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).
Related
Basically, I have a model with various classes that load my data in different collections, including collections of collections (e.g. Cart has a collection of Bundle, which has collections of Product) . If my understanding of MVVM is correct, I would need to make one ViewModel class that implements INotifyPropertyChanged (directly or through inheritance from a base class) for each of my model classes. Though I must admit it seems to me that it implies a lot of duplicated code, just, having the ViewModel associate each property of the model class with a OnPropertyChanged call.
Just like is shown in this article for instance.
Am I getting this right ?
I'm currently trying to understand the basics of MVVM, so I try to fully implement it in my programs without any additional framework (MVVM Light and others).
You don't need a ViewModel-Class for each Model-Class you have. Your Model-Classes should implement the INotifyPropertyChanged-Interface.
You need ViewModels to interact with your Views. In the ViewModels you can have instances of your Model-Classes.
Btw.: To avoid writing the code for INotifyPropertyChanged every time in each ViewModel and Model i've created an abstract base class where everything is derived from. This class looks like:
public abstract class NotifyBase : INotifyPropertyChanged
{
private readonly Dictionary<string, object> mapping;
protected NotifyBase()
{
mapping = new Dictionary<string, object>();
}
protected void Set<T>(T value, [CallerMemberName] string propertyName = "")
{
mapping[propertyName] = value;
OnPropertyChanged(propertyName);
}
protected T Get<T>([CallerMemberName] string propertyName = "")
{
if(mapping.ContainsKey(propertyName))
return (T)mapping[propertyName];
return default(T);
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemeberName] string propertyName = null)
{
PropertyChangedEventHandler handler = PropertyChanged;
if(handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
I am making simple mvvm binding with picker field in xamarin.forms. I am following this guide xamarin guide setting a picker's bindings
So I made a model:
public class Operation
{
public int Number { get; set; }
public string Name { get; set; }
}
An ViewModel:
private List<Operation> _operations;
public List<Operation> Operations
{
get { return _operations; }
set
{
_operations = value;
OnPropertyChanged();
}
}
and View:
<Picker
ItemsSource="{Binding Operations}"
ItemDisplayBinding="{Binding Number}"
SelectedItem = "{Binding SelectedOperation}"/>
<Entry x:Name="HelpEntry"
Text="{Binding SelectedOperation.Name}" />
In the Pickers list items are displayed correctly, but when I Select an item number, then binding inside a Entry is not displayed.
Ouestion is, what am I doing wrong?
By the way.. I am doing this because I need to get an selected Operation's Name as variable in my code-behind section, by using HelpEntry.Text. It's not a smartest way and do u have better idea to do that?
Any help would be much appreciate.
Your ViewModel should also contain the SelectedOperation property that should also call the OnPropertyChanged method in its setter.
Also you should consider using ObservableCollection instead of List in you view models.
Be sure that your ViewModel implements the INotifyPropertyChanged interface. The way to do this easily is to create a BaseViewModel that implements the interface and then inherit all of your concrete view model classes from this base class.
public class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
public class MainPageVM : ViewModelBase
{...}
Can someone explain me why need to use implementation of INotifyPropertyChanged when using binding in wpf?
I can bind properties without implementation of this interface?
For example i have code
public class StudentData : INotifyPropertyChanged
{
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
string _firstName = null;
public string StudentFirstName
{
get
{
return _firstName;
}
set
{
_firstName = value;
OnPropertyChanged("StudentFirstName");
}
}
}
And binding in .xaml
<TextBox Text="{Binding Path=StudentFirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
Grid.Row="1"
Grid.Column="2"
VerticalAlignment="Center" />
this code from .xaml.cs
StudentData _studentData = new StudentData { StudentFirstName = "John", StudentGradePointAverage = 3.5};
public MainWindow()
{
InitializeComponent();
this.DataContext = _studentData;
}
why we need to use INotifyPropertyChanged in this case?
It is not my code.
You need INotifyPropertyChanged if you want a wpf form to be automatically updated when a property changes through code. Also some controllers might want to know if edits have been made in order to enable/disable a save-button, for instance. You also might be displaying the same property on different views; in this case INotifyPropertyChanged helps to immediately update the other view when you edit a property.
If you think that your form behaves well without INotifyPropertyChanged, then you can drop it.
Note that binding works even without INotifyPropertyChanged. See: Why does the binding update without implementing INotifyPropertyChanged?
I would implement the properties like this. In some rare cases it can help to avoid endless circular updates. And it is more efficient by the way.
private string _firstName;
public string StudentFirstName
{
get { return _firstName; }
set
{
if (value != _firstName) {
_firstName = value;
OnPropertyChanged("StudentFirstName");
}
}
}
Starting with C#6.0 (VS 2015), you can implement OnPropertyChanged like this:
private void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
When you bind to a property of StudentData such as the StudentFirstName then the binding class tests to see if the StudentData instance provides the INotifyPropertyChanged interface. If so then it will hook into the PropertyChanged event. When the event fires and it fires because of the StudentFirstName property then it knows it needs to recover the source value again because it has changed. This is how the binding is able to monitor changes in the source and reflect them in the user interface.
If you do not provide the INotifyPropertyChanged interface then the binding has no idea when the source value changes. In which case the user interface will not update when the property is changed. You will only see the initial value that was defined when the binding was first used.
It does need to be implemented in order for binding to work but that doesn't mean you always have to do it yourself. There are other options like Castle Dynamic Proxy (which wraps your classes in a proxy and injects INPC into all virtual properties) and Fody (which adds it to the IL in a post-processing step). It's also possible to implement yourself while at the same time reducing code bloat, as demonstrated in my answer to this question.
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.
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 :)