I have a WPF window with controls I wish to bind to my model. The model implements INotifyPropertyChanged to notify the view when the Properties change value. The Properties are primitives backed by fields, e.g:
private bool m_isRunning;
public bool IsRunning
{
get { return m_isRunning; }
private set
{
m_isRunning= value;
OnPropertyChanged("IsRunning");
}
}
protected virtual void OnPropertyChanged(string propertyName)
{
if (String.IsNullOrEmpty(propertyName))
{
return;
}
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
This works fine for primtiive values. I also have a composite object with various primitive properties and a hash table. Or, I'd like to bind to an ObservableCollection's Count property. I would like to bind my View controls to properties within the composite object, but I do not think this is possible. Something like:
<Run Text="{Binding Path=CompositeObject.SomeInnerProperty, Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" />
This doesn't seem possible, but is there some other way? I feel like my only option is to expose properties that map in to the inner property of the composite object, but this seems like a lot of repetiion, e.g.:
public bool SomeInnerProperty
{
get { return m_myComposite.SomeInnerProperty; }
private set
{
m_myComposite.SomeInnerProperty= value;
OnPropertyChanged("SomeInnerProperty");
}
}
There's nothing wrong with binding to something like CompositeObject.SomeInnerProperty, however if CompositeObject does not implement INotifyPropertyChanged, then your UI won't get notified of the change and know that it needs to update when SomeInnerProperty changes.
Also, note that you can only bind to properties (with get and set methods), and not fields. So you can bind to public string SomeValue { get; set; } but you can't bind to public string SomeValue;
In regards dealing with repetitive code, I personally use some Visual Studio macros to write my public properties for me, so perhaps you could look into doing something like that if you don't want to implement INotifyPropertyChanged on your CompositeObject class. It should be noted that Macros were removed from Visual Studio 2012 though, so if you have a newer version you might need to use some other alternative like creating an add-in to run your macros
I had this same problem some time ago. Look at how I solved it:
MVVM INotifyPropertyChanged conflict with base class PropertyChange
Basically I created a Base class that implemented INotifyPropertyChanged and I made all my classes inherit from that base class and data binding worked fine.
You have two options:
If your model classes do not implement INPC, then create wrapper properties in your ViewModel like you suggested, or
Implement INPC in your model and just expose your main object in the ViewModel, you can bind as deep as you want as long as inner properties notify changes.
Related
I am very new to WPF, although I have already heard some concepts of it, like Model-View-ViewModel. I want to start learning WPF by doing a small program.
In the program I have a TreeView to display directories (e.g. My Documents, Computer etc.). I can easily develop a class for the data, which is the DirectoryModel
class DirectoryModel : INotifyPropertyChanged
{
private DirectoryInfo directory;
private FileInfo file;
private DirectoryModel(DirectoryInfo directory)
{
//...
}
private DirectoryModel(FileInfo file)
{
//...
}
public static DirectoryModel[] RootDirs()
{
//...
}
public DirectoryModel[] SubDirs()
{
//...
}
public DirectoryModel[] Files()
{
//...
}
public override string ToString()
{
if (this.directory != null)
return this.directory.Name;
else
return this.file.Name;
}
public event PropertyChangedEventHandler PropertyChanged;
}
Now I want to bind this model to the tree view. I have read several articles about binding data, from MSDN and CodeProject, but I still don't catch the key! I read something about creating a binding in XAML, and there one just writes something like <c:MyData x:Key="myDataSource"/>, but where is this myDataSource come from? As the first letter is not capitalized, I assume it is a variable of some type, but there is no definition or declaration or mention about this.
I have read this article, but there are many things not clear to me (like ItemTemplate, ItemContainerStyle etc.). My feeling is, it seems there are many different ways to do the binding, and different blogs or article use different approaches.
So my question is, how do I define a model class, and create a varible of that class and bind it to a control (here tree view)? Or I want to know, what is the standard approach to bind data in WPF, if there is any?
To start with you can only bind to the public properties (at the moment you only have methods in your VM). The properties should implement the property changed event.
In your View.xaml.cs in constructor:
this.DataContext = new DirectoryModel ();
and obviously in your view.xaml you have to create the bindings e.g.
<TextBlock Text="{Binding MyPropertyFromViewModel}"/>
I'm using the MVVM pattern in our WPF application to allow for comprehensive unit testing. The MVVM pattern itself is working great, however I'm struggling to adapt the pattern in a way that means I can use the design-time data support of WPF.
As I'm using Prism the ViewModel instances are generally injected into the constructor of the view, like so
public MyView(MyViewModel viewModel)
{
DataContext = viewModel;
}
Dependencies for the ViewModel are then injected into the constructor, like so
public class MyViewModel
{
public MyViewModel(IFoo foo, IBar bar)
{
// ...
}
// Gets and sets the model represented in the view
public MyModel { get; set; }
// Read-only properties that the view data binds to
public ICollectionView Rows { get; }
public string Title { get; }
// Read-write properties are databound to the UI and are used to control logic
public string Filter { get; set; }
}
This is generally working really well except when it comes to design data - I wanted to avoid compiling design-data specific classes into my released assembly and so I opted to use the {d:DesignData} approach instead of the {d:DesignInstance} approach, however in order for this to work correctly my ViewModel now needs to have a parameterless constructor. In addition, I also often need to change additional properties either to have setters or to be modifiable collections in order to be able to set these properties in XAML.
public class MyViewModel
{
public MyViewModel()
{
}
public MyViewModel(IFoo foo, IBar bar)
{
// ...
}
// Gets and sets the model represented in the view
public MyModel { get; set; }
// My read-only properties are no longer read-only
public ObservableCollection<Something> Rows { get; }
public string Title { get; set; }
public string Filter { get; set; }
}
This is worrying me:
I have a parameterless constructor that is never intended to be called and isn't unit tested
There are setters for properties that only the ViewModel itself should be calling
My ViewModel is now a jumbled mixture of properties that should be modified by the view, and those that shouldn't - this makes it tricky to tell at a glance which piece of code is responsible for maintaining any given property
Setting certain properties at design time (e.g. to see styling on the Filter text) can actually end up invoking ViewModel logic! (so my ViewModel also needs to be tollerant of otherwise mandatory dependencies being missing at design time)
Is there a better way to get design-time data in a WPF MVVM application in a way that doesn't compromise my ViewModel in this way?
Alternatively should I be building my ViewModel differently so that it has more simple properties with the logic separated out somewhere else.
First, I would recommend you to have a look at this video where Brian Lagunas provides several best practices about MVVM. Brian is - at least - involved in the development of Prism, as his name appears in the nuget packages information. Didn't check further.
On my side I only use bits of Prism, and my Model and ViewModel always offer blank constructors (like what Brian shows), the data context is assigned in the view's XAML, and I set the properties values like :
<MyView.DataContext>
<MyViewModel />
</MyView.DataContext>
and
public void BringSomethingNew()
{
var myView = new View();
(myView.DataContext as ViewModel).Model = myModel;
UseMyView();
}
Another benefit with this approach is that the ViewModel is created once, with the same path at design and run time, so you create less objects and save GC efforts. I find this elegant.
With regards to the setters, the design data will still work if you make them private, like:
public string MyProp { get; private set; }
Ok, customize it to manage NotifyPropertyChange at your convenience, but you've got the idea.
Now, I don't have yet a solution to manage ObesrvableCollections (I face the same problem, although putting multiple values in XAML sometimes work... ???), and yes, I agree that you have to manage the case when the properties are not set, like setting default values in the constructor.
I hope this helps.
I too have worked with NUnit testing with WPF and MVVM implementation. However, my version is reversed from yours. You are creating the view first, then creating the model to control it.
In my version, I create the MVVM model FIRST and can unit test it till the cows come home and not worry about any visual design... if the model is broken, so too will the visual implementation.
in my MVVM model, I have a method to "GetTheViewWindow". So, when I derive from my MVVM baseline, each view model has its own view its responsible for. So via a virtual method, each instance will do its own new view window when being applied for production.
public class MyMVVMBase
{
private MyViewBaseline currentView;
public MyMVVMBase()
{ // no parameters required }
public virtual void GetTheViewWindow()
{ throw new exception( "You need to define the window to get"; ) }
}
public class MyXYZInstanceModel : MyMVVMBase
{
public override void GetTheViewWindow()
{
currentView = new YourActualViewWindow();
}
}
Hope this helps as an alternative to what you are running into.
I understand that ViewModel shouldn't have any knowledge of View, but how can I call MediaElement.Play() method from ViewModel, other than having a reference to View (or directly to MediaElement) in ViewModel?
Other (linked) question: how can I manage View's controls visibility from ViewModel without violating MVVM pattern?
1) Do not call Play() from the view model. Raise an event in the view model instead (for instance PlayRequested) and listen to this event in the view:
view model:
public event EventHandler PlayRequested;
...
if (this.PlayRequested != null)
{
this.PlayRequested(this, EventArgs.Empty);
}
view:
ViewModel vm = new ViewModel();
this.DataContext = vm;
vm.PlayRequested += (sender, e) =>
{
this.myMediaElement.Play();
};
2) You can expose in the view model a public boolean property, and bind the Visibility property of your controls to this property. As Visibility is of type Visibility and not bool, you'll have to use a converter.
You can find a basic implementation of such a converter here.
This related question might help you too.
For all the late-comers,
There are many ways to achieve the same result and it really depends on how you would like to implement yours, as long as your code is not difficult to maintain, I do believe it's ok to break the MVVM pattern under certain cases.
But having said that, I also believe there is always way to do this within the pattern, and the following is one of them just in case if anyone would like to know what other alternatives are available.
The Tasks:
we don't want to have direct reference from the ViewModel to any UI elements, i.e. the the MediaElement and the View itself.
we want to use Command to do the magic here
The Solution:
In short, we are going to introduce an interface between the View and the ViewModel to break the dependecy, and the View will be implementing the interface and be responsible for the direct controlling of the MediaElement while leaving the ViewModel talking only to the interface, which can be swapped with other implementation for testing purposes if needed, and here comes the long version:
Introduce an interface called IMediaService as below:
public interface IMediaService
{
void Play();
void Pause();
void Stop();
void Rewind();
void FastForward();
}
Implement the IMediaService in the View:
public partial class DemoView : UserControl, IMediaService
{
public DemoView()
{
InitializeComponent();
}
void IMediaService.FastForward()
{
this.MediaPlayer.Position += TimeSpan.FromSeconds(10);
}
void IMediaService.Pause()
{
this.MediaPlayer.Pause();
}
void IMediaService.Play()
{
this.MediaPlayer.Play();
}
void IMediaService.Rewind()
{
this.MediaPlayer.Position -= TimeSpan.FromSeconds(10);
}
void IMediaService.Stop()
{
this.MediaPlayer.Stop();
}
}
we then do few things in the DemoView.XAML:
Give the MediaElement a name so the code behind can access it like above:
<MediaElement Source="{Binding CurrentMedia}" x:Name="MediaPlayer"/>
Give the view a name so we can pass it as a parameter, and
import the interactivity namespace for later use (some default namespaces are omitted for simplicity reason):
<UserControl x:Class="Test.DemoView"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:ia="http://schemas.microsoft.com/expression/2010/interactivity"
x:Name="MediaService">
Hookup the Loaded event through Trigger to pass the view itself to the view model through a Command
<ia:Interaction.Triggers>
<ia:EventTrigger EventName="Loaded">
<ia:InvokeCommandAction Command="{Binding LoadedCommand}" CommandParameter="{Binding ElementName=MediaService}"></ia:InvokeCommandAction>
</ia:EventTrigger>
</ia:Interaction.Triggers>
last but not least, we need to hookup the media controls through Commands:
<Button Command="{Binding PlayCommand}" Content="Play"></Button>
<Button Command="{Binding PauseCommand}" Content="Pause"></Button>
<Button Command="{Binding StopCommand}" Content="Stop"></Button>
<Button Command="{Binding RewindCommand}" Content="Rewind"></Button>
<Button Command="{Binding FastForwardCommand}" Content="FastForward"></Button>
We now can catch everything in the ViewModel (I'm using prism's DelegateCommand here):
public class AboutUsViewModel : SkinTalkViewModelBase, IConfirmNavigationRequest
{
public IMediaService {get; private set;}
private DelegateCommand<IMediaService> loadedCommand;
public DelegateCommand<IMediaService> LoadedCommand
{
get
{
if (this.loadedCommand == null)
{
this.loadedCommand = new DelegateCommand<IMediaService>((mediaService) =>
{
this.MediaService = mediaService;
});
}
return loadedCommand;
}
}
private DelegateCommand playCommand;
public DelegateCommand PlayCommand
{
get
{
if (this.playCommand == null)
{
this.playCommand = new DelegateCommand(() =>
{
this.MediaService.Play();
});
}
return playCommand;
}
}
.
. // other commands are not listed, but you get the idea
.
}
Side note: I use Prism's Auto Wiring feature to link up the View and ViewModel. So at the View's code behind file there is no DataContext assignment code, and I prefer to keep it that way, and hence I chose to use purely Commands to achieve this result.
I use media element to play sounds in UI whenever an event occurs in the application. The view model handling this, was created with a Source property of type Uri (with notify property changed, but you already know you need that to notify UI).
All you have to do whenever source changes (and this is up to you), is to set the source property to null (this is why Source property should be Uri and not string, MediaElement will naturally throw exception, NotSupportedException I think), then set it to whatever URI you want.
Probably, the most important aspect of this tip is that you have to set MediaElement's property LoadedBehaviour to Play in XAML of your view. Hopefully no code behind is needed for what you want to achieve.
The trick is extremely simple so I won't post a complete example. The view model's play function should look like this:
private void PlaySomething(string fileUri)
{
if (string.IsNullOrWhiteSpace(fileUri))
return;
// HACK for MediaElement: to force it to play a new source, set source to null then put the real source URI.
this.Source = null;
this.Source = new Uri(fileUri);
}
Here is the Source property, nothing special about it:
#region Source property
/// <summary>
/// Stores Source value.
/// </summary>
private Uri _Source = null;
/// <summary>
/// Gets or sets file URI to play.
/// </summary>
public Uri Source
{
get { return this._Source; }
private set
{
if (this._Source != value)
{
this._Source = value;
this.RaisePropertyChanged("Source");
}
}
}
#endregion Source property
As for Visibility, and stuff like this, you can use converters (e.g. from bool to visibility, which you can find on CodePlex for WPF, SL, WP7,8) and bind your control's property to that of the view model's (e.g. IsVisible). This way, you control parts of you view's aspect. Or you can just have Visibility property typed System.Windows.Visibility on your view model (I don't see any pattern breach here). Really, it's not that uncommon.
Good luck,
Andrei
P.S. I have to mention that .NET 4.5 is the version where I tested this, but I think it should work on other versions as well.
In my .NET 4.0 project I've got an object that has public fields and this object neither implements INotifyPropertyChanged nor inherits DependencyObject, and it will never do. However, I need a mechanism to "bind" to fields of this object in my WPF control. I know I can't do it directly as binding requires a dependency property (or at least, properties and notifying property changes), so what can I do to implement the binding functionality I need. I've tried something like this in my WPF control:
void FirePropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
public float Friction
{
get
{
if (CurrentObject != null)
{
return CurrentObject.Friction;
}
else
{
return 0.0f;
}
}
set
{
if (CurrentObject != null)
{
CurrentObject.Friction = value;
FirePropertyChanged("Friction");
}
}
}
public PlatformObjectTemplate CurrentObject
{
get
{
return GetValue(CurrentObjectProperty) as PlatformObjectTemplate;
}
set
{
SetValue(CurrentObjectProperty, value);
FirePropertyChanged("Friction");
FirePropertyChanged("CurrentObject");
BindShapes();
IntersectionComboBox.SelectedItem = CurrentObject.IntersectionStaticMethod;
}
}
public static readonly DependencyProperty CurrentObjectProperty = DependencyProperty.Register("CurrentObject", typeof(PlatformObjectTemplate), typeof(PlatformStaticObjectPropertyEditor), new PropertyMetadata(null));
My WPF control implements INotifyPropertyChanged, and my PlatformObjectTemplate does not have properties, just public fields like Friction. I need to bind to my object in XAML as such:
(in my control): //DoubleUpDown is from the WPF toolkit.
<tk:DoubleUpDown Margin="91,10,7,0" Name="doubleUpDown1" VerticalAlignment="Top" Value="{Binding Friction, ElementName=window, FallbackValue=0}" />
(in my main window):
<my:PlatformStaticObjectPropertyEditor x:Name="platformStaticObjectPropertyEditor1" CurrentObject="{Binding CurrentObject, ElementName=window}" />
I put a breakpoint in the getter of Friction property, and it tries to bind before the CurrentObject is bound, and because it is null, I can't read the correct friction value from the object. I've tried to fire Friction property changed in the setter of the CurrentObject, to populate the Friction when CurrentObject gets set, but that doesn't work either.
Ok, here are two requirements:
PlatformObjectTemplate will not use properties. It will have public fields.
I need a declarative way of binding as usual, just as I used in the XAML above.
I probably have got things over-complicated, and I must be missing some stuff. What is the most "correct" and "declarative" way of doing this right, within the constraints of my requirements just above?
Thanks,
Can.
object neither implements INotifyPropertyChanged nor inherits DependencyObject, and it will never. However, I need a mechanism to "bind" to fields of this object in my WPF control
poyra, I have this same situation. Because one cannot bind to instance fields, your best option is to create wrapper classes which implement INotifyPropertyChanged.
Do folks have any guidance on when a simple .NET property that fires INotifyPropertyChanged.PropertyChanged is sufficient in a view model? Then when do you want to move up to a full blown dependency property? Or are the DPs intended primarily for views?
There are a few approaches:
1. The dependency property
While you using the dependency property it makes the most sense in elements-classes that have a visual appearance (UIElements).
Pros:
WPF do the logic stuff for you
Some mechanism like animation use only dependency property
'Fits' ViewModel style
Cons:
You need to derive form DependencyObject
A bit awkward for simple stuff
Sample:
public static class StoryBoardHelper
{
public static DependencyObject GetTarget(Timeline timeline)
{
if (timeline == null)
throw new ArgumentNullException("timeline");
return timeline.GetValue(TargetProperty) as DependencyObject;
}
public static void SetTarget(Timeline timeline, DependencyObject value)
{
if (timeline == null)
throw new ArgumentNullException("timeline");
timeline.SetValue(TargetProperty, value);
}
public static readonly DependencyProperty TargetProperty =
DependencyProperty.RegisterAttached(
"Target",
typeof(DependencyObject),
typeof(Timeline),
new PropertyMetadata(null, OnTargetPropertyChanged));
private static void OnTargetPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
Storyboard.SetTarget(d as Timeline, e.NewValue as DependencyObject);
}
}
2. The System.ComponentModel.INotifyPropertyChanged
Usually, when creating a data object, you’ll use this approach. It is simple and neat solution for Data-like stuff.
Pros and Cons - complementary to 1. You need to to implement only one event (PropertyChanged).
Sample:
public class Student : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(PropertyChangedEventArgs e)
{
if (PropertyChanged != null)
PropertyChanged(this, e);
}
}
private string name;
public string Name;
{
get { return name; }
set {
name = value;
OnPropertyChanged(new PropertyChangedEventArgs("Name"));
}
}
3.PropertyNameChanged
Rising an event for each property with specified name(f.e. NameChanged). Event must have this name and it is up to you to handle/rise them. Similar approach as 2.
4. Get the binding
Using the FrameworkElement.GetBindingExpression() you can get the BindingExpression object
and call BindingExpression.UpdateTarget() to refresh.
First and second are the most likely depending what is your goal.
All in all, it is Visual vs Data.
As far as I know, DependencyProperty is only required when you need
PropertyValue inheritence
you need to allow the property to be set in Style setters
Use animation for the property
etc.
These features will not be available with normal properties.
DependencyProperty is required if you want to allow a binding to be set on the property. Usually this is for custom UIElements you create. You want to allow people to be able to bind data to your UIElements.
<local:MyUIElement MyProperty={Binding Path=SomethingToBindTo} />
To do this requires that MyProperty is a dependancy property
The main problem I see with INotifyPropertyChanged is if you viewmodel is complex containing many nested types it appears that you have to bubble the PropertyChanged event up through the hierarchy.
As the other answers have already said enough about when to create dependency property. i.e.
PropertyValue inheritence
you need to use binding on a property
Use animation for the property
The one more perspective/question on this is "In a WPF application is makes sense to create dependency properties in a control cause they are likely to change during user interaction like Height,width, text,content, background etc but what about other classes like Behaviors Classes(Non UI classes). Do properties in those classes need to be a dependency property?"
I won't say for very absolute or emphasis on some set of rules here but you should create your properties as DP. As from design perspective if a property is DP it's always in default form of WPF to use/bind.i.e.
As a DP is much more fast/natural in reflecting changes compare to a normal CLR property.
A DP has validation mechanism to validate the value assigned and a default structure to revert the value.
A DP has Coerce value callback to control the limits of property.
A DP has meta data associated with it unlike CLR property.
In terms of practices I've seen people doing many mistakes in nested bindings and then raising changes these kind of faults doesn't happen with a DP cause of it's design and compatibility of raising change itself. So with a little extra syntax you put a flexibility/performance/ easeness to your application. So go for it wherever affordable.
Still can't say sure for ViewModel classes/other helper classes. will update the answer if found convincing reasons in future.
Just a post worth reading on this topic