I have a rather interesting case with ComboBox control - CustomComboBox;
In the style of this ComboBox, Popup contains one custom control that requests a DataContext;
<ctrl:CustomGrid DataContext="{TemplateBinding DataContext}" GridName="{Binding Preferences.CurrentGridName}"/>
The idea:
to use this control several times on one page
to use it in a masterpage container
the masterpage control needs to have different DataContexts regarding the Page it is on
The logic:
In the overriden OnApplyTemplate I am getting the grid and connecting few eventhandlers
The problem:
The masterpage control is triggering OnApplyTemplate only once
The first appearance of the CustomComboBox is as expected.
However, every next apearance is with same DataContext, even when changing the datacontext of the CustomComboBox
These changes don't reach to change my CustomGrid DataContext
I am sure that something on the bindings or the presentation logic is bad...
Please throw some thoughts on, I would appreciate a hint here
Thanks
OnApplyTemplate is called when a ControlTemplate is applied to the control that overrides the method (neither its parent, nor children). If OnApplyTemplate is entered once, the overriding control must also be created once. I mean you simply have a single masterpage instance. This shouldn't be unexpected.
Speaking about Popups and DataContext, there often are issues with bindings from a Popup to outside it. So, I would rather write some code-behind to deliver correct context to Popups, instead of relying on Bindings. There sure is a problem of DataContextChanged event absence prior to SL5. To workaround this one, you should define your custom DependencyProperty on your CustomComboBox, bind it to the CustomComboBox's context and assign its value to the Popup in the PropertyChangedCallback.
Related
I have a ComboBox which uses ListBox to show items. ListBox’s ItemSource is bound with a CollectionViewSource.
Issue: Once I open ComboBox and scroll through the items and leave it in middle or at bottom. Once I reopen ComboBox, even though I refresh or reload ItemSource (CollectionViewSource), the Scrollbar remains at the same place where I left it last time. I want it to be as default (at top) each and every time I reload ItemSource.
Is there is any way of doing this in XAML itself? Another thing, I cannot use Behavior or Attach property. I want any template or style for this.
In order for a Style on a ListBox to embody some behavior for the <ScrollViewer>, you would need to use an Attached Property / Attached Behavior to control the ScrollViewer's "grabber" position. This is because your collection is bound to your ListBox and notifying when it is updated needs to drive a behavior that isn't natively on the ListBox. It may be possible to reset the scroll position with a <ControlTemplate> for the <ScrollViewer> itself, but I imagine it would be difficult as it would likely involve manipulating Transforms / StoryBoards based on DataTriggering with your ItemsSource, but again that may cause a dependency on needing to use an Attached property, which for some reason you can't use...
If you simply want to get a result now, and you don't care about testability or re-usability, I would handle the TargetUpdated event in the code-behind. It's ultimately what the Attached Behavior would end up doing. On the other hand, if you do care about re-usability then you need to evaluate and challenge why you can't use an Attached Behavior (they are also testable, too); an Attached Behavior would be also easier than trying to edit a ControlTemplate.
Here is the code-behind approach:
.xaml:
<ListBox x:Name="myListBox"
ItemsSource="{Binding MyItemsSource, NotifyOnTargetUpdated=True}"
TargetUpdated="ListBox_TargetUpdated"/>
.xaml.cs:
private void ListBox_TargetUpdated(object sender, DataTransferEventArgs e)
{
if (myListBox.Items.Count > 0)
myListBox.ScrollIntoView(myListBox.Items[0]);
}
Edit: On the flip-side, if you are using MVVM, you can do something like this SO post suggests and set IsSynchronizationWithCurrentItem="True" and when you refresh your ItemsSource, simply set your SelectedItem to the first in the list and handle the SelectionChanged event in your vm.
I'm looking for a solution and/or the rationality behind why a Binding instance is shared within a DataTemplate. This ultimately comes down to the fact that within a DataTemplate, there's seemingly no way to force a new instance of a Binding on a DependencyProperty for each control generated. That is probably a fair and good assumption in all cases except when there are ValidationRules representing something specific about the instance of that control.
To elaborate (I can provide code, but I don't believe it's necessary), I use a DependencyPropertyDescriptor on IsEnabled to update one or more ValidationRules that belong to a TextBox.Text Binding, DatePicker.Text Binding, or a ComboBox.SelectedValue Binding, etc. The idea is that validation will be different or undesired when a control is not enabled.
Because of this, the ValidationRule's state of IsEnabled is specific to the individual control and because the ValidationRule Collection is a part of the Binding and the Binding instance is being shared - each control that ends up sharing that binding will update/override the previous IsEnabled value that was applied by the previously generated control's IsEnabled value.
IsEnabled is only one of at least two properties in the ValidationRule (another custom IsRequired DependencyProperty) that represent the state of the control to which the Binding is applied. When working outside of a DataTemplate (IE: The Binding instance is not shared), this works very well and ignores/alters the validation logic based on the state of the control. I'm not closed off to alternatives but do feel this has been (this issue aside) a very flexible and dynamic option that allows the Binding instance ValidationRule's and the control-changed state of the Rules to evolve effortlessly. This has also allowed me to avoid other obvious but much uglier options such as creating several Bindings, each representing one of the combinations of the ValidationRule's control properties and switching the whole binding out at the time the DependencyPropertyDescriptor fires. shivers
Any thoughts are MUCH appreciated!
I suggest you to use x:Shared attribute in the DataTemplate definition.
<DataTemplate x:Key="DataTemplateKey" DataType="{x:Type YourType}" x:Shared="False">
...
</DataTemplate>
Since you use it WPF will create a new DataTemplate instance on every request.
I have a Silverlight UserControl which uses the ContentPropertyAttribute to exposes the Children property of one of it's child panels. This allows me to add child controls to the panel on my page:
<local:MyUserControl>
<TextBox Name="tbTest" />
</local:MyUserControl>
This works, apart from the 'tbTest' field of the page being present, but not initialised. On closer inspection, the InitializeComponent method does try to locate the TextBox (with FindName), but fails to do so (returning null).
After some investigation, I've found that namescopes are the problem - the UserControl has it's own namescope, thus it's children can't be located with the page's FindName but can with the UserControl's FindName method.
How can I alter my UserControl so that the child controls are locatable by the InitializeComponent method? The standard Panels (StackPanel, Grid, etc.) don't seem to have any problem doing so, so there must be a solution?
Thanks
It may be difficult to do at this point but the best course of action would probably be to derive your control from ItemsControl instead of UserControl. Then you wouldn't have the problem with name scopes.
I suppose as a workaround you could do a dive into the control with VisualTreeHelper to manually set the tbTest field.
I have a dependency property on my ViewModel which is the DataContext for my View. The ViewModel has no reference to the View. The property on the ViewModel is going to reference a control on the view, but I need to be able to set this property in XAML.
How is this possible? One thought I had was to develop a custom control which has a Property property and a Value property, so you could do something like this in the View to set the property on the ViewModel:
<PropertySetter Property="{Binding MyViewModelDependencyProperty}" Value="{Binding ElementName=aControlOnMyView" />
Before I went down this route, I wanted to check if there was any other approach I could take?
Thanks for the detailed reply Ray, but if I give you a bit more detail about the problem I'm trying to solve, you might get a better idea of why I mentioned the approach I did.
Basically, what I'm trying to do is set the focus to a textbox when the user hits a button. I've written an attached property which you can attach to the Button control, specify what the trigger event is (in this case the 'Click' event), and then what control to focus on. This works really nicely, and keeps everything in XAML.
However, I now have a use case where the focus should be set to an arbitrary text box from the click event on a button which is part of a toolbar. This toolbar is itself a user control which is sitting inside another user control, which is inside another user control! This toolbar needs to be reusable across various different forms, and each time, the control to set focus on after you click the button will be different per form.
That's why I had the idea of making the focus control (i.e. a textbox) a property on the view model itself (on my ViewModel base to be precise), and have the ViewModel base code (which the toolbar is bound to), set the focus to the control when the button is clicked (and the e.g. Add/Edit method is called on the ViewModel base).
In unit test land, the control to focus on property will be null, so it's .Focus() method just won't be called. So I can't see an issue there. My problem is then how you set the focus control property from XAML, which is why I had the PropertySetter idea.
I don't like the fact that the ViewModel has any reference to controls sitting on the view, but I can't see another way to achieve what I need. What if the logic that dictates whether to set focus to the control is quite complex? This would sit in the ViewModel surely? Therefore, is there any harm in the ViewModel having this UIElement property? It still knows nothing about the specific View it is bound to, it just knows that there is a control which it needs to set focus to when some action happens on the ViewModel.
My first reaction (and it's a strong one) is so say "Don't do that!" By giving your view model a reference to a part of your UI you are breaking the encapsulation that makes view models so powerful and useful.
For example, what if you want to unit test your view model or serialize it to disk? In each case the piece of your UI will not be present, because there will be no view at all. Your tests will miss coverage and your reconstitution will be incomplete.
If your view model actually needs references to UI objects and there is no better way to architect it, the best solution is to have the view model itself construct those controls it requires a reference to. Then your view can incorporate that control as the Content of a ContentPresenter via binding and provide a Style to configure the control, including a ControlTemplate to provide its content. Thusly:
public class MyViewModel
{
public ListBox SpecialControl { get; set; }
public MyViewModel()
{
SpecialControl = new ListBox();
}
}
and
<DataTemplate TargetType="{x:Type local:MyViewModel}">
<DataTemplate.Resources>
<Style TargetType="ListBox" ... />
</DataTemplate.Resources>
...
<ContentPresenter Content="{Binding SpecialControl}" />
</DataTemplate>
Other possibilities are:
Have the view model actually derive from the Control class, then override OnApplyTemplate() and use GetTemplateChild to find a template item whose name starts with "PART_"
Implement an attached property that takes a property name, finds that property in the DataContext, and sets it to the DependencyObject to which the property is attached.
Implement your PropertySetter idea
My option #2 would look like this:
<DataTemplate TargetType="{x:Type MyViewModel}">
...
<TextBox local:PropertyHelper.SetViewModelToThis="SpecialControl" />
...
</DataTemplate>
The code in the SetViewModelToThis PropertyChangedCallback would get the view model from the DataContext, reflect on it to find the "SpecialControl" property, then set it to the TextBox. Note that the implementation of SetViewModelToThis must take into account the possiblity that DataContext is not set right away, and that it maybe changed requiring the old setting to be removed and a new one made.
First of all, the DataContext of the control should be the ViewModel object and not a property of it. Second, when you TwoWay bind a property of ViewModel to your control, changes in the control's value will update (in your case, 'set') the value of ViewModel's property.
I have a few usercontrols loaded into a tabcontrol via MVVM in WPF.
Within the XAML for the usercontrol I am setting focus to a textbox using the FocusManager, however this appears to only work when the first instance of the usercontrol is created.
Just to test I added a loaded event handler to the usercontrol - this is only called on the first instance.
I'm using data templates for the user controls as follows:
<DataTemplate DataType="{x:Type local:UserTypeViewModel}">
<local:UserTypeView />
</DataTemplate>
The textbox is focused as follows:
FocusManager.FocusedElement="{Binding ElementName=txtName}"
Additionally I'm using a global event handler (for the textbox GotFocus event) which selects all the text using a dispatcher.
If anyone has any tips on how to achieve focus with every usercontrol I'd be very grateful.
Thanks, Ben.
Remember that a visual element can only receive focus, if:
It is visible (in a TabControl only one tabitem can be visible at a time
IsFocusable must be set to true (is default false for UserControls)
It has finished loading (as you write - do it in the Loaded event))
I think the first reason is why it only works for the first element.
As for how to achieve it for all controls - you can use a style with an EventSetter for the Loaded event. You need to make a style per type of control though to avoid having to set it for each control.