Develop Reference

React on (before) datatemplate change - Or prevent listbox item change at this moment

In my application I'm using a simple view selector using datatemplates, I basically have a content control and datatemplates like
<UserControl.Resources>
<DataTemplate DataType="{x:Type viewmodels:OverviewViewModel}">
<local:OverviewView></local:OverviewView>
</DataTemplate>
<DataTemplate DataType="{x:Type viewmodels:DetailViewModel}">
<local:DetailView></local:DetailView>
</DataTemplate>
</UserControl.Resources>
<Grid>
<ContentControl Content="{Binding CurrentView}" Grid.Column="0"/>
</Grid>
Simple, efficient and easily extendable. (Key point is that these usercontrol views can be recursive). The main view also holds a few button with commands behind them to go to "another view".
The problem is, the DetailView contains a ListBox (ListView to be exact), at which "selected items" is of actual importance for the application state.
Now I notice that if I switch from the DetailView to the OverviewView the SelectionChanged event fires - actually unselecting all elements. Looking through the callstack this is due to the fact that the actual elements inside the ListView are updated, the listview is being cleared when the datatemplate changes.
Now since this actually changes the application state this is bad. (The user didn't actually click to unselect the elements, the selection is one of the main reasons to actually open the detail view).
I've tried catching the Unloaded event from the ListBox: this event is indeed fired. But also AFTER the selectionchanged event is fired, so it doesn't really help here.
So can I hook up to a "this UserControl will be unloaded momentarily" event? The bigger UserControl (and the ViewModel for that) should be considered a black box from the smaller DetailView/DetailViewModel.

Navigating through multiple child views contained in Main Window

I am working on a an WPF MVVM application where I need to have a Main Window with just a logo and it has to show child views inside it. I don't have any controls in Main Window all the controls reside in child view for example Buttons like Next, Back, Cancel and some text blocks etc. Now If users select Next button on the child view I have to draw or load the next child view inside the Main Window. If Back button is clicked I have to go back to the previous child view. So basically I am changing the child views depending on which button is clicked. Also I am maintaining different view models for every child view. Now the problem is I am not able to figure how should I link the child views to there respective view models. This application is similar to some Installation applications where different dialogs are shown depending on the selection and the button clicked by the user.I am new to this wpf and don't want to use MVVM Light , Prism etc. Any detailed help will be greatly appreciated. Thanks in advance.

One of the easiest ways to associate any data type with XAML controls is to use a DataTemplate. Therefore, you can simply add something like this into your Application.Resources and as long as you do not set the x:Key properties on the DataTemplates, then they will be explicitly applied by the Framework whenever it comes across instances of your view models:
<DataTemplate DataType="{x:Type ViewModels:HomeViewModel}">
<Views:HomeView />
</DataTemplate>
...
<DataTemplate DataType="{x:Type ViewModels:MainViewModel}">
<Views:MainView />
</DataTemplate>
Then displaying the view is as simple as this:
<ContentControl Content="{Binding YourViewModelProperty"} />
In code behind, or your view model:
YourViewModelProperty = new MainViewModel();
It's often handy to create a base class for your view models and then the YourViewModelProperty can of that type and you will be able to interchange them using the same property and ContentControl.
UPDATE >>>
The general idea is that you have one MainViewModel class with one BaseViewModel property data bound to one ContentControl in MainWindow.xaml... the navigation controls should also be in MainWindow.xaml and not in the views themselves. In this way, the MainViewModel class is responsible for changing the property to the relevant view model instances when it receives navigation Commands from the MainWindow.xaml.

How to Create a View within a View WPF

I explain my issue as I'm quite new to UI design :
I have a main View which displays a TreeView on its left part. When an element is selected I'd like to show a description of the Item on the right on the same window. The design of this description depends on the nature of the Item.
So I created a View per Item Type corresponding to the different possible design.
Now When I click on the TreeView I have no idea how to show the corresponding view on the right of the same window. (I'm not asking about catching the event, just displaying a view within another view, like if I dynamically plotted a control).
Is it possible ? If not what kind of approach would you suggest ?
Many Thanks.

This seems like a great candidate for a Data Template.
Basically, create a content presenter and bind its content property to the TreeView's SelectedItem property. Now, create data templates for each of your types (using the DataType property) in the ContentTemplate property.
Now, the correct data template will be chosen with the correct data whenever you select something in your tree view.
As far as a separate dispatcher goes, I'm not sure, but I'm also not sure what scenario would require one.
More information can be found in this SO question.
Sample:
<ContentPresenter Content="{Binding Path=SelectedItem, ElementName=TreeView}">
<ContentPresenter.ContentTemplate>
<DataTemplate DataType="{x:Type Type1}">
<!-- Bunch of stuff-->
</DataTemplate>
<DataTemplate DataType="{x:Type Type2}">
<!-- Bunch of stuff-->
</DataTemplate>
</ContentPresenter.ContentTemplate>
</ContentPresenter>

How to translate ViewModel to Canvas in MVVM and WPF

I am new to WPF and MVVM, and would appreciate help with the following problem.
I want to create an application in which a user specifies – through a dialog - how he would like to layout N number of chart objects on a page, and the application shows him this layout on a canvas. When satisfied with the layout he sees in the canvas, the user persists it for later use.
All chart objects can be visualized as rectangles. User can also define a header, which too is a rectangle.
A typical layout could be the header at the top of the page, below which are three charts side-by-side. User would be able to specify this layout as well as dimensions and location of each child in a dialog, and then hit the ‘Apply’ button expecting to see this specification in graphical form on the canvas.
In my view model I would have a tree where the parent is the canvas, having one child of type header, and 3 children of chart type.
The user might not like what he sees, and make changes in the dialog which would then effect changes in the view model.
I kind of understand the View-ViewModel interaction between the dialog and the view model. But don’t know how to implement the Canvas-ViewModel interaction. Meaning that when the user requests in the dialog say a header rectangle of a given size at a given coordinate, I know how to add that header object in the tree in the view model, but I do not know how to then update the canvas from the ViewModel's tree. How would the canvas get drawn to reflect the object tree in the viewmodel, and then get re-drawn each time the viewmodel changes (as a result of user's interaction with the dialog)?

One option is to add the viewmodels to a collection, and then bind those to an ItemsControl. If you provide the appropriate datatemplates in the XAML, the views are automatically bound to the data. The Itemscontrol I have looks like this:
<ItemsControl x:Name="WorksetPresenter"
ItemsSource="{Binding ElementName=RootWindow, Path=TableauItems}"
>
<ItemsControl.Resources>
<DataTemplate DataType="{x:Type viewModels:AnalysisViewModel}">
<wg:AnalysisView DataContext="{Binding DescriptiveAnalysis}"/>
</DataTemplate>
<!-- more datatemplates for more view/viewmodel pairs -->
</ItemsControl.Resources>
<ItemsControl.ItemsPanel>
<ItemsPanelTemplate>
<Canvas />
</ItemsPanelTemplate>
</ItemsControl.ItemsPanel>
</ItemsControl>
TableauItems is an ObservableCollection<>. As soon as an ViewModel is added to the collection, it is rendered on the Canvas according to the View specified in the datatemplate. For positioning you can use e.g. the Canvas.Left and Canvas.Top properties (mind the alignment!), or a rendertransform.

You shouldn't store graphical settings such as size and coordinates of your controls in your viewModel.
If I were you I would approch this a bit differently.
In the View, Use DragAndDrop operations on the canvas to let the user change the location of the charts and header.
You can use GridSplitter to make them user-resizable
Then, when user hits Apply, Save the canvas object using XamlWriter.Save method
When you need it for later use, load it using XamlReader.Load method
In your ViewModel have a command that gets the canvas as a parameter and handles the Save operation.
Example
view:
<Canvas x:Name="mainCanvas">
<StackPanel>
<TextBlock Text="My Header ..."/>
<!-- Charts goes here .... -->
<Button Content="Apply">
<i:Interaction.Triggers>
<i:EventTrigger EventName="Click">
<i:InvokeCommandAction Command="{Binding ApplyCommand}" CommandParameter="{Binding ElementName=mainCanvas}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
</Button>
</StackPanel>
</Canvas>
view model:
public class MainWindowViewModel
{
public MainWindowViewModel()
{
ApplyCommand = new DelegateCommand<Canvas>(canvas =>
{
string userLayout = XamlWriter.Save(canvas);
// save userLayout for later use ...
});
}
public DelegateCommand<Canvas> ApplyCommand { get; set; }
}
Hope this helps

If the application specifically deals with changes in layout and layout information is the data you are presenting, then putting layout information in your view model is certainly appropriate. However, simple presentation information does not belong in your view model.
For that you need a different solution. Consider this.
If I need to locate view model template location on screen, how do I do it? My view model cannot know about the visual tree! Bugger. To solve this, I tag elements with attached properties and use a custom layout behavior or control to query the attached properties.
This is very similar to how jQuery allows a javascript programmer to grab DOM elements from a web page.

Changing Main Viewing Area

I'm wondering how to go about creating different views in the main window when a button is pressed. I'm not sure of the correct terminology, so that has hampered my google fu.
I'm thinking that the main viewing area would be a content control, that I could change when a event happens. I made a small drawing to help illustrate my idea / thought.
Any input will be appreciated. Thanks!

It would be really easy to implement this senario using MVVM approach....
Make a ViewModel for you MainView. Then Define Properties of the ViewModels of your UserControls
For Example You have Two UserControl as FirstView and SecondView then make a properties in your viewmodels as ViewToLoadProperty of the type ViewModel (usually called as ViewModelBase)
Set bindings as
<!-- Panel For Hosting UserControls -->
<Border Grid.Column="2">
<ContentControl Name="userControlContentControl"
Content="{Binding Path=ViewToLoadProperty,
}">
<ContentControl.Resources>
<DataTemplate DataType="{x:Type ViewModelLayer:FirstViewModel}">
<ViewLayer:FirstView/>
</DataTemplate>
<DataTemplate DataType="{x:Type ViewModelLayer:SecondViewModel}">
<ViewLayer:SecondView />
</DataTemplate>
</ContentControl.Resources>
</ContentControl>
</Border>
<!-- Panel For Hosting UserControls -->
Then when you click the button Use a command to set the respective ViewModel Intance to this(ViewToLoadProperty) property...(Use RelayCommannds or something like it)
DataTempates would do the rest of the job by selecting the right View according to the right type of ViewModel
YOu can use MVVMLight toolkit if you are implementing MVVM Pattern.. :)

On the right you could have a frame. Then the button would bind a different page or user control to the content of that frame.