There seems to be conflicting thoughts on whether INotifyPropertyChanged should be implemented in the Model or not. I think that it should be implemented in the ViewModel, but I can't figure out how it would be accomplished. There are plenty of mentions of this same idea all over stackoverlow.com ( In MVVM model should the model implement INotifyPropertyChanged interface?, In MVVM should the ViewModel or Model implement INotifyPropertyChanged?), but I can't find any example to show how to do it.
Let's say for example I have a model Person:
Public Person {
public int Age { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public void NameChange( string newName );
}
How would I implement the ViewModel so that changes in Age, FirstName, or LastName are all recognized?
Public PersonViewModel : INotifyPropertyChanged {
Person _person;
public event PropertyChangedEventHandler PropertyChanged;
void OnPropertyChanged(string propertyName) {
if(this.PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
//ctor, Properties, etc...
}
EDIT - Clarification:
So without changing the Person model how do I modify the ViewModel to get notified of the updates?
Is that even possible? If not, how are those that subscribe to the "INPC in the model is baaaad" get notified of changes in the model?
ViewModel should definitely implement INotifyPropertyChanged. I don't have a strong opinion on whether it should be implemented in the Model as well. I don't think you need it when the model properties don't change independently from the ViewModel while it is bound to the View.
Anyway, this is how I'd implement INotifyPropertyChanged in the ViewModel when it is not already implemented in the Model:
public class PersonViewModel : INotifyPropertyChanged
{
private Person person;
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string propertyName)
{
if(PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public PersonViewModel(Person person)
{
this.person = person;
}
public int Age
{
get { return person.Age; }
set
{
if (value != person.Age)
{
person.Age = value;
OnPropertyChanged("Age");
}
}
}
public string FirstName
{
get { return person.FirstName; }
set
{
if (value != person.FirstName)
{
person.FirstName = value;
OnPropertyChanged("FirstName");
}
}
}
public string LastName
{
get { return person.LastName; }
set
{
if (value != person.LastName)
{
person.LastName = value;
OnPropertyChanged("LastName");
}
}
}
}
Seeing how you updated you question, I need to add that without having INotifyPropertyChanged (or a similar custom notification event) implemented in the model, you can't get notified about the changes in the model that happen in it independently from the ViewModel. I guess you should be able to avoid that. Otherwise just implement INotifyPropertyChanged in it. There's nothing wrong with that if you need it.
Interesting question. I've read about MVVM for more than a year now, and I'm still not sure about it.
If your application is representing a state of a process for example, and this state is modified internally without any interaction of the user, then your model needs to be able to notify your viewmodel that it changed.
So if your model implement INotifyPropertyChanged, and your viewmodel only pass the same informations to the view, then... does your viewmodel really need to exist...?
In our company, we consider two main cases:
We structure our software with a quite strict UML analysis before developping (not so agile). When we then want to display our objects on screen, they return us their different views, which are used when needed with Bindings (using ContentControl or so). Most of the views we need for our software display these kinds of object, that implement INotifyPropertyChanged and are therefore also kind of ViewModels.
To build the software main Views (view structure), we create global views and ViewModels for them. That's when we really follow the MVVM practices.
Maybe I missed a point about MVVM, but in my experience, it's not a pattern that you absolutely have to always follow. It's a very good way of thinking to develop WPF applications, but creating ViewModels for each and every view seems to me like a big overhead.
What do all of you think of this way of doing?
Best regards,
Antoine
EDIT 31.03.2012
I have found a very interesting article explaining how to handle your model properties in the viewmodel, without having to implement a proxy property in the viewModel for each one of them.
Also the writer say some words about having INPC implemented in the model, and the viewmodel listening to it.
I think this is the most practical oriented article I've read about MVVM so far.
Check it out :
http://msdn.microsoft.com/en-us/magazine/ff798279.aspx
In my experience, Model objects don't have to (and probably shouldn't) know that they are being constructed in a View. Often, Model objects are entities that should never be allowed to be in an invalid state. ViewModel objects are the things that construct the Model objects.
So, since you never want to create a person who is very old or very young, and every person needs a name, your Person class might look like this:
public class Person {
public int Age { get; private set; }
public string Name { get; private set; }
public Person(int age, string name) {
if (age < 0 || age > 150) throw new ArgumentOutOfRangeException();
if (string.IsNullOrEmpty(name)) throw new ArgumentNullException();
Age = age;
Name = name;
}
}
And your PersonViewModel might look like this::
class PersonViewModel : INotifyPropertyChanged {
private int _Age;
private int _Name;
public int Age {
get { return _Age; }
set {
if (_Age.Equals(value)) return;
_Age = value;
RaisePropertyChanged("Age");
}
}
public string Name {
get { return _Name; }
set {
if (_Name.Equals(value)) return;
_Name = value;
RaisePropertyChanged("Name");
}
}
public Person CreatePerson() {
return new Person(_Age, _Name);
}
}
You can then put whatever values you want in your PersonViewModel without worrying about creating an invalid Person object. You can also implement validation in the PersonViewModel that may be more strict than the validation in the Person class (for example, restricting the age range to adults over 18 years old (see IDataErrorInfo)).
Save for the typos you pretty much have it ;)
All you would need to add is your constructor and property definitions:
public class PersonViewModel : INotifyPropertyChanged
{
Person _person;
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
var handler = PropertyChanged;
if (handler != null)
{
var e = new PropertyChangedEventArgs(propertyName);
handler(this, e);
}
}
public PersonViewModel(Person person)
{
_person = person;
}
public int Age
{
get
{
return _person.Age;
}
set
{
_person.Age = value;
OnPropertyChanged("Age");
}
}
}
If you have a choice, I would definitely recommend implementing INotifyPropertyChanged in the Model because you won't havae to worry about translating Models to ViewModels and back.
But if you can't, see above :)
Related
If I wanted to create a Attribute (derived from System.Attribute) that hooks into the .NET Build process and translates/converts a standard C# auto property like:
[Notify]
public string Name { get; set; }
to this code, which then is compiled:
private string _nameField;
public string Name
{
get => _nameField;
set
{
if (!EqualityComparer<string>.Default.Equals(_nameField, value))
{
_nameField = value;
NotifyPropertyChanged(nameof(Name));
}
}
}
How would I achive it? What would I have to do?
How can I let the attribute hook into the Build?
As you can see I have no clues at all of the Build process, nor of Roslyn.
But I want to get rid of superfluous MVVM boilerplate code and no longer spent too much time for dull repetitive typing....
Thx, Chris
There's a Fody weaver for this, called PropertyChanged.
Fody is a system for modifying your code at the end of the compile process. The PropertyChanged weaver automatically impements change notification for all properties in all classes that implement INotifyPropertyChanged. It has a number of ways to control the generation using attributes, implementing methods, etc.
The example from their GitHub project page starts like this:
public class Person : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public string GivenNames { get; set; }
public string FamilyName { get; set; }
public string FullName => $"{GivenNames} {FamilyName}";
}
The weaver interprets the above and generates code like this:
public class Person : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
string givenNames;
public string GivenNames
{
get => givenNames;
set
{
if (value != givenNames)
{
givenNames = value;
OnPropertyChanged(InternalEventArgsCache.GivenNames);
OnPropertyChanged(InternalEventArgsCache.FullName);
}
}
}
string familyName;
public string FamilyName
{
get => familyName;
set
{
if (value != familyName)
{
familyName = value;
OnPropertyChanged(InternalEventArgsCache.FamilyName);
OnPropertyChanged(InternalEventArgsCache.FullName);
}
}
}
public string FullName => $"{GivenNames} {FamilyName}";
protected void OnPropertyChanged(PropertyChangedEventArgs eventArgs)
{
PropertyChanged?.Invoke(this, eventArgs);
}
}
internal static class InternalEventArgsCache
{
internal static PropertyChangedEventArgs FamilyName = new PropertyChangedEventArgs("FamilyName");
internal static PropertyChangedEventArgs FullName = new PropertyChangedEventArgs("FullName");
internal static PropertyChangedEventArgs GivenNames = new PropertyChangedEventArgs("GivenNames");
}
Of course you won't have access to that version, since it happens somewhere around the end of the compilation pass and your source is unaffected. Debugging the code can be a little difficult.
To help with that the PropertyChanged weaver looks for pre-implemented methods matching various rules and generates code to call those. If you have an OnPropertyChanged method in the class it'll call that instead of generating a boilerplate version. Or you can add an OnFamilyNameChanged method to the class above and that will be called before the OnPropertyChanged method.
You can use Roslyn source generators for that: Source Generators Cookbook
There's an INotifyPropertyChanged example.
I've been using MVVM Light for a while now - it's extremely useful and almost always the first library I add to a new project!
I'm wondering what the implications would be of developing a class that implements INotifyPropertyChanged to encapsulate a bindable property (example below).
public class BindableProperty<T> : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private T mValue;
public T Value
{
get { return mValue; }
set
{
if (!EqualityComparer<T>.Default.Equals(mValue, value))
{
mValue = value;
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs("Value"));
}
}
}
}
public BindableProperty(T default_value)
{
mValue = default_value;
}
}
With this class, I have to change my Xaml, but I believe my ViewModel might be more readable (below) - especially when the number of properties grows.
<TextBox Text="{Binding FirstName.Value, UpdateSourceTrigger=PropertyChanged}"/>
public class MainVM
{
public BindableProperty<string> FirstName { get; private set; }
public BindableProperty<string> LastName { get; private set; }
public MainVM()
{
FirstName = new BindableProperty<string>("");
LastName = new BindableProperty<string>("");
}
}
I know MVVM Light is designed to be extremely flexible, lightweight, and provide complete control (which it does very well). I can of course combine implementations and use the BindableProperty class above for some properties and more explicit ViewModelBase code for other properties in more complex situations.
Am I missing something obvious? What are some of the trade offs for this design that I might not be aware of (e.g. implications of changing xaml binding, data validation...)?
There is no extra value in encapsulating a property. This may work for very simple scenarios, but as soon as your ViewModel becomes more complex, you'll end up wiring your encapsulated classes in odd ways.
This approach won't work very well with validation neither it will if you have properties that depend on each other, i.e. FirstName, LastName and FullName where FullName is just a public string FullName { get { return FirstName+" "+LastName; } }.
Same applies for validation (with IDataErrorInfo). With the base class your code looks like
public string FirstName
{
get { return firstName; }
set
{
if(string.IsNullOrEmpty(value))
{
// where errors is a Dictionary<string, string>
errors.Add(nameof(FirstName), "First name can't be empty.");
return;
}
if(value.Length <2)
{
errors.Add(nameof(FirstName), "First name must be at least 2 characters long.");
return
}
Set(ref firstName, value);
errors.Remove(nameof(FirstName));
}
}
This will be a pain to implement in encapsulated properties
I am developing a ViewModel based on BindableBase.
This VM holds an instance of a domain model.
The VM exposes a property, say Name from which I want to not use local storage (i.e. storage in VM), but rather delegate to the model object's property.
I would like to use SetProperty(), but the storage reference cannot be a property.
Do I have to implement INotifyPropertyChanged my self ?
Is it at all a good idea to delegate to the model this way ?
Would it be possible to extend BindableBase (or team to add) to cover for this ?
So first you must chose how you will architect your VMs and Models. As you mentioned, there are a few options. The easiest and the way I recommend, is to just expose your Model as a property and then bind your View to the model properties:
public class MyViewModel : BindableBase
{
private Person _myPerson;
public Person Person
{
get { return _myPerson; }
set { SetProperty(ref _myPerson, value); }
}
}
If you don't want to do that and would rather wrap each individual model property, you would do it like this.
public class MyViewModel : BindableBase
{
private Person _myPerson;
private string _name;
public string Name
{
get { return _myPerson.Name; }
set { _myPerson.Name = value }
}
}
Keep in mind, your Person model object still has to implement INPC.
EDIT: If you don't have control over your models and need them to implement INPC, you could try to use IL weaving, or create a façade/decorator for your models and wrap them individually.
public class MyPersonFacade : BindableBase
{
private Person _myPerson;
private string _name;
public string Name
{
get { return _myPerson.Name; }
set
{
_myPerson.Name = value;
OnPropertyChanged();
}
}
}
Then use this as your Model in your VM.
I am creating the program-side architecture of a software developped in WPF, I designed the architecture as being compliant with the MVVM pattern.
For many sakes (design, coherence, reusability, maintainability, scalability, etc) I created the class BaseViewModel implementing the interface INotifyPropertyChanged and some other methods:
public class BaseViewModel: INotifyPropertyChanged
{
private PropertyChangedEventHandler property_changed;
public event PropertyChangedEventHandler PropertyChanged
{
add { property_changed += value; }
remove { property_changed -= value; }
}
//Here several methods using PropertyChanged and easing the usage of ViewModels
public BaseViewModel() { }
}
The above-defined class BaseViewModel is used as a base class for all the other ViewModels of the application (or, at least, is meant to be so), for example:
public class SampleViewModel : BaseViewModel
{
//private PropertyChangedEventHandler property_changed;
//public event PropertyChangedEventHandler PropertyChanged
//{
// add { property_changed += value; }
// remove { property_changed -= value; }
//}
public String Name
{
get { return name; }
set
{
if(value != name)
{
name = value;
var handler = PropertyChanged;
if(handler != null)
{
handler(this, new PropertyChangedEventArgs("Name"));
}
}
}
}
private String name = "";
public SampleViewModel ()
: base() { }
}
I use the class SampleViewModel as the DataContext of SampleUserControl which bares a DependencyProperty:
public partial class SampleUserControl : UserControl
{
#region ViewModel
public SampleViewModel ViewModel
{
get { return view_model; }
}
private SampleViewModel view_model = new SampleViewModel();
#endregion
#region DependencyProperty
public String Text
{
get { return (String)GetValue(TextProperty); }
set { SetValue(TextProperty, value); }
}
public static readonly DependencyProperty TextProperty =
DependencyProperty.Register("Text", typeof(String), typeof(SampleUserControl),
new FrameworkPropertyMetadata(String.Empty, FrameworkPropertyMetadataOptions.AffectsRender,
new PropertyChangedCallback(TextPropertyChangedCallback)));
private static void TextPropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
SampleUserControl sender = d as SampleUserControl;
if (sender != null)
{
sender.ViewModel.Name = (String)e.NewValue;
}
}
#endregion
public SampleUserControl()
{
InitializeComponent();
LayoutRoot.DataContext = ViewModel;
ViewModel.PropertyChanged += new PropertyChangedEventHandler(ViewModel_PropertyChanged);
}
void ViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
SampleViewModel viewmodel = sender as SampleViewModel;
if (viewmodel != null)
{
switch (e.PropertyName)
{
case "Name":
SetValue(TextProperty, viewmodel.Name);
break;
default:
break;
}
}
}
}
To sum up, the data relative to SampleUserControl are contained at three locations : the instance of SampleViewModel, within TextProperty and within the property Text of a TextBox in the xaml part of SampleUserControl(this property Text is twoway-bound through Binding with the field Name of ViewModel).
To synchronize the three values, I added the methods TextPropertyChangedCallback and ViewModel_PropertyChanged which update the fields which need to be updated.
The above code works and the three above-mentionned locations are kept up-to-date, events fire and so on, things are fine when SampleUsercontrol is consumed with data-binding.
But SampleViewModel fires the event BaseViewModel.PropertyChanged, and since BaseViewModel is meant to be extensively used, I would like each ViewModel to have its own event PropertyChanged, at least in order to avoid overlapping events.
So I uncomment the code of SampleViewModel thus redefining the event PropertyChanged but it breaks down the synchronization between the field Name of the instance of SampleViewModel and the property TextProperty of SampleUserControl.
Am I making some mistakes on the conception side?
Do you have any guidance for me?
What is the best economic way of defining a different event PropertyChanged for each ViewModel inheriting from BaseViewModel while still using the general-purpose methods defined within that base class (such methods use PropertyChanged)?
(I would like to avoid having heavy pieces of code to copy-paste.)
I know that it is more about optimization, but such optimizations can make a difference between a slow software and a fast one. I am at the stage of code-factoring, so I fancy nicely-shaped, elegant and factorized code.
End of the day happening, I may miss some obvious solutions.
Thanks in advance for any clue,
Julien
TL;DR: Basically, I would double-check that you are doing your DC/DP on that user control correctly, and toss out any concept of multiple definitions of PropertyChanged
In detail:
You defined PropertyChanged in the base class, which is great. There is no reason to ever redefine it anywhere else. Really, you are just asking for trouble by doing this.
Related to that, you should really just make a method to do the event invocation rather than doing the whole handler bit in the settter. Insta-reduction of copy paste.
The fact that you are having to use TextPropertyChanged is a huge red flag here. Which relates to the real problem, that you are probably abusing your dependency property. DPs are used to allow parent controls to bind to a property of your user control. You typically won't use them in conjunction with a data context internal to the control because, as you have seen, keeping them in sync is a nightmare.
In general, user controls should only have their own data context if they are set up to stand apart from any other control (ie, a sub-view). If they are just a fancy control, then giving them a view model rarely gets you anything.
I'm relatively new to MVVM and I'm trying to understand how INotifyPropertyChanged interface works and how to implement it in my models. The approach that I decided to take was to implement it in each of my Business Object classes. The problem with that approach is that when I bind my View to a property in a Base class the PropertyChanged event in that base class never gets initialized (is null) and therefore the View does not refresh the data for that element when my Model changes. I was able to reproduce the problem with the example below.
I have a Person Base class:
public class Person : INotifyPropertyChanged
{
#region INotifyProperty
public event PropertyChangedEventHandler PropertyChanged;
public void RaisePropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
#endregion
public String Name
{
get
{
return _name;
}
set
{
_name = value;
RaisePropertyChanged("Name");
}
}
private String _name;
}
And I have an Employee class inheriting from my Person Base class:
public class Employee : Person,INotifyPropertyChanged
{
#region INotifyProperty
public event PropertyChangedEventHandler PropertyChanged;
public void RaisePropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
#endregion
public String EmployeeID
{
get
{
return _employeeId;
}
set
{
_employeeId = value;
RaisePropertyChanged("EmployeeID");
}
}
private String _employeeId;
}
Here my View Model:
public class ViewModel : ViewModelBase<ViewModel>
{
private Employee _employee;
public ViewModel()
{
ChangeModelCommand = new RelayCommand(param=>this.ChangeModel() , param=>this.CanChangeModel);
Employee = new Employee()
{
Name = "BOB",EmployeeID = "1234"
};
}
public ICommand ChangeModelCommand { get; set; }
public Employee Employee
{
get
{
return _employee;
}
set
{
this._employee = value;
NotifyPropertyChanged(m=>m.Employee);
}
}
public void ChangeModel()
{
MessageBox.Show("CHANGING MODEL");
this.Employee.Name = "MIKE";
this.Employee.EmployeeID = "5678";
}
public bool CanChangeModel
{
get{ return true;}
}
}
And finally my View:
<Window.Resources>
<MVVM_NotificationTest:ViewModel x:Key="Model"></MVVM_NotificationTest:ViewModel>
</Window.Resources>
<Grid DataContext="{StaticResource Model}">
<StackPanel>
<Label Content="Employee Name"/>
<TextBox Text="{Binding Path=Employee.Name,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
<Label Content="Employee ID"/>
<TextBox Text="{Binding Path=Employee.EmployeeID,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
<Button Content="Change Model" Height="30" Width="100" Margin="5" Command="{Binding Path=ChangeModelCommand}"/>
</StackPanel>
</Grid>
In this example I initialize my Employee VM Property in the VM constructor and then I have a command to modify the EmployeeID (from Employee class) and Name (from Person Class). However, the only UI element in the View that gets updated is the EmployeeID and not the Name (I expected Bob to update to Mike). While debugging I found that PropertyChanged event was always null in my base class (Person). I also noticed that when I remove the whole #INotifyProperty region from my Employee class everything works fine since it is using the Base Type event and methods.The problem I have with that is that all my current model classes implement INotifyPropertyChanged explicitly. They all define a PropertyChanged event and implement the RaisePropertyChanged method, which obviously will impact my bindings in my MVVM application. Lastly, I want to clarify that I do not want wrap my Model properties in my ViewModel and rely on the VM INPC mechanism. I would like to use my Model INPC implementation already in place whithout having to conditionally remove the INPC implementations depending on whether I am inheriting or not from a base type.
In summary, I would like to know what's the best way to implement the INPC in my deeply hierarchical model so that inheritance doesn't break the PropertyEvent propagation as we saw in this example and so my independent classes can be self sufficient as well. Any ideas or suggestions will be greatly appreciated :)
Simply make RaisePropertyChanged protected and move it into the base class. Currently you will have a lot of duplication that is not necessary.
Something like this:
protected virtual void RaisePropertyChanged(string propertyName);
Many MVVM frameworks provide this for you. For example PRISM has a NotificationObject ViewModel base class.
You should only implement INPC once, you can use the same raising method in the subclasses.
I would also change the raise property changed method to use reflection instead of passing in hard coded strings. I see you did it in your view model but not in your models (where most of the errors tend to occur).