I have a userControl named SensorControl which I want to bind its dataContext to a viewModel class named SensorModel.
The binding is successful and the initial values of viewModel is visible in the userControl, but the problem is that when I change the viewModel's properties, the userControl does not update, and I have to manually update that (by assigning null to its dataContext and assigning the viewModel again).
Here is a simplified example of my code.
SensorControl.xml:
<UserControl ...[other attributes]... DataContext={Binding Model}>
.
.
.
<Label Content="{Binding Title}"/>
.
.
.
</UserControl>
SensorControl.xml.cs (code-behind):
public partial class SensorControl : UserControl
{
public SensorModel model;
public SensorModel Model
{
get { return model; }
set { model = value; }
}
public SensorControl(SensorModel sm)
{
Model = sm;
}
}
MainWindow.xml.cs:
public partial class MainWindow : Window
{
public SensorModel sensorModel_1;
public SensorControl sensorControl_1;
public MainWindow()
{
InitializeComponent();
sensorModel_1 = new SensorModel(...[some arguments]...);
sensorControl_1 = new SensorControl(sensorModel_1);
mainGrid.Children.Add(sensorControl_1);
}
private void Button_Click(object sender, RoutedEventArgs e)
{
sensorModel_1.Title = "myTitle";
//The UserControl does not update
}
0) I implemented INotifyPropertyChanged in SensorModel
1) The reason I need this, is that there is only one single concept of 'Sensor' in my project (it is a real electronic sensor) and therefore I have a single model for it (that deal with the real sensor, the database, etc), but in the UI I have multiple userControls for presenting different aspects of Sensor. So I have to create one instance of model (SensorModel) for each real sensor, and multiple userControls must bind to that (each one uses different parts of model).
2) I'm not that new to WPF, but I'm kind of new to MVVM and it's possible that I misunderstand something essential, so I would appreciate if someone could clearly explain the correct approach.
Thanks in advance
In your UserControl, remove the DataContext attribute and add an x:Name attribute. Then in your Label, bind like this:
<UserControl x:Name="uc">
<Label Content="{Binding ElementName=uc,Path=Model.Title}" />
</UserControl>
I believe the issue is the DataContext can't be set to Model because binding works off the parent's DataContext which will be based on mainGrid when it gets added as a child to that. Since the property "Model" doesn't exist in maiGrid's DataContext no binding will occur so your update won't reflect. Getting the DataContext of a UserControl properly can be tricky. I use the ElementName quite a bit or create DependencyProperties on the UserControl and then set them from the parent who will be using the control.
You need to set the DataContext to your ViewModel class in your View, and if you're applying the MVVM pattern, you should use ICommand for actions. Maybe it would be better If you'd implement a MainView class that does the logic in the background instead in the MainWindow class.
public MainWindow()
{
InitializeComponent();
DataContext = new MainView();
// sensorModel_1 = new SensorModel(...[some arguments]...);
// sensorControl_1 = new SensorControl(sensorModel_1);
// mainGrid.Children.Add(sensorControl_1);
}
Your MainView class :
public class MainView {
public SensorControl Control {get; internal set;}
...
}
And in your xaml change the binding :
<Label Content="{Binding Control.Model.Title}"/>
Thanks to all of you guys, I took your advices and finally I found a way.
1) I implemented an event in the SensorModel that fires every time any of properties changes (name ModelChanged)
2) Then as Merve & manOvision both suggested, I declared a dependency property in the SensorControl (of type SensorModel) and bind ui elements to that (Binding Path=Model.Title)
3) Then I used the ModelChanged event of this dependency property in the SensorControl and raise an event (of type INotifyPropertyChanged) so the bindings update their value
It works fine.
Related
Databinding is a difficult concept that I still can't quite grasp despite reading through dozens of topics already.
I want to have a TextBox that would change its text every time a field 'status' is being changed in my code-behind, for debugging purposes.
Here's what I have so far:
public partial class ReviewsControl : UserControl
{
private Status status = MainControl.AppStatus;
public string StatusDisplay
{
get
{
return status.ToString();
}
}
}
And here's my take on the view:
<TextBlock x:Name="StatusBlock" HorizontalAlignment="Left" Margin="10,450,0,0" TextWrapping="Wrap" Text="{Binding StatusDisplay, Source=ReviewsControl, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Height="40" Width="205"/>
The code above doesn't even show anything, let alone do that dynamically. What should I change if I want XAML to detect changes in the in my C# code and change the textbox accordingly?
I too had issues early on. Your view (display to end-users) does not care how or where things come from, you just know what will be exposed in your View Model controller to bind to. To have things updated in your view, the most common is binding and having your view model include the INotifyPropertyChanged interface. This is so you can force raising the event when a property changes, whatever is "listening" for it will update itself..
Simple class, you can just grab from the : INotify, the event exposed to allow things to get registered to, and your method to actually RAISE the event to pass up stream to those listening for changes.
public class SomeBaseClass : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void RaisePropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
Once that is done, your going to expose properties in your class by making as public with getter/setters. You don't bind to a field.
public string SomeThingYouWantToExpose {get; set; }
And in your code, however you are getting data, label refresh, whatever, you would set and raise the property-changed
public void GettingSomeData()
{
// … doing something to get / prepare / whatever...
SomeThingYouWantToExpose = "some new label";
// Now raise which the view bound to this property will updated itself
RaisePropertyChanged( "SomeThingYouWantToExpose" );
}
Now, in your view, you would bind to whatever your view model object is and then the property on the class. Don't know if you specifically need the x:Name reference which basically makes this a field in your control. Not necessary in this example unless you are trying to bind other controls in same display as a result of this field..
<TextBlock Height="40" Width="205" Margin="10,450,0,0"
HorizontalAlignment="Left" VerticalAlignment="Top"
DataContext="{Binding YourViewModelObjectName}"
Text="{Binding SomeThingYouWantToExpose}"
TextWrapping="Wrap" />
Hopefully these pieces within your scenario will make sense and move you forward in your project. Any additional clarification, let me know.
CLARIFICATION on the DATA CONTEXT BINDING
The way I have implemented in my apps, I would have a
MyView -- via the visual declaration... be it a window, grid, complex user control with many controls, whatever...
MyDataModel - a class that is used to query data from whatever data source, such as SQL-Server.
MyView_ViewModel - a custom class that has the INotifyPropertyChanged incorporated and where I expose different properties and other objects I want to expose / make available to the view
So, in the MyData_ViewModel, I would create the view and also create my view model. After creating the view, I would set the overall view's DataContext to the "MyView_ViewModel"
public class MyData_ViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void RaisePropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public void LoadingMyDataAndView()
{
// Controller I use to get whatever data I need from ex: SQL
_myDataModel = new MyDataModel();
// the view / window / control I want to present to users
_myView = new MyView();
// Now, set the data context on the view to THIS ENTIRE OBJECT.
// Now, anything on THIS class made as public can be have a binding
// directly to the control in the view. Since the DataContext is
// set here, I can bind to anything at this level or lower that is public.
_myView.DataContext = this;
}
private MyView _myView;
private MyDataModel _myDataModel;
// such as example exposed property
public string SomeThingYouWantToExpose {get; set; }
public void GettingSomeData()
{
var something = _myDataModel.GetSomeData();
// … doing something to get / prepare / whatever...
SomeThingYouWantToExpose = "some new label";
// Now raise which the view bound to this property will updated itself
RaisePropertyChanged( "SomeThingYouWantToExpose" );
}
}
Hopefully this EXAMPLE shows how the pieces tie together. The view would no longer need the individual DataContext set since the whole view is set, just needs to bind to the individual public property.
Assuming that the TextBlock is a child element of the UserControl, i.e. that
<TextBlock x:Name="StatusBlock" Text="{Binding StatusDisplay}" ... />
is declared in the UserControl's XAML, the Binding's RelativeSource should be set to the parent UserControl like this:
<TextBlock Text="{Binding StatusDisplay,
RelativeSource={RelativeSource AncestorType=UserControl}}" />
Since StatusDisplay is a property of a UserControl, i.e. a DependencyObject, it should be declared as a dependency property:
public partial class ReviewsControl : UserControl
{
public static readonly DependencyProperty StatusDisplayProperty =
DependencyProperty.Register(
nameof(StatusDisplay), typeof(string), typeof(ReviewsControl);
public string StatusDisplay
{
get { return (string)GetValue(StatusDisplayProperty); }
set { SetValue(StatusDisplayProperty, value); }
}
}
So I currently have a Window with a TabControl. The MainWindow has its own ViewModel and all the TabItems have their own ViewModels also.
I can easily change tabs from the MainWindow ViewModel through a bound SelectedIndex property. What I would like to do is change to another tab from code that runs within ANOTHER tab viewmodel. Since the Tabs are not part of the MainWindowViewModel, I am looking for a clean way to change the Tab without resorting to code behind to do it.
There are also cases, where I might need to change the tab from something such as a message prompt. I thinking my only way is to create and event and subscribe to that from MainWindowViewModel.
So I solved this with an EventAggregator.
public static class IntAggregator
{
public static void Transmit(int data)
{
if (OnDataTransmitted != null)
{
OnDataTransmitted(data);
}
}
public static Action<int> OnDataTransmitted;
}
First ViewModel sends data.
public class ModifyUsersViewModel : INotifyPropertyChanged
{
private void change_tab(int data)
{
IntAggregator.Transmit(data);
}
}
Second ViewModel receives data and then does something with it.
public class MainWindowViewModel : INotifyPropertyChanged
{
private int _Tab_SelectedIndex = 0;
public int Tab_SelectedIndex
{
get
{
return _Tab_SelectedIndex;
}
set
{
_Tab_SelectedIndex = value;
OnPropertyChanged(new PropertyChangedEventArgs("Tab_SelectedIndex"));
}
}
public MainWindowViewModel()
{
IntAggregator.OnDataTransmitted += OnDataReceived;
}
private void OnDataReceived(int data)
{
Tab_SelectedIndex = data;
}
}
Rather than trying to bind to SelectedIndex, if the TabItems have their own view models, then you can create a property for each of those view models: IsSelected and then bind the TabItem.IsSelected property to that:
<TabItem IsSelected="{Binding IsSelected}">
This prevents the view models from needing to know the index of their corresponding TabItem, something I would argue is a detail that should be specific to the view and something the view model should not concern itself with. What if you add another TabItem or want to change the order? Now you've got changes to make in the view models for something that could be just simple change to the view.
I'm trying to learn on MVVM. I've understand the concept, however, i'm confused about the binding. I'm not sure where to bind my Fill property. Please help. Tqvm in advanced.
View - name: MainScreen.xaml
<Path Fill="{Binding mainScreenClass, Converter={StaticResource colorConverter}}"/>
inCodeBehind
DataContext = new vmMainScreen();
ViewModel - name:vmMainScreen
public ICommand cmdMouseEnterNav { get; private set; }
public mMainScreen mainScreenClass { get; set; }
public vmMainScreen()
{
mainScreenClass = new mMainScreen();
mainScreenClass.propNaviconFill = new SolidColorBrush(Colors.White);
naviconMouseEventChecker();
}
private void naviconMouseEventChecker()
{
cmdMouseEnterNav = new SimpleCommand
{
ExecuteDelegate = x => mainScreenClass.propNaviconFill = (SolidColorBrush)(new BrushConverter().ConvertFrom("#c5a02b"))
};
}
Model - name:mMainScreen
public class mMainScreen : INotifyPropertyChanged
{
private Brush _NaviconFill = new SolidColorBrush(Colors.White);
public Brush propNaviconFill
{
get
{
return this._NaviconFill;
}
set
{
this._NaviconFill = value;
NotifyPropertyChanged("propNaviconFill");
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string info)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}
}
I understand that when i breakpoint on my colorConverter, I'm getting the class. Not the property of propNaviconFill. If i create another property with the Brush class on my ViewModel and bind it to Fill, there is no problem. But that means I'm not following the correct structure of MVVM. Thanks again.
You should bind to the property of your view model.
<Path Fill="{Binding propNaviconFill, Converter={StaticResource colorConverter}}"/>
Use the view model implementing the INotifyPropertyChanged as the data context of your view.
DataContext = new mMainScreen();
If you really want to use vmMainScreen as your data context, then vmMainScreen should implement INotifyPropertyChanged there and you should study how NotifyPropertyChanged was used to notify the view that the view model property has changed.
Keep in mind there are two basic types of MVVM:
1. View First
2. View Model First
Based on your example you are trying to do View First. This is easier to implement but comes with drawbacks on larger projects since the view controls the creation of the ViewModel it's harder to inject data or state into the ViewModel.
For all MVVM patterns you have the three parts:
Model - Basically a state bag. This thing is just like a customer class which most of the time implements INotifyProperty changed.
ViewModel - This is like the controller class in MVC. It has all the real logic and does the work.
View - This is your XAML and it only holds presentation logic. The code-behind class ie: MyWindow.xaml.cs should not be used except to setup the ViewModel if your going View First. (there are exceptions of course but generally it should basically be empty)
For View First
Your Window (or control) should create the ViewModel in the constructor and assign it to the DataContext.
Your ViewModel will have ICommand's, ObservableCollections and such that can be bound to controls in the View. So when your constructor fires you fill up your data and put it into the necessary structures; because of databinding this gets related to the View and shown.
Your Model (you usually have more than one, can have Customer, Order, StockTicker or whatever.) These are created by the ViewModel and put into things such as ObservableCollections for the View to databind to.
I'm trying to access the view model of a view in code-behind using the DataContext property. However, no matter at what point in the view lifecycle I'm trying to access it, the property is always null.
Simple dummy setup:
// shell
internal class ShellViewModel : Conductor<IScreen>.Collection.OneActive
{
public ShellViewModel() {
ActivateItem(new MyTestViewModel());
}
}
// view code-behind
public partial class MyTestView : UserControl {
public MyTestView() {
InitializeComponent();
}
protected override void OnInitialized(EventArgs args) {
var vm = DataContext as MyTestViewModel;
Debug.Assert(vm != null); // is always null!
}
}
The view get's correctly initialized, the view model is being called and both can be bound together. When I bind a property of the view model to a view control, it can be accessed without problems. Only during initialization, the DataContext of the view is always null. What do?
MVVM framework: Caliburn.Micro
public partial class MyTestView : UserControl {
public MyTestView() {
InitializeComponent();
}
public MyTestViewModel ViewModel() {
return (MyTestViewModel)Datacontext;
}
}
Doing this for whatever reason sort of breaks the idea behind the pattern since the viewmodel shouldn't be hardcoded into the view. Understandable for testing purposes but for any actual use; bad form.
--Edit -- I was thinking view only actions, but for some reason it was late and wasn't thinking about accessing data from within the view not accessing view from within ViewModel.
This should get what you need. Only after the Framework binds the view with the viewmodel (i.e. datacontext is set, once Loaded has been hit) will this work. If you need to do something beforehand I am not sure what else would be a better solution.
This is more of a conceptual question rather than a practical one. I'm just starting to learn the MVVM concept for developing UI , and I've come across a dillema I'm not sure the answer to:
Say I have a main window and a little pop-up window (meaning it's a small window with some UI elements in it). The structure of the program will look something like this:
MainWindow
model <-- MainWindowViewModel.cs <-- MainWindowView.xaml (containing no code-behind)
PopUpWindow (A UserControl)
model <-- PopUpWindowViewModel.cs <-- PopUpWindowView.xaml (containing no code-behind)
*the model is just a bunch of BL classes that are irrelevant for this question.
Now , lets say I want to create a new PopUp window from inside the MainWindowViewModel (or even save an instance of it in a private data-member). What is the correct way of doing so?
The way I see it I can't do something like this :
PopUpWindow pop = new PopUpWindow()
Because it kind of defeats the purpose of abstracting the view from the view model(What if a year from now i'll want to create a better version of the PopUpWindow using the same PopUpWindowViewModel?).
On the other hand , I can't initialize a new instnace of the PopUpWindow using just it's view model (The viewModel as I understand is not supposed to know anything about the view that will use it).
Hope it all makes sense... so what would you do in that situation?
*Just to clarify it further , let's say for argument's sake that the situation I'm describing is a button on the MainWindowView that upon clicking will open a PopUpWindowView.
Thanks in advnace.
I had somewhat a similar dilemma and I'll explain how I solved it.
Let's say you have MainWindow and a SettingsWindow, which you want to display when the SettingsButton is clicked.
You have two respective view models, MainWindowViewModel and SettingsViewModel, which you will be passing as their Window.DataContext properties.
Your MainWindowViewModel should expose an ICommand property named SettingsButtonCommand (or similar). Bind this command to the SettingsButton.Command.
Now your command should invoke something like this:
void OnSettingsButtonClicked()
{
var viewModel = new SettingsViewModel();
var window = new SettingsWindow();
window.DataContext = viewModel;
window.Show();
}
There is a slight issue when you want to use Window.ShowDialog(), because you need to resume execution.
For these cases I have an asynchronous variant of the DelegateCommand:
public sealed class AsyncDelegateCommand : ICommand
{
readonly Func<object, Task> onExecute;
readonly Predicate<object> onCanExecute;
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public AsyncDelegateCommand(Func<object, Task> onExecute)
: this(onExecute, null) { }
public AsyncDelegateCommand(Func<object, Task> onExecute, Predicate<object> onCanExecute)
{
if (onExecute == null)
throw new ArgumentNullException("onExecute");
this.onExecute = onExecute;
this.onCanExecute = onCanExecute;
}
#region ICommand Methods
public async void Execute(object parameter)
{
await onExecute(parameter);
}
public bool CanExecute(object parameter)
{
return onCanExecute != null ? onCanExecute(parameter) : true;
}
#endregion
}
You've specifically said that the popup is a UserControl so you can use basic data templating. First create view models for your main window and popup control:
public class MainViewModel : ViewModelBase
{
private PopUpViewModel _PopUp;
public PopUpViewModel PopUp
{
get { return _PopUp; }
set { _PopUp = value; RaisePropertyChanged(() => this.PopUp); }
}
}
public class PopUpViewModel : ViewModelBase
{
private string _Message;
public string Message
{
get { return _Message; }
set { _Message = value; RaisePropertyChanged(() => this.Message); }
}
}
The MainViewModel's PopUp member is initially null, we'll set it to an instance of PopUpViewModel when we want the popup to appear. To do that we create a content control on the main window and set it's content to that member. We also use a data template to specify the type of child control to create when the popup view model has been set:
<Window.Resources>
<DataTemplate DataType="{x:Type local:PopUpViewModel}">
<local:PopUpWindow />
</DataTemplate>
</Window.Resources>
<StackPanel>
<Button Content="Show PopUp" Click="Button_Click_1" HorizontalAlignment="Left"/>
<ContentControl Content="{Binding PopUp}" />
</StackPanel>
I'm doing a big no-no here by creating the view model in the code-behind along with a click handler, but it's just for illustrative purposes:
public partial class MainWindow : Window
{
MainViewModel VM = new MainViewModel();
public MainWindow()
{
InitializeComponent();
this.DataContext = this.VM;
}
private void Button_Click_1(object sender, RoutedEventArgs e)
{
this.VM.PopUp = new PopUpViewModel { Message = "Hello World!" };
}
}
That's it! Click the button, popup window appears underneath it showing the content. Now it may not always be this simple, sometimes you may want to create multiple children on a parent control...in that case you'd set up an ItemsControl, set its panel to a Grid (say) and modify the data templates to set the Margin etc on each element to position them. Or you may not always know what type of view model is going to be created, in which case you need to add multiple data templates for each type you're expecting. Either way you still have good separation of concerns because it is the views that are deciding how to display the content in the view models. The view models themselves still don't know anything about the views and they can be unit-tested etc independently.