I'm using a 3rd party control (in this case, Telerik's RadChart control), which has a complex inner ControlTemplate that encompasses several elements of the chart. Unfortunately, not all parts are exposed explicitly for me to style, and I find myself wanting to set a property on a particular sub-element inside the ControlTemplate, either via XAML or via code. After struggling with the XAML for a while, I settled for a code-centric approach, but I'm still stuck.
Using Snoop, I can see that the top-most ChartArea control (which I can access) renders an AxisX2D object named PART_AxisX, which in turn has a template which contains the PART_AxisLabels object, which is the one I need to style. (Incidentally, the property I want to set is AlternationCount, but I couldn't find a less hacky way of getting to it).
I tried to use the ControlTemplate.FindName method (as shown here) to get the AxisX2D object, and then again on that object to get the AxisLabels object, but FindName always returns null:
var chartArea = myChart.DefaultView.ChartArea;
var visualAxisX = chartArea.Template.FindName("PART_AxisX", chartArea) as AxisX2D;
even though Snoop confirms that chartArea is, in fact, the AxisX2D's TemplatedParent. I checked in the debugger and I could find PART_AxisX in chartArea's internal Template property.
So my questions are:
1) What am I doing wrong here?
2) Is there a better way to approach this that isn't as roundabout as this?
The solution is to wait till after the Loaded event has occurred on the Control.
When this event has occurred you know that the Template has been applied and the visual tree has been built for the control, and so you can then access the elements using FindName.
http://blogs.msdn.com/b/mikehillberg/archive/2006/09/19/loadedvsinitialized.aspx
Slightly related link given here for case when using a Content Template on a ContentControl.
Why would 'this.ContentTemplate.FindName' throw an InvalidOperationException on its own template?
Related
WPF has a method called NameScope.GetNameScope(control). How can you get the same thing in UWP/Windows 8.1? The runtime must be doing it internally somehow or else ElementName bindings wouldn't work
XAML namescopes do exist in UWP, but currently there's no public API similar to WPF's GetNameScope(control).
Depending on what you're trying to achieve, you will most likely have to fall back on using FindName or GetTemplateChild methods or ultimatly the VisualTreeHelper.
Because of the separate XAML namescopes, finding named elements within
a template from the scope of the page where the template is applied
requires a different technique. Rather than calling FindName on some
object in the object tree, you first obtain the object that has the
template applied, and then call GetTemplateChild.
At what time of object lifecycle are bindings resolved for the first time?
It is a simple question but I cannot find any information neither in books nor through Google.
It's not that simple actually, you won't get a straight answer for this question. It depends on the context.
Here are two simple examples :
If the bounded property is owned by a WPF control that is not inside a ControlTemplate, the binding will most likely be resolved for the first time when the UpdateLayout method is called for the first time, if the DataContext is already set.
If the DataContext was not set, it will try to be resolved after the control is Loaded: see the DataBindEngine.RequestRun() below
private void RequestRun()
{
base.Dispatcher.BeginInvoke(DispatcherPriority.DataBind, new DispatcherOperationCallback(this.Run), false);
base.Dispatcher.BeginInvoke(DispatcherPriority.Loaded, new DispatcherOperationCallback(this.Run), true);
}
If the bounded property is owned by a WPF control that is inside a ControlTemplate however, it will be resolved for the first time during the first layouting pass that will trigger an ApplyTemplate and lead to resolving the binding.
Those are only specific examples, if you want to fully understand the binding mechanisms, you should use reflector to take a look at MS.Internal.Data.DataBindEngine and System.Windows.Data.BindindExpression classes. Those are the classes responsible for pushing the correct data when using bindings on dependency properties.
I am new to Silverlight, but I couldn't find anything about this when I googled it.
I have a button that I am trying to set the style programatically. I have the style defined in a XAML file and I want to pull the style into C# so I can dynamically create a button and assign it this style. So far, this is what I am trying:
button.Style = (Style)Resources["CloseButtonStyle"];
However, it just makes the button have no style. Is there an easy way to do this? I feel like this should be obvious, but I can't get it to work.
You are assuming that your Resources property on the current object is the one that contains the defined style. However, I assume, given the symptoms of your issue, that CloseButtonStyle is actually defined further up the control hierarchy.
Instead, you need to traverse your control hierarchy until you find the resource (or if you know the object that defines it, just refer directly to that object). Unfortunately, Silverlight doesn't include FindResource call like WPF, but it's not too difficult to implement your own.
I can call button1.Style = (Style)Resources["NonExistentKey"]; and it makes my button have no style at all as well, point being that the resource is probably not being found, you won't get an exception.
You directly access the Resources property, but is the style really in the immediate resource dictionary of your Window/UserControl/whatever-you-have?
Some months ago I started developing a Silverlight application on my own. I quickly discovered that I was unable to get expected garbage collection for most of my controls. I struggled for about a week with WinDBG and ANTS memory profiler and then found the "DataTemplate memory leak" strand on the Silverlight forum (http://forums.silverlight.net/forums/t/171739.aspx).
Given that so many people seemed to be frustrated with various memory issues I decided to delay further investigation into the memory situation until the most obvious issue was resolved.
Anyway, now I'm looking into the issue again and I realise that the problem I'm having is much more fundamental than I had first thought. I simply don't have a paradigm for writing garbage collectable Silverlight controls when: a) the control has dependency properties that can be bound to, and b) the control can be unloaded from one control and then subsequently loaded again.
I'm starting to think that the second of these demands is too great. Can anyone confirm this?
To give a tiny bit more detail, the most robust pattern I can come up with for writing good, garbage collectable Silverlight controls is as follows:
1) When a Control's Template is applied (in the OnApplyTemplate override) I setup any internal bindings between local properties and TemplateParts. For example, I might setup a Binding between a local property called CanSearch and a button.
if (x_Button_Search != null)
{
Binding b = new Binding("CanSearch");
b.Source = this;
this.x_Button_Search.SetBinding(Button.IsEnabledProperty, b);
}
2) When the Control raises the Unloaded event, I clear the internal bindings and un-wire any eventhandlers.
if (x_Button_Search != null)
{
this.x_Button_Search.ClearValue(Button.IsEnabledProperty);
}
This seems to be the cleanest way of ensuring that no trailing references exist between the x_Button_Search element and the Control. I don't know if this is strictly necessary.
3) Again, when the Control raises the Unloaded event, I clear bindings to existing dependency properties.
this.ClearValue(SearchParametersProperty);
If I fail to do this I can cause leaks. For example, if the SearchParameters property was bound to some INotifyPropertyChanged object then a reference to the Control remains in the PropertyChanged event on the INotifyPropertyChanged object to which I am bound even after the control has been unloaded i.e. the View will stay around as long as the Model and that may not be desired.
4) I 'flicker' the Template value so that next time the control is loaded, the template is reapplied and the OnApplyTemplate method is fired again.
var oldTemplate = this.Template;
this.Template = null;
this.Template = oldTemplate;
The reason to do 4 is that I need to reinstate bindings when the Control is reloaded onto a page. In Silverlight, there are two points of entry through which to do this: in the OnApplyTemplate override or after the control fires the Loaded event. As I want to enforce binding values before the control has been loaded (to avoid flickering), there is only one available entry point available, OnApplyTemplate. I have to flicker the template in order to force the template to reapply when the control is reloaded.
It appears this pattern up to point 3 is the bare minimum for providing garbage collected controls.
My problem comes when you want to unload your control (remove it from a Panel for example) and subsequently reload it. Any dependency properties on the control have been set to null in point 3. For example, imagine there is a binding on the declaration of the control e.g. . As far as I can tell, there is no way of reinstating this Binding once the value of SearchParameters has been set to null, it's not part of a Template after all. The result is that when the control is re-loaded it's as if the value of SearchParameters was null. So I either skip out step 3 in the pattern and get a reloadable control that is not garbage collected, or I keep 3 and get an unreloadable control.
What you do in 1) seems really strange. Why initiating a binding to the template in code and not in xaml?
We have solved lots of memory leak issues in silverlight using this software
http://memprofiler.com/
EDIT
For more control over the binding, you can use
{Binding Property, RelativeSource={RelativeSource TemplatedParent}}
That way the implicit converters are used as expected and you can also specify your own. And I believe that BindingMode TwoWay works as well.
Good luck!
I've got 2 DataTemplates defined for a Listbox Control. 1 Template is for the UnSelected State and the other one is for the Selected State(showing more detail than the UnSelected State).
I followed the example here:
Link
about how to access the Elements inside the DataTemplates from Code behind.
I get it right, but it only finds and returns an element of the UnSelected DataTemplate. But when i search for an element in the Selected DataTemplate i get a NullReferenceException.
What could i be doing wrong?
Setting keyboard focus might be one reason you need to access the datatemplate elements. MVVM will not solve that issue and the FocusManager doesn't set keyboard focus.
What you are doing wrong?
I would say what you are doing wrong is trying to access elements inside the DataTemplate from code-behind. Naughty, naughty!
All joking aside, 99.9% of the time I see someone trying to access an element inside a DataTemplate from code, it is because their application is poorly designed, with no (or few) bindings and no view model at all. Many beginners tend to store their data directly in UI elements rather than using a view model. I think it is because their minds have been corrupted by experience VB, WinForms, and ASP.NET where it was the "normal" way to do it. There are a thousand reasons to use a view model and bind your data instead of storing them in UI elements. Look up "model view view model" online for more details.
Now to answer your question:
Any given ListBoxItem can only have one DataTemplate at a time. This is because its ContentPresenter has only one ContentTemplate property, and this property cannot have two different values.
Because of this, the visual tree under a ListBoxItem will always be generated from one a specific template, not a combination of several templates. If you change the ItemTemplate of the ListBox or otherwise update ListBoxItem.ContentTemplate, the visual tree produced by the old template will be thrown away and a new one built.
Let me say that again: If you change data templates, the visual tree produced by the old data template will be thrown away and a new visual tree built.
You can have a hundred data templates defined and usable on a given ListBoxItem, but only one at a time can have a visual tree instantiated for it. And these are the only elements that actually exist as part of the visual tree. All other templates exist only as templates - there are no actual elements created for them.
To put it another way: It is meaningless to ask about how to find elements in two different visual trees instantiated by two different templates on the same control, because a single control cannot have two different templates active at the same time.
Hope this clears things up for you.
Final advice: Do read up on MVVM, and stop trying to access elements inside DataTemplates ASAP. However if you think you might be in that 0.1% who actually do have valid reasons to access elements inside templates, write back with your actual reason for wanting to do so and maybe I can provide further guidance.