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.
Related
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.
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?
Advance apologies for the event-style explanation; there's a lot of factors that I feel all play a role of their own. WPF is not my native framework of choice, and it probably shows. :)
Old situation: I had a window with several controls. Depending on their selections, I used multibindings and a converter to determine whether certain controls needed to be shown that inform the user about the implications of their changes before they'd eventually confirm them by OK (or simply dismissed by using Cancel). This worked perfectly.
Problem: Too many controls as time went by, too much clutter.
Solution: Put stuff in different Pages so it becomes easier to browse for the user. In order to have changes-to-be persist as a user arbitrarily browses between the pages, I create these dynamically and put them in a cache (Dictionary<string, BasePage>, see below), from which they will be pulled as the user chooses them.
Consequence: I need to decouple the bindings to the notification controls as the different options are now on different pages.
Solution? I put a BasePage class in that exposes certain abstract read-only properties that define the various aspects that the window needs to know about in order to do its notifications. For example, a bool requiresReboot property defines whether the current state of things on that page requires a reboot to take (full) effect. A specific page implements the property based on its controls.
Problem: I do not know how to keep create a proper binding that properly gets updated as the pages are changed. I tried giving my notification controls a binding to the Dictionary<string, BasePage> with a converter that checks all pages and the relevant property.
Questions:
1) How do I create a proper property for this purpose? I presume I need a DependancyProperty as I did a fair bit of reading on MSDN, but I can't figure out how this fits together.
2) How do I make a link between my custom property so that it allows (multiple) control(s) on a page to change that property? Do I use INotifyPropertyChanged somehow? My old example bound against several CheckBox.IsChecked properties in XAML. I am trying to avoid putting tons of events (OnChange, etc) on the controls as the original code did not need it and I have been told it makes for a messy solution for as far WPF is concerned.
3) Finally, I suspect I may need to change my Dictionary<string, BasePage> class to a custom implementation that implements some sort of INotifyPropertyChanged but for Collections? Observable Collection is the term I am looking for, I believe.
I hope someone is able to bridge the gap in my understanding of WPF (property) internals; I would very much appreciate it. A basic sample would be even better, but if it is too complicated, just a nudge in the right direction will do. Thank you. :)
It's been a while since I solved this, and while I cannot remember the exact cause of the problems, there were a few different issues that made up the bulk of the trouble I ran into.
I ended up making the Property in question a non-abstract DependencyProperty in the base class; it was the only way in which I could properly delegate the change notifications to the interface. Derived classes simply ended up binding it to their controls (with a proper Converter in the case extra logic was necessitated).
As Dictionary<string, BasePage> does not support any sort of change notification, I made an extra collection of ObservableCollection<BasePage> which I used for binding purposes.
However, such a collection does not propagate a change event when items inside of it has a property changed. Since this situation required that, and I was binding to the collection itself in the context of a property that does not have a Master<->Detail relationship like a DataGrid (which basically add their own OnPropertyChanged handlers to the binded object), I ended up subclassing a VeryObservableCollecton<>. This one listens to its own items, and throws a proper change event (I think it was an OnPropertyChanged from the INotifyPropertyChanged interface) so that the binding (or in this case a multi-binding) would refresh properly and allow my interface to update.
It is hardly the prettiest code, and it feels over-engineered, but at least it allows me to properly bind the UI to the data in this manner.
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 have a Window that uses DataTemplates to display a different UserControl (view) in a ContentPresenter based on the type of its Content property, which is bound to a property that holds the current viewmodel. In this way, by changing the viewmodel property with an event, I can facilitate the basic back/forward navigation I need.
When creating a new viewmodel, it is passed a reference to the current one. Going back to the old viewmodel instance works fine for a CheckBox control, but not for a UserControl I made that contains a TextBlock and a ComboBox.
The problem is that, when the view containing the ComboBox gets unloaded, the ComboBox's ItemsSource gets nulled, which triggers it to clear its SelectedItem/Text properties, which are for some reason still bound to my viewmodel--thus clearing the data it stores. I don't know how to manually unbind them at the appropriate time. (Again, the CheckBox works just fine.)
I have read that other users have had this exact same problem. For them, changing the declaration order of the ItemsSource and SelectedItem/Text bindings so that the attributes for the latter are placed before the former fixes the issue. However, in my case, it does not. Others have also fixed the issue by ignoring null/empty values, but this won't work in my case.
I could work around the issue by copying the interesting data to a separate object, and reloading it from that, but I would need to add code to trigger reloading the data = more data linkage code to maintain.
I could also avoid using DataTemplates and manually add the UserControls in the codebehind, which would allow me to break the data binding before removing the UserControl. But this runs counter to the point of MVVM.
I'm not above modifying my very non-MVVM UserControl to handle any events on the ComboBox it contains to work around this issue.
UPDATE:
I have narrowed down the issue a little bit. I refactored the code so that it manually creates and adds the view UserControl based on which viewmodel has been set. The issue now only occurs when I set the DataContext of the view UserControl to null. If I simply replace the view without removing the reference, it no longer erases the values in question. Is this a usable workaround, or does it create issues like memory leaks?
Maybe something that would "open mind" for a simpler solution... If I understand your problem, it's similar to a past problem we had. In our case, we simply made the assumption that it's not possible to set a specific value to null when accessed by the bound property, so we tweaked the appropriate ViewModel Properties a bit:
public MyItem SelectedItem {
get {
return Model.MyItem;
}
set {
if (value != null) {
// Set and notify if not null
Model.MyItem = value;
OnPropertyChanged("SelectedItem");
}
else // just notify when trying to set to null
OnPropertyChanged("SelectedItem");
}
}
Using such tweaked properties we were able to block any try to set the value to null, by calling OnPropertyChanged(..) insead, the existing value was recalled by the UI. If there is a need to be able to set a value to null, you have to provide a seperate property allowing that.
Not sure if this applies to your problem.
Good luck.
UPDATE
oh, I see probably this describes same method as "Others have also fixed the issue by ignoring null/empty values" which seems not to work in your case. But I dont unterstand why it shouldn't.
This is a known bug in the early versions of WPF caused by event leapfrogging. It was fixed for the Selector-derived controls in .NET 4.0.
See this blog post for more details: http://blogs.interknowlogy.com/2011/03/09/event-leapfrogging/
I have worked around the issue by adding a property Active and corresponding Activate()/Deactivate() methods to my base viewmodel class, and calling these as appropriate when swapping out viewmodels. This fits into my application pretty well. I'm still open to other suggestions, of course.