I have a user control written in C# & WPF using the MVVM pattern.
All I want to do is have a property in the bound ViewModel exposed to outside of the control. I want to be able to bind to it and I want any changes to the property to be picked up by anything outside the control that is bound to the exposed value.
This sounds simple, but its making me pull out my hair (and there is not much of that left).
I have a dependency property in the user control. The ViewModel has the property implementing the INotifyPropertyChanged interface and is calling the PropertyChanged event correctly.
Some questions:
1) How do I pick up the changes to the ViewModel Property and tie it to the Dependency Property without breaking the MVVM separation? So far the only way I've managed to do this is to assign the ViewModels PropertyChanged Event in the Controls code behind, which is definitely not MVVM.
2) Using the above fudge, I can get the Dependency property to kick off its PropertyChangedCallback, but anything bound to it outside the control does not pick up the change.
There has to be a simple way to do all of this. Note that I've not posted any code here - I'm hoping not to influence the answers with my existing code. Also, you'd probably all laugh at it anyway...
Rob
OK, to clarify - code examples:
usercontrol code behind:
public static DependencyProperty NewRepositoryRunProperty = DependencyProperty.Register("NewRepositoryRun", typeof(int?), typeof(GroupTree),
new FrameworkPropertyMetadata( null, new PropertyChangedCallback(OnNewRepositoryRunChanged)));
public int? NewRepositoryRun
{
get { return (int?)GetValue(NewRepositoryRunProperty); }
set
{
SetValue(NewRepositoryRunProperty, value);
}
}
private static void OnNewRepositoryRunChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
if (e.OldValue != e.NewValue)
{
}
}
public GroupTree()
{
InitializeComponent();
GroupTreeVM vm = new GroupTreeVM();
this.DataContext = vm;
}
Viewmodel (GroupTreeVM.cs)
private int? _NewRepositoryRun;
public int? NewRepositoryRun
{
get
{
return _NewRepositoryRun;
}
set
{
_NewRepositoryRun = value;
NotifyPropertyChanged();
}
}
And now for my weekly "don't do that" answer...
Creating a ViewModel for your UserControl is a code smell.
You're experiencing this issue because of that smell, and it should be an indication that you're doing something wrong.
The solution is to ditch the VM built for the UserControl. If it contains business logic, it should be moved to an appropriate location in another ViewModel.
You should think of a UserControl as nothing more than a more complex control. Does the TextBox have its own ViewModel? No. You bind your VM's property to the Text property of the control, and the control shows your text in its UI.
Think of UserControls in MVVM like this--For each model, you have a UserControl, and it is designed to present the data in that model to the user. You can use it anywhere you want to show the user that model. Does it need a button? Expose an ICommand property on your UserControl and let your business logic bind to it. Does your business logic need to know something going on inside? Add a routed event.
Normally, in WPF, if you find yourself asking why it hurts to do something, it's because you shouldn't do it.
Perhaps I've misunderstood, but it seems like you're trying to use binding in the code behind?
public MyUserControl()
{
InitializeComponent();
// Set your datacontext.
var binding = new Binding("SomeVMProperty");
binding.Source = this.DataContext;
SetBinding(MyDependencyProperty, binding);
}
Related
I am a C++ developer and new to WPF and MVVM. please bear with me if I choose any wrong word to ask my question
I have my Main application in MFC/C++ which is passing some data to C# library(CLI is used as middle layer).
In C# library, there is a section of code where a dialog is opened , data is filled and user selection is notified to the calling object in below way -
public classA()
{
MyDialog dlg = new MyDialog(param1, param2, param3)
if(dlg.ShowDialog().GetValueOrDefault())
{
var name = dlg.name;
var roll = dlg.roll;
}
else
{
var name = string.Empty;
var roll = string.Empty;
}
}
Now Dialog has been modified and implemented using MVVM pattern.
I have created below files as part of implementation-
1
MyDialogView.Xaml
MyDialogView.xaml.cs
MyDialogViewModel.cs
MyDialogModel.cs
My question is, how to instantiate the new dialog now from my classA so that data is filled using the parameters passed to dialog in same way as previously it was doing and record user selection without loosing any data and safely closing the view.
Standard MVVM approach works like this (at least when using MVVM Light):
You have a VM layer, a Class Library.
You have a View layer, a WPF Controls Library or WPF Application.
View layer adds reference to VM layer. VM layer doesn't know anything about View.
You create a normal public class for your dialog's VM. Call it DialogVM or whatever. Make sure this class inherits from MVVM Light's built-in ViewModelBase. This will get you access to change notification methods provided by MVVM Light. Might look like this in your case:
public partial class DialogVM : ViewModelBase
{
private string _Name;
public string Name
{
get { return _Name; }
set { Set(ref _Name, value); }
}
private string _Roll;
public string Roll
{
get { return _Roll; }
set { Set(ref _Roll, value); }
}
}
VM layer has a global static class called ViewModelLocator. This class performs IoC/DI and provides public static properties to expose different VMs. (In your case your dialog's VM goes to the VM project and the ViewModelLocator looks something like this:
using System;
namespace VMLayer
{
public class ViewModelLocator
{
static ViewModelLocator()
{
SimpleIoc.Default.Register<DialogVM>(true);
}
public static DialogVM MyDialog => SimpleIoc.Default.GetInstance<DialogVM>();
}
}
Your dialog box (a Window) goes to View layer and uses this exposed property MyDialog to provide a DataContext for the dialog:
<Window x:Class="GlasshouseTestingShell.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:vm="clr-namespace:VMLayer;assembly=VMLayer"
DataContext="{x:Static vm:ViewModelLocator.MyDialog}"
d:DataContext="{d:DesignInstance Type=vm:DialogVM}">
</Window>
Look how cleanly we have created View layer's DataContext without writing a line of C# code in the View layer. This is also elegant in the sense that you get all design-time Intellisense in Binding expressions and elsewhere.
You now bind all your UI stuff (textboxes, buttons etc.) to the public properties and commands exposed by your dialog's VM. Still no lines in the code-behind. Might look like this in your case:
<TextBox Text="{Binding Name}" />
Rest of the stuff is in C++:
You add reference to your View and VM DLLs in your C++ project.
Create an object of your dialog. It will automatically instantiate its VM and perform binding. You call ShowDialog() to bring it to screen.
Use takes actions in the dialog and finally presses OK or Cancel.
You capture dialog result and then access your dialog object's DataContext property, which is an object of DialogVM class. You can access user-supplied values from therein as Binding has updated those properties for you in the VM.
I'm not sure I follow all of your requirements but this is roughly how I'd approach such a task:
Instantiate the view and viewmodel in class A.
Set whatever parameters you want on your viewmodel. Either as properties or via constructor injection.
Set the datacontext of the view to the viewmodel.
Everything you need to bind should then bind between them.
showdialog the view.
The user edits in the view and changes persist to the viewmodel properties.
They finish editing and you then work with the viewmodel properties. Maybe one of them is the model you mention. Maybe the model is instantiated by the viewmodel to get data or by classA if that is more convenient. In the latter case you probably have to pass that model to the viewmodel.
Bearing in mind the above.
Some rough code:
public class ClassA
{
MyDialogViewModel vm = new MyDialogViewModel { Name = "X", Roll = "Y" };
MyDialog dlg = new MyDialog();
dlg.ShowDialog();
var name = vm.Name;
var roll = vm.roll;
// Do something to persist your data as necessary. Either here or in a model within the viewmodel
}
Name and Roll presumably bind to some textboxes Text properties in the view or some such.
If it's as simple as obtaining two string values then I see no advantage to actually having a model at all. On the other hand, if processing is more involved then of course the viewmodel might instantiate a model.
MyDialogViewModel should implement inotifypropertychanged and anything you need to bind should be a public property. Not sure if you'll need propertychanged notification but always implement it. Optionally raise propertychanged from property setters.
I am developing a text editor with WPF which should also be usable for blind people.
For screenreaders to read the program interface, the Helptext property must be set for components.
In XAML, this is how it works:
AutomationProperties.HelpText = "Here the accessibility of the editor can be activated or deactivated"
But how do I set the property HelpText in C#?
Attached properties conventionally have a static method that is the property name prefixed with "Set" to update from code. In this case
SetHelpText(DependencyObject, value)
AutomationProperties.SetHelpText(ChkBarrierefrei, "Here the accessibility of the editor can be activated or deactivated")
As you've asked for an example on how to use bindings to achieve your goal, here is a simple one.
We first need to create a class that works as our base class for every so-called ViewModel.
public class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void RaisePropertyChanged([CallerMemberName] string propertyName)
{
this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
We can now derive specific ViewModels from that class, namely the one we want to use to hold the BarriereFrei Help Text.
public class MyViewModel : ViewModelBase
{
private string barriereFreiHelpText = "Here the accessibility of the editor can be activated or deactivated.";
public string BarriereFreiHelpText
{
get { return this.barriereFreiHelpText; }
set
{
if (value == this.barriereFreiHelpText)
{
return;
}
this.barriereFreiHelpText = value;
this.RaisePropertyChanged(); // This line makes sure the UI is updated
// whenever a new help text is set.
}
}
}
In the View (I assumed it's called MainWindow) we can now use the previously defined ViewModel like shown below:
<Window x:Class="MyProject.Views.MainWindow"
xmlns:viewModel="clr-namespace:MyProject.ViewModels">
<Window.DataContext>
<viewModel:MyViewModel />
</Window.DataContext>
<Grid>
<CheckBox x:Name="ChkBarrierefrei"
Content="Editor ba_rrierefrei"
ToolTip="Hier kann die Barrierefreiheit des Editors aktiviert oder deaktiviert werden"
AutomationProperties.HelpText="{Binding BarriereFreiHelpText}"
Margin="10"
Click="ChkBarrierefrei_CheckedChanged"/>
</Grid>
</Window>
Did you notice the
AutomationProperties.HelpText="{Binding BarriereFreiHelpText}"
part? This is where the magic happens. What this line does is it ensures that the HelpText property is always equal to what you set in the ViewModels BarriereFreiHelpText property.
To make this all work correctly, somewhere in your application you need to wire up the ViewModel and the Window like this:
MyViewModel viewModel = new MyViewModel();
MainWindow window = new MainWindow();
window.DataContext = viewModel;
and you can now change the help text at runtime using
(window.DataContext as MyViewModel).BarriereFreiHelpText = "Whatever you want it to be";
and the UI is automatically updated with the new help text.
This is definitely a better solution than setting the CheckBoxes AutomationProperties.HelpText property directly, as it is much cleaner and less tightly coupled (you are changing the property of a ViewModel without knowing about any UI).
I understand that this all might be overwhelming, but I strongly recommend you follow the so-called MVVM approach, not only for the CheckBoxes help text, but for everything at all. You will save yourself much time, lines of code and nasty bugs.
Further reading:
MVVM und WPF (german) explains how to use MVVM and is way more detailed and easier to understand than my answer,
The World's Simplest C# WPF MVVM Example is a very nice, short and comprehendible article covering everything you need to know in order to create robust and reusable WPF applications,
MVVM Step by Step from Basic to Advanced another nice article, covering why MVVM makes sense.
I hope it's not to late to go with the MVVM approach, and I wish you lots of success and fun with MVVM - it really is nice and fun once you got a hang of it, believe me ;)
I am creating a user control (a textbox that only accepts integers). The control has to have properties to specify max/min values and whether to allow negative values etc.). I am using MVVM, in my view I have public properties e.g.
const string EXAMPLE = "Example";
string example;
public string Example
{
get { return example; }
set
{
if (value == example) return;
example = value;
OnPropertyChanged(EXAMPLE);
}
}
These properties are in my View so that someone using the control will be able to easily set them. In my ViewModel I have an identical property, I need these properties to be bound together so that they and their backing fields always have the same value. I hate the code repetition too.
To be honest the whole approach feels wrong and usually that is a good indication that I am approaching the whole thing from the wrong direction or misunderstanding something fundamental.
I have used WPF before but this is a first attempt at a custom control.
The first thing I want to make sure is that you're truly trying to make a CustomControl and not a UserControl. I believe this question basically is the same as yours except worded differently.
A UserControl lends itself to the MVVM pattern way more readily than a CustomControl because you would have a .xaml (and .xaml.cs) file along with a .cs file to serve as the ViewModel. On the other hand, a CustomControl is never done with MVVM, as the visual appearance (view) is defined and overridable via a ControlTemplate.
Since you said you have a View and ViewModel, let's think about how you would achieve the behavior you want with your textbox. Your textbox will have to validate and reject user input outside the range of values you desire. This means your View code-behind has to have properties and logic that control the restrictions in the input values of the textbox defined in your View. You have already violated MVVM here.
When you said you have a View, that makes me think you're writing a UserControl. But your requirements (a custom behavior for textbox) suggest that you really need a CustomControl, for which you do not use MVVM.
If you agree that you need a CustomControl, here's a quick and dirty example:
public class RestrictedTextBox : TextBox
{
public static readonly DependencyProperty MaxValueProperty = DependencyProperty.Register("MaxValue", typeof(int), typeof(RestrictedTextBox), new PropertyMetadata(int.MaxValue));
public RestrictedTextBox()
{
PreviewTextInput += RestrictedTextBox_PreviewTextInput;
}
public int MaxValue
{
get
{
return (int)GetValue(MaxValueProperty);
}
set
{
SetValue(MaxValueProperty, value);
}
}
private void RestrictedTextBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
{
int inputDigits;
RestrictedTextBox box = sender as RestrictedTextBox;
if (box != null)
{
if (!e.Text.All(Char.IsDigit))
{
// Stops the text from being handled
e.Handled = true;
}
else if (int.TryParse(box.Text + e.Text, out inputDigits))
{
if (inputDigits > MaxValue)
e.Handled = true;
}
}
}
}
XAML Usage:
<local:RestrictedTextBox MaxValue="100"></local:RestrictedTextBox>
There is no MVVM in a Custom Control.
Any Control is only in the View layer. So what you need to do is expose relevant DP to a consumer that you have no knowledge of.
In the Custom Control you need to define your Control behavior, how it reacts to change in DP value and what should be available to a consumer. In the default Template you define how you want to display this Control.
The consumer may want to set or get some dp values so he'll have to bind your Custom Control'dp to a property in his ViewModel but that's up to him.
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I don't even know if that's the right title for this but anyway. I'm switching from WinForms and trying to learn WPF and the MVVM methodology.
I have a custom class, Incident, used to store data about incidents that occur that my team must respond to. I am building a View to display the data in instances of that class to a user, as well as allow that user to manipulate it. There are several pieces of DateTime data that need to be displayed - Start, End, Notification_Received, Actions_Taken. I need to have a small button that will put DateTime.Now into each associated TextBox as well as change the underlying value of the currently loaded instance of the Incident class.
I'm trying to figure out how to do this. With WinForms, I would've just set TextBox.Text and Incident.Start (etc) to DateTime.Now all in the same Button_Click function, but my understanding of MVVM is that I'm not supposed to do this, instead I should bind the TextBox to the VM and update the value of the VM.
This is where I'm stuck. I'm pretty sure I'm good on how to do the binding, but not the part where I change the value of the VM from my Button_Click function. Please assist?
You're correct - the view-model should control the change, and the textbox should update through a binding.
In the MVVM pattern, code-behind is rarely used. Instead of a Button_Click method, you need a command binding:
<Button Command="{Binding SetAllDatesToNowCommand}"/>
The command will be executed when the button is pressed. SetAllDatesToNowCommand is a command handler - it should be an ICommand property on your view-model:
public ICommand SetAllDatesToNowCommand { get; private set; }
I generally tend to use RelayCommand from the MVVM Light Toolkit to create command handlers, because the syntax is clean and very simple. The command handler is initialized in your view-model's constructor. The handler method passed to RelayCommand is where you should set properties on the selected Incident object:
public YourViewModel()
{
this.SetAllDatesToNowCommand =
new RelayCommand(this.ExecuteSetAllDatesToNowCommand);
}
...
public void ExecuteSetAllDatesToNowCommand()
{
this.selectedIncident.Start = DateTime.Now;
// etc.
}
If the bindings on your textboxes are correctly set up, and the properties that are being set are firing appropriate PropertyChanged events, they should be updated when the properties are set in the command execution method.
However, I'd suggest that you should have a view-model for Incident, which implements the INotifyPropertyChanged interface. The command outlined above would be a property on that view-model. Setting, for example, the Start property on that view-model should set the property on the Incident object it is the view-model for (the "model" object), and should also raise a PropertyChanged event. Otherwise, your Incident class will have to implement INotifyPropertyChanged, and the line between model and view-model classes becomes less clear.
I assume you've bound the form to your ViewModel. Therefore you have a property in your ViewModel of Start. You want to bind a field to that
<TextBlock Text={Binding Start}/>
or
<TextBlock Text={Binding Incident.Start}/>
Depending on how you expose Incident. To update Start you have to do two things. Use a command on the button.
<Button Command="{Binding TestCommand}">Test</Button>
In your ViewModel you'd define the command.
private RelayCommand _testCommand;
public RelayCommand TestCommand
{
get
{
return _testCommand ?? (_testCommand = new RelayCommand(TestUpdate, CanRunTest));
}
set
{
if (_testCommand == value) return;
_testCommand = value;
}
}
public bool CanRunTest()
{
return some boolean test that defines if the command can run now;
}
private void TestUpdate()
{
Incident.Start = DateTime.Now;
}
RelayCommand is a helper method you can find in MVVMLight. Also see Josh Smith for info on RelayCommand.
Second, you will need to implement INotifyPropertyChanged on the Incident model or look up ObservableObject and make your Start property look like this.
public class Incident : ObservableObject
{
private ObservableCollection<WordLetter> _start;
public virtual ObservableCollection<WordLetter> Start
{
get { return _start; }
set
{
if (value == _start) return;
_start = value;
NotifyPropertyChanged();
}
}
}
I"m trying to wrap my head around MVVM. I understand a lot of it, but I'm having difficulty grasping one aspect: Setting DataContext.
I want to show an view with a particular object. The user doesn't get to decide what is visible, so I need to create the view in code. Then, I want to set the DataContext of the view to an object (for binding). I'm trying not to put code in the View for this, but View.LayoutRoot.DataContext isn't public.
What am I missing?
trying to avoid this:
public class View
{
public View(object dataContext)
{
InitializeComponent();
LayoutRoot.DataContext = dataContext;
}
}
with something like this:
public class ViewModel
{
...
public UIElement GetView()
{
UIElement *element = new View();
element.LayoutRoot.DataContext = element;
return element;
}
}
Don't forget that the View should know about the ViewModel, and not the other way around.
So in your case puting code in the ViewModel to create the view isn't the best way around.
Josh Smith's article on MVVM has a section on applying the View to the ViewModel. He recommends using WPF's DataTemplates to select your View in XAML.
If you use a XAML control or Window (which should be the case if you use MVVM), LayoutRoot (Grid by default) is public. In your example, you use just a plain class for View, so it is hard to tell what is going on.
Also, I second Cameron's opinion - nor View or ModelView should deal with assigning DataContext. It can be done in different ways (DataTemplate, dependency injection, special builder class, plain code) but normally it happens on the application level.