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Transition Selection

We had many requests on tips how to change the transitions based on some application logic. Such logic is for example:

  • Navigating back and forth between UserControls that follow some sequence – slide left or right

  • Implement History – slide left or right

  • Drill down – zoom data in or out

The Transition property of the RadTransitionControl is of type TransitionProvider so it works pretty much as a DataTemplate that can create object when asked. Setting properties on it will not change the instances that have been generated. It is not a DependencyObject so its properties are not bindable. In other words you can’t bind the Direction property of the SlideAndZoomTransition.

One of the possible solutions to set the Transition property of the RadTransitionControl to new TransitionProvider such as SlideAndZoomTransition with Direction set to Left or Right based on some custom logic.

  • This could happen in code behind.

  • In MVVM application you could bind the Transition to property of your ViewModel and use a IValueConverter implementation that will convert to the right TransitionProvider.

  • You could create a custom TransitionProvider implementation that would use the parameters passed to an override of CreateTransition to generate a Transition.

All approaches have pros and cons. For small applications the first one is recommended as you could easily drop a few lines of code on the back of your UserControl and alter the RadTransitionControl.

In large projects it is a common practice to separate code from presentation so you may have the RadTransitionControl placed within a ControlTemplate in a ResourceDictionary where it would be hard to add codebehind. In such cases the second and the third approach are preferred. Which one to choose? It depends on that whether you are willing to keep navigation state in your ViewModel and if the data items or controls passed in the Content property of the RadTransitionControl are enough to select the proper transition.

Switch Transition in Code Behind

For example if you have a ListBox and a RadTransitionControl with its Content bound to the ListBoxSelectedItem you could easily handle the SelectionChanged event of the ListBox and switch the Transition property of the RadTransitionControl.

Switch through a Binding and a Converter

You may not wish to add navigation and state data to your ViewModel and desire to keep it focused on application logic and data. Yet in complex application such data gets in your application through implementation of history tracking, deep link functionality etc. In such case you could use that data In RadTransitionControl.

For example you may have a list of items and keep the currently selected item as well as history of the previously selected items in your ViewModel. In such case you could easily add property that indicates whether the last navigation was Backward or Forward. If you have for example a bool property IsNavigatingForward you could bind the Transition of the RadTransitionControl to that boolean property and convert it to a Transition via a IValueConverter.

Such converter would look like:

public class BooleanToTransitionConverter : IValueConverter 
    public TransitionProvider TransitionForward { get; set; } 
    public TransitionProvider TransitionBackward { get; set; } 
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) 
        if (!(value is bool) || (bool)value) 
            return TransitionForward; 
            return TransitionBackward; 
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) 
        throw new NotImplementedException(); 

And could be used in XAML:

<telerik:RadTransitionControl Content="{Binding SelectedImageSource}"> 
        <Binding Path="IsNavigatingForward"> 
                        <telerik:SlideAndZoomTransition SlideDirection="LeftToRight" /> 
                        <telerik:SlideAndZoomTransition SlideDirection="RightToLeft" /> 

Switch in a Custom TransitionProvider

This could be a very MVVM friendly approach if the parameters provided in the TransitionContext instance in the CreateTransition method of the TransitionProvider are enough. If you are binding the Content property of RadTransitionControl to objects from your ViewModel and you could compare these objects, then you could implement a very decent solution to transition switching.

For example if you drill down you could check if the new Content contains larger or smaller data range and respectively pick a zoom in or zoom out animation.

If you navigate between indexed items you could check the indices of the items and slide left or right.

If you need some additional data for the comparison this approach won’t work and you will have to stick to the CodeBehind transition selection.

In general implementing a custom TransitionProvider would use quite a few lines of code:

public class WizzardPageTransition : TransitionProvider 
    public TransitionProvider BackTransition { get; set; } 
    public TransitionProvider ForwardTransition { get; set; } 
    public override Transition CreateTransition(TransitionContext context) 
        WizzardPage oldPage = context.OldContent as WizzardPage; 
        WizzardPage newPage = context.CurrentContent as WizzardPage; 
        if (oldPage == null || newPage == null) 
            return null; 
        else if (newPage.PageIndex > oldPage.PageIndex) 
            return this.ForwardTransition.CreateTransition(context); 
            return this.BackTransition.CreateTransition(context); 

And in XAML you could use it like:

<telerik:RadTransitionControl Content="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=SelectedContent}" > 
                <telerik:SlideAndZoomTransition SlideDirection="LeftToRight"/> 
                <telerik:SlideAndZoomTransition SlideDirection="RightToLeft"/> 

If the Content is non-UI element you could define simple properties that can be compared in the if-statements.

If the Content is UI you could create a class that extends UserControl or Control and add properties there. Also an AttachedProperty could be used so you could attach data for comparison on Controls without having to extend them.

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