This dialog makes no sense to me
http://img576.imageshack.us/img576/4223/50709706.gif
And I'm having trouble finding good tutorials on it. Most of the examples aren't detailed enough, or do stuff via code, but I'd like to take advantage of the IDE as much as possible.
Whats the difference between ItemsSource and DataContext?
I'd like to bind it to just a List for starters. I don't need SQL or databases or anything fancy. Where would I declare my list? In MainWindow.xaml.cs? How do I get it to appear in that dialog?
Think of "DataContext" as the default value for "Source" in a binding.
When you create a binding, you can specify the path and source, like this (I'll use TextBox as an example):
<TextBox Text="{Binding Path=Foo,Source={StaticResource Bar}}" />
So my TextBox.Text property is bound to a Foo property on an object called Bar (a resource somewhere in the application).
However, if you have a whole bunch of things that you want to bind to properties on Bar, it's easier to set Bar as the DataContext of the parent container. A Binding without a Source will just use the DataContext by default, and DataContext flows through to child controls from the parent. So:
<StackPanel DataContext="{StaticResource Bar}">
<TextBox Text="{Binding Path=Foo}" />
<TextBox Text="{Binding Path=Fizz}" />
<TextBox Text="{Binding Path=Buzz}" />
</StackPanel>
All of the TextBoxes are still binding to properties on Bar, but they're doing it without setting it as a Source explicitly.
So let's have another look at the dialog you posted. It's giving you several options for the "source" of the ItemsSource binding. When you choose "DataContext", you're telling Visual Studio that the ItemsControl doesn't need to know the source - it'll pick it up from the DataContext of the parent container (maybe even the Window itself).
If you chose one of the other options (ElementName, RelativeSource or StaticResource) then you'd be setting the binding's source explicitly for that ItemsControl.
Once you've told it that it's binding to the DataContext, you'll need to drop into the "Path" section of the dialog and tell it which property to bind the items of the control to. In the end, the markup would look something like this (assuming it's a ListBox):
<ListBox ItemsSource="{Binding Path=Foos}" />
So the items in the ListBox are coming from a property called "Foos", and that property is on an object that's set in the DataContext somewhere higher in the logical tree (perhaps on the Window itself).
You rarely need to use the data context of a control outside of the control. The most common use case for setting DataContext(DataContext = this;) is within UserControl's code-behind to make all controls within the UserControl to bind to the control's properties.
When you use a ListBox, setting ItemsSource is sufficient, unless you are doing something funky.
This is a pretty good walkthrough: http://windowsclient.net/learn/video.aspx?v=315275
Specifically, you need to set the DataContext first to tell it where to look for the ItemsSource. The easiest way is to set this on the Window through the XAML:
<Window.DataContext>
<controllers:DownloadManager />
</Window.DataContext>
Related
I have UI-elements like a textbox and I want to bind them to a ViewModel. I need to access many properties of the textbox like the text- or the IsEnabled-property.
Is it possible that I bind the textbox directly to another textbox in the ViewModel with all their properties instead of binding every single property to properties?
Yes, using an ElementName, but you still bind all properties though.
<!-- Bound to ViewModel -->
<TextBox Name="tbOne" IsEnabled="{Binding OneIsEnabled}" Text={Binding TextOne}/>
<TextBox Name="tbTwo" IsEnabled="{Binding ElementName=tbOne, Path=IsEnabled}" Text={Binding ElementName=tbOne, Path=Text}/>
There is no built-in way to bind all dependency properties of the TextBox to another. Personally, I would prefer binding directly to the ViewModel.
An alternative solution would be to create a UserControl that internally clones the TextBox with all bindings:
<CloneControl Target="{Binding ElementName=tbOne}"/>
Here, the ControlControl would inspect the target, and have code that create a new TextBox, and sets the bindings in code. This is only useful if you are doing this very often, and there is a slightly performance price to pay, as you are adding another level of controls to the UI tree.
As a continuation of the question Linking DataContext with another property in WPF.
At the very end of the research I was very surprised to find out that when one writes something like that:
<Label Content="{Binding Path=Name}" />
The DataContext against which the Content property is binded is of the Label control itself! The fact that it still works is due to the default inheritance of the DataContext value from the nearest parent.
But if you have this label wrapped in a custom control, and you don't want to bind your data to the DataContext property of that control, you would more likely love to have:
<Controls:SearchSettings Settings="{Binding Path=Settings}" />
And here you are. Now you need to set Settings as the DataContext for the SearchSettings control, for Label inside to bind against, but you can't, because that will trigger re-binding of Settings property.
I can't see the point in mixing binding properties using different sources: DataContext, by ElementName, etc.
So why would I ever use DataContext?
When you write
<Label name="myLabel" Content="{Binding Path=Name}" />
you are binding to myLabel.DataContext.Name, and not myLabel.Name.
The XAML in WPF is just a pretty user interface to display and interact with the actual data, otherwise known as the DataContext. The purpose of other binding sources (RelativeSource, ElementName, etc) is to point to another property that doesn't exist in the current control's DataContext
So suppose you have a Window. Without setting the DataContext, the window still displays but there is no data behind it.
Now suppose to set myWindow.DataContext = new ClassA();. Now the data that the window is displaying is ClassA. If ClassA has a property called Name, I could write a label and bind it to Name (such as your example), and whatever value is stored in ClassA.Name would get displayed.
Now, suppose ClassA has a property of ClassB and both classes have a property called Name. Here is a block of XAML which illustrates the purpose of the DataContext, and an example of how a control would refer to a property not in it's own DataContext
<Window x:Name="myWindow"> <!-- DataContext is set to ClassA -->
<StackPanel> <!-- DataContext is set to ClassA -->
<!-- DataContext is set to ClassA, so will display ClassA.Name -->
<Label Content="{Binding Name}" />
<!-- DataContext is still ClassA, however we are setting it to ClassA.ClassB -->
<StackPanel DataContext="{Binding ClassB}">
<!-- DataContext is set to ClassB, so will display ClassB.Name -->
<Label Content="{Binding Name}" />
<!-- DataContext is still ClassB, but we are binding to the Window's DataContext.Name which is ClassA.Name -->
<Label Content="{Binding ElementName=myWindow, Path=DataContext.Name}" />
</StackPanel>
</StackPanel>
</Window>
As you can see, the DataContext is based on whatever data is behind the UI object.
Update: I see this question so often from new WPF users that I expanded this answer into a post on my blog: What is this “DataContext” you speak of?
From CodeProject by kishore Gaddam:
DataContext is one of the most fundamental concepts in Data Binding. The Binding object needs to get its data from somewhere, and there are a few ways to specify the source of the data like using Source property directly in the Binding, inheriting a DataContext from the nearest element when traversing up in the tree, setting the ElementName and RelativeSource properties in the Binding object.
Detailed example on CodeProject: http://www.codeproject.com/Articles/321899/DataContext-in-WPF
In that particular case, you could do:
<Controls:SearchSettings DataContext="{Binding Path=Settings}" Settings="{Binding}" />
Assuming you want everything that may be content of the SearchSettings to use Settings as it's data context. Basically, the DataContext affects the element itself an any descendants that don't explicitly override it.
In most cases you do want to bind to the DataContext, in some templates on ItemsControls it is the only way to bind to the currently templated item for example. Further bindings to the DataContext are nice to write and read as they are concise.
In your example you can still set the DataContext, you only need to modify the binding on the Settings respectively:
<Controls:SearchSettings DataContext="{Binding Settings}" Settings="{Binding}"/>
I have a an object created in Xaml:
<Grid>
<MyObject/>
</Grid>
I need someway to bind the object myObject back to a property in my view model. I dont know whether this is possible, everything ive seen so far binds properties together, but any help would be greatly appreciated.
I am assuming what you want is your ViewModel to hold the actual visual control MyObject in it and your Grid to display it via MVVM.
This is possible through ContentControl in WPF.
Assuming your ViewModel has a property MyObjectView which holds MyObject...
<Grid>
<ContentControl Content="{Binding MyObjectView}" />
</Grid>
Having said that you must take caution that same MyObjectView is not bound to any other content control as that will result in an error
"Specified element is already the logical child of another element.
Disconnect it first"
And if that requirement is possible then you must excercise ContentTemplate option.
Let me know if this helps.
It is possible. It kinda breaks mvvm though.
You can attach an InvokeCommandAction to this object, and bind the CommandParameter to it via ElementBinding. Then in the callback of the command which you defined in the viewmodel, you will have a reference to this object from the CommandParameter.
I use MVVM for my application, the DataContext of controls is assigned in my c# code (not in XAML).
Therefore the XAML controls have no idea to which instance type its DataContext is set to. The consequence is that there is no refactoring support and intellisense for the bound properties of my viewmodel in XAML.
Is there a way to tell a control in XAML to which type its DataContext is linked?
So when I modify a property name in my ViewModel or search for all references of that property, I want that this property in XAML bindings gets considered too.
There is no framework support, the best you can do is tell the VS designer the 'shape' of the DataContext so that it will give you hints for the properties. If you want to make your solution more refactor-proof, I would recommend Daniel's T4 metadata solution:
http://www.codeproject.com/KB/codegen/T4Metadata.aspx
This generatesmetadata for your view models which you can reference in the XAML:
<StackPanel DataContext="{Binding Source={StaticResource Person}}">
<TextBlock >Name:</TextBlock>
<TextBox Text="{Binding Path={x:Static Metadata:PersonMetadata.NamePath}}"/>
</StackPanel>
Colin E.
No, as the DataContext can change at runtime, it doesn't make sense to tie this to a particular type.
I have a large number of ViewModel classes. For each of these classes, there is a corresponding .xaml file which is a 'UserControl'. In my App.xaml, I have them registered as DataTemplates, like so:
<DataTemplate DataType="{x:Type viewModel:MainMenuViewModel}">
<view:MainMenuView/>
</DataTemplate>
With the idea being that WPF will be able automatically swap in the necessary user controls at runtime. For example, this works:
<Grid>
<StackPanel>
<TextBlock Text="SuperApp" />
<ItemsControl>
<ViewModels:MainMenuViewModel/>
</ItemsControl>
</StackPanel>
</Grid>
In that the entry "MainMenuViewModel" is automatically replaced by the MainMenuView, bound to the MainMenuViewModel. Great. My current goal is now this: I want to have a button, on, say, a view embedded in the MainMenuView, which opens a popup window, which will have a new ViewModel inside. The idea is to set it up so that I have a single 'generic' popup form, in which I embed an arbitrary ViewModel, and let WPF handle actually rendering it with DataTemplates, similar to the above. So I have a command bound to a button, like so:
<Button Command="{Binding Path=LaunchInStandaloneForm}" Content="Rip Out"/>
Which successfully creates a new window, sets the dataContext equal to the appropriate ViewModel, and shows the window.
The question is: How do I set up the XAML of this popup window so that it will render the appropriate DataTemplate for the ViewModel which is the DataContext? I've tried:
<Grid>
<ItemsControl ItemsSource="{Binding Path=.}">
</ItemsControl>
</Grid>
, but it comes up blank. Any pointers?
To set the ItemsSource to the DataContext, use ItemsSource={Binding}. That assumes that the DataContext is an enumerable collection of your View Model objects.
Updating with correct answer:
Use a ContentControl :)
Hope that helps.
The accepted answer here shows how to change templates at runtime. You should be able to dig out the answer from that. Any questions just shout.
How to modify silverlight combobox data display
Hope that helps