interested in the answer to this one of the MS engineers on the WPF forum cleared it up for me. Instead of binding to an event directly you bind to a wrapper object that encapsulates that event. The behaviour can then grab the reference to the wrapper from its DP and do whatever it wants with it i.e. subscribe to the event, trigger it etc.

Silverlight 4: How to trigger an Animation when TextBlock’s Text is changed during a DataBinding?

I have a user control which datacontext is set to a view model. The user control contains some textblocks and textboxes which are bind to view model's properties. I would like to do a small animation when a view model's property's value changes but I can't seem to find a way to do this in Silverlight.
In WPF we can use properties like NotifyOnTargetUpdated and Binding.TargetUpdated to capture the change but they seem to be missing from Silverlight. In my case the animation isn't based on the property's value, meaning I don't want to start the animation when some property's value is for example 5. Instead the animation should start every time the property's value is changed (to highlight the changed content for the user).
Here's the answer on how to do this in WPF: Animate WPF Text when binding updates, how?
But how can I do the same with Silverlight 4?
you should be able to get this done using the new trigger stuff included in Expression Blend 4. There's a whole bunch of ne behaviors/triggers etc. that let you react to changes in the ViewModel, for instance.
From the Expression Blend feature page:
New Behaviors
Expression Blend includes the new TranslateZoomRotateBehavior multi-touch Behavior, and a PanningItems control that you can use to scroll between items by using touch. Expression Blend also has a new trigger that responds to a frame change or the pausing or completion of a SketchFlow animation, called the SketchFlowAnimationTrigger. Expression Blend has new sets of Behaviors for dragging items between list boxes, for modifying menu navigation, and for preserving screen states, such as SetDataStoreValueAction and DataStoreChangedTrigger.
An exciting enhancement has been made to the FluidMoveBehavior: if you apply it to different views of the same data item, when the item moves from one view to another (for example, from a list view to a details view), it fluidly animates the transition between the two views.
New Behaviors for use with applications that use the Model-View-ViewModel pattern include the following: CallMethodAction, InvokeCommandAction, and DataStateBehavior. You can use these Behaviors to invoke behavior on your ViewModels, or to respond to changes to their properties.
Conditional Behaviors and the data store
You can now build conditional logic into your prototypes and production applications without the need to write code. In fact any action can be associated with a set of conditions that must be met in order to execute the action. The new data store feature enables application variables, such as the current user's role, for example, can be read from and written to so that, effectively, different parts of your application can communicate via this shared state.
New behavior components introduced as part of this feature include the conditions editor that appears in the Properties panel for every action, a SetDataStoreValueAction action that allows you to manipulate values in your data store, and a DataStoreChangedTrigger trigger that fires whenever a chosen property inside the data store is changed.
http://www.microsoft.com/expression/products/Blend_Features.aspx
Cheers, Alex

WPF Binding to IDictionary<int,MyType>.Values Doesn't Respond to Property Changes?

I am binding a ListView a property that essentially wraps the Values collection (ICollection) on a generic dictionary.
When the values in the dictionary change, I call OnNotifyPropertyChanged(property). I don't see the updates on the screen, with no binding errors.
When I change the property getter to return the Linq extension dictionary.Values.ToList(), without changing the signature of the property (ICollection) it works with no problem.
Any reason why the Values collection bind and notify properly without projecting to an IList<>?
Calling OnNotifyPropertyChanged() isn't exactly correct in this case, since the collection is still the same, however the items in the collection have changed. I don't know exactly how the WPF binding code works, but it might do a quick check to see if the reference it is binding to has changed, and if not it won't update the UI.
The reason that ToList() works is because each time it is called, a new instance of List<T> is returned, so when OnNotifyPropertyChanged() is fired, WPF picks up on the change and updates all of its bindings.
I don't know if this is possible or not, but the ideal solution would be to use a collection for bindings that implements INotifyCollectionChanged. This will fire events that WPF monitors so that items can be added, removed, etc. from the UI as appropriate.

Validating collection elements in WPF

I would like to know how people are going about validating collections in WPF. Lets say for example that I have an observable collection of ViewModels that I am binding to the items source of a grid, and the user can add new rows to the grid and needs to fill them.
First of all I need to validate on each row to ensure that required fields of each ViewModel are filled in. This is fine and simple to do for each row.
However, the second level of validation is on the collection as a whole. For example i want to ensure that no two rows of the collection have the same identifier, or that no two rows have the same name. Im basically checking for duplicate properties within different rows. I also have more complex conditions where I must ensure that there is at least one item within the collection that has some property set.
How do I get a validation rule that would allow me to check these rules, validating on the whole collection rather than the individual items. I also want to print any validation error above the datagrid so that the user can fix the problem and the message will update or disappear as the user fixes each different rule.
Anyone have any experience of the proper way to do this?
The trick is to place your collection validation logic such that it's called when the ItemsControl's ItemsSource property changes. If you're using IDataErrorInfo on your view-model, then set ValidatesOnDataErrors=True on the ItemsSource binding and, when the bound collection property's name is passed into the interface's error indexer, run the logic to determine if the property is still valid or not. If you're using custom validation rules, then putting the rules into the ItemsSource binding should be fine, to.
Next, in your view-model, raise the PropertyChanged event for the ItemsSource-bound property whenever an event occurs which changes the collection's valid/invalid state. For example, if the collection needs a certain number of elements, then listen to the CollectionChanged event. Whenever the collection changes, raise the PropertyChanged event for the ItemsSource-bound property. This tells WPF that the property changed, which leads to its revalidation. Thus, your collection validation logic will run whenever the collection changes and, if the collection is invalid, WPF displays the error adorner or, if the collection becomes valid, WPF removes the adorner.

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