How to set the DataContext value later in a WPF application?. Here in the below code I am setting it at the beginning (Startup). Later I would like to set another DataContext value to the view. What is the best way to do it?
public partial class App : Application
{
private void OnStartup(object sender, StartupEventArgs e)
{
MainWindow view = new MainWindow();
MainViewModel mainViewModel = new MainViewModel();
view.DataContext = mainViewModel;
view.Show();
}
}
Simply telling, there is a ObservableCollection inside the MainViewModel and it is assigned when the user do some button press action only. I need that data to be updated in the view. Please let me know if you have any questions
EDIT
::::
I have an additional question too. Do I need to reassign the entire DataContext or Can I do something through INotifyCollectionChanged event? Please clarify
Your mainViewModel is tightly coupled to your class MainViewModel. Try to use interface. you can create IViewModel and implement it to your MainViewModel. This site will help you
dofactory
you can also use some dependency injection like ninject.
Related
I am trying to share the following object between my view model and code behind
Dictionary<ItemTypeType, Dictionary<string, Dictionary<int,List<ConfigParameter>>>> ItemToConfigParametersValues
This object is not used in the XAML. It is used in the code behind for several dynamically generated UI Elements.
What is the MVVM Light way of doing this?
Well, the ViewModel is typically set as the "DataContext" of the View. Code-behind is part of the View.
So... just expose the data from your ViewModel. In your code-behind, you can access it using the DataContext property (with appropriate casting).
I would store the viewmodel in a variable so I didnt need to cast DataContext everytime... In the MainWindow.xaml.cs code behind for example:
private MainWindowViewModel _vm;
public MainWindow()
{
InitializeComponent();
this._vm = new MainWindowViewModel();
// this._vm.MyProperty = ... (or does the vm instantiate MyProperty?)
this.DataContext = this._vm;
}
private void HandleSomeEvent(object sender, RoutedEventArgs e)
{
var sharedObject = _vm.MyProperty;
}
I need to connect methods in View (WPF window) to events in ViewModel. Is it violation of MVVM pattern to DirectCast Object DataContext in view to concrete VM type and connect its events? If yes, is there better way to do it?
First look at what the methods in the view do. If they manipulate the view, consider adding properties to the viewmodel that you change in the events in the viewmodel and bind the view to. This way, by binding the view to properties you eliminate the need for code in the view.
If the methods contain other logic consider moving that logic to the viewmodel.
In other cases casting a DataContext to a viewmodel or interface can be a valid option and is not a violation of the MVVM pattern.
When adding code to a view, do consider testing. Automated/unit testing a view is harder than testing a viewmodel.
It is not a violation of the MVVM pattern, but the more abstract the better, of course (not because of MVVM but as a general good practice).
If you're setting your DataContext on XAML, you may be able to keep it abstract by using Interactivity EventTrigger and CallMethodAction... Maybe. But if you're setting it on code-behind (via injection or whatever), you're left with either casting the DataContext to a known type, or using Reflection (I wouldn't >_>).
Generally, creating an interface for your ViewModel, so you keep a decent level of abstraction and only expose what the view needs to know instead of its whole implementation, is good enough for most scenarios.
public interface IMyViewModel
{
event EventHandler MyEvent;
}
public class MyViewModel : IMyViewModel
{
public event EventHandler MyEvent;
// More viewmodel related stuff
protected virtual void OnMyEvent(EventArgs e)
{
if (MyEvent != null)
MyEvent(this, e);
}
}
public class MyWindow : Window
{
public MyWindow(IMyViewModel viewModel)
{
this.DataContext = viewModel;
InitializeComponent();
(this.DataContext as IViewModel).MyEvent += MyEventHandler;
}
private void MyEventHandler(object sender, EventArgs e)
{
// Do view related stuff
}
}
I've got a usercontrol (MyUC) that is programatically added to a page (MainPage) several times.
In MyUC I set the DataContext to a view model like this:
private void UserControl_Loaded(object sender, RoutedEventArgs e)
{
DataContext = new MyUCViewModel();
}
In my view model (MyUCViewModel) I have a collection of data items:
public MyDataItemCollection MyDataItems { get; private set; }
And in the constructor I have:
public MyUCViewModel()
{
this.MyDataItems = new MyDataItemCollection();
this.MyDataItems.ChosenItems.CollectionChanged += new NotifyCollectionChangedEventHandler(ChosenItemsChanged);
this.MyDataItems.Add(new DataItem());
}
From the above MyDataItems has another collection in it for ChosenItems and I added a NotifyCollectionChangedEventHandler to it.
Other parts of my code add and remove from the ChosenItems collection and that part seems to work ok.
Next I have the event handler method in the view model:
private void ChosenItemsChanged(object sender, EventArgs e)
{
MessageBox.Show("Chosen Items Changed");
}
This also works and I get a messagebox everytime the user makes a change in the UI that affects the ChosenItems collection.
The part I'm trying to figure out now is how do I set it up so that my MainPage does something when the ChosenItemsChanged event fires in my user controls. What I want to do is have the MainPage loop through the generated MyUC controls on the page and make each usercontrol call a method.
You can add more event listeners in the MainPage like this:
MyUCViewModel viewModel = myUC.DataContext;
viewModel.MyDataItems.ChosenItems.CollectionChanged
+= new NotifyCollectionChangedEventHandler(MainPage_ChosenItemsChanged);
This is based on the comment as the question was a little misleading:
While not strictly MVVM, as your question appears to be, your should write your User Controls as if it was a third-party control and simply expose a custom event on it. User Controls should always be a black-box with a public interface. For a reusable control that is self-contained (as many are) MVVM is overkill.
e.g.
in your User Control add:
public event EventHandler<MyEventArgs> MyEvent
Create a MyEventArgs class deriving from EventArgs and get it to hold useful parameters (like the selected item).
In your main page add a handler to MyEvent on each User Control you dynamically add.
I actually think the MVVM model is flawed and all this sort of controlling logic and event handlers belong in a Controller class (MVCVM!), but that's another story. :)
New to WPF. I am creating UserControls that need read access to the ViewModel state to do their thing. I currently use the following technique:
public partial class ControlBar : UserControl
{
private static readonly DependencyProperty URLProperty =
DependencyProperty.Register("URL", typeof(string), typeof(ControlBar),
new UIPropertyMetadata(null));
public ControlBar()
{
InitializeComponent();
SetBinding(URLProperty, "CurrentPage.URL");
Pin.Click += Pin_Click;
}
private void Pin_Click(object sender, RoutedEventArgs e)
{
var URL = (string)GetValue(URLProperty);
}
}
Is this the correct way and is it not overkill to set up a long-term binding for each variable I need access to? Or can you do something like:
GetValue(new Path("CurrentPage.URL.....
I made up the above obviously.
Thanks!
In general data-binding is the way to go. However sometimes when you are creating controls that have view-specific concerns for which data-binding will not be appropriate.
In those cases you will want to be able to interact with the DependencyProperty to set it and know when it changes. I have been following a pattern that I picked up from a Charles Petzold article in MSDN magazine.
My answer to another question shows the pattern for creating a DependencyProperty for a UserControl Stack Overflow: Dependency Property In WPF/SilverLight
Again, data-binding to a view model will likely solve your problem, but a DependencyProperty may come in useful depending on the situation.
Update in response to comment:
In many situations you can data bind your in a UserControl without using a DependencyProperty. For example if you have a TextBlock that displays a name you would put a TextBlock in the XAML of the UserControl
<TextBlock Text="{Binding Path=NameString}" />
In the view model which is present in the DataContext you would have a property NameString and if the TextBlock is to update the display when the NameString property changes the view model should implement INotifyPropertyChanged and the property should fire the PropertyChanged event with the name of the property sent along with the event.
protected string _NameString;
public string NameString
{
get { return _NameString; }
set { _NameString = value: Notify("NameString"); }
}
Where Notify is a method that checks the PropertyChanged event for null and sends the event if not null.
This works well if everywhere that you want to use the UserControl has a view model with a Name property. The great thing is that the UserControl can pick up on the DataContext of wherever it is hosted and bind to an external view model.
When you want to start binding the same UserControl to different properties is one place that you may want to use a DependencyProperty. In that case you could make a UserControl with a DependencyProperty and bind it to different properties
<my:SampleControl NameString="{Binding Path=GivenName}" />
<my:SampleControl NameString="{Binding Path=FamilyName}" />
And then have an internal view model that the DependencyProperty change handler updates when the bound property changes.
Update: No DependencyProperty or binding
You can always add an ordinary C# property to the UserControl and pass the data in that way.
public MyClass Data { get; set; }
Then in the code-behind of the UserControl you can simply use the property:
if (this.Data != null)
{
this.textBox1.Text = Data.NameString;
}
Update in response to comment:
Another way to access the view model in code is to cast the DataContext to your view model type:
MyClass data = this.DataContext as MyClass;
if (data != null)
{
// do something
this.textBox1.Text = data.NameString;
}
I have a user control where the XAML of the control can bind to the appropriate properties from the parent's data context like normal (the data context propagates in xaml).
For example, I have a window whose DataContext I am setting to ObjectA for example. My user control within the window is then try to access the properties within the dataContext
So my window's xaml and code behind can both see a non-null DataContext.
My control that DataContext propagates to can see a non-null DataContext in the Xaml but not in the code behind.
What is the proper way of handling this?
failing that if you need to check whether the DataContext is being set you can use the DataContextChanged
public partial class UserControl1 : UserControl
{
public UserControl1()
{
InitializeComponent();
DataContextChanged += new DependencyPropertyChangedEventHandler(UserControl1_DataContextChanged);
}
void UserControl1_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
// You can also validate the data going into the DataContext using the event args
}
}
Note it wont enter UserControl1_DataContextChanged until DataContext is changed from null to a different value.
Not sure if this answers your question but can be quite handy to use in debugging issues.
I think you are checking the 'DataContext' in the constructor of the UserControl. It will be null at the Constructor since the user control hasnt yet created while execution is in the constructor code. But check the property at Loaded event you will see the object properly.
public partial class UserControl1
{
public UserControl1()
{
this.InitializeComponent();
//DataContext will be null here
this.Loaded += new RoutedEventHandler(UserControl1_Loaded);
}
void UserControl1_Loaded(object sender, RoutedEventArgs e)
{
//Check DataContext Property here - Value is not null
}
}
I would check to see whether you are having a binding error at runtime. Add this namespace to your XAML:
xmlns:debug="clr-namespace:System.Diagnostics;assembly=System"
and check the debugger's Output window for relevant error messages.
Alternatively, can you show us more code?