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.
Related
I'm not sure how to make navigation using mvvm. I'm a beginner so I haven't used any framework like mvvm light.
I found good example https://rachel53461.wordpress.com/2011/12/18/navigation-with-mvvm-2/. But it is not exactly what I'm looking for because in my app each view will cover all window. So when I will change page i will have no controls access from the mainview.
So I decided to make one MainViewModel for changing ViewModels (as in Rachel Blog) but each ViewModel should know about MainViewModel to execute change view. So when I create PageViewModel, I pass in constructor MainViewModel with public method, for example, changeview().
Is it a good way of doing this? Or, maybe, there's a better way to achieve this?
The child viewmodels should not know about main viewmodel.
Instead they should raise events with names like Forward or Back and so forth. ChangeView is the only example you give, so we’ll go with that.
We'll have the child viewmodel expose commands that cause the events to be raised. Buttons or MenuItems in the child view's XAML can bind to the commands to let the user invoke them. You can also do that via Click event handlers calling viewmodel methods in the child view code behind, but commands are more "correct", because at the cost of a little more work in the viewmodel, they make life a lot simpler for creators of views.
Main viewmodel handles those events and changes the active page viewmodel accordingly. So instead of child calling _mainVM.ChangeView(), child raises its own ChangeView event, and the main VM’s handler for that event on the child calls its own method this.ChangeView(). Main VM is the organizer VM, so it owns navigation.
It’s a good rule to make code as agnostic as possible about how and where it’s used. This goes for controls and viewmodels. Imagine if the ListBox class required the parent to be some particular class; that would be frustrating, and unnecessary as well. Events help us write useful child classes that don’t need to know or care anything about which parent uses them. Even if reuse isn’t a possibility, this approach helps you write clean, well-separated classes that are easy to write and maintain.
If you need help with the details, provide more code, and we can go through applying this design to your project.
Example
public class MainViewModel : ViewModelBase
{
public MainViewModel()
{
FooViewModel = new FooViewModel();
FooViewModel.Back += (object sender, EventArgs e) => Back();
}
public FooViewModel FooViewModel { get; private set; }
public void Back()
{
// Change selected page property
}
}
public class FooViewModel : ViewModelBase
{
public event EventHandler Back;
private ICommand _backCommand;
public ICommand BackCommand {
get {
if (_backCommand == null)
{
// It has to give us a parameter, but we don't have to use it.
_backCommand = new DelegateCommand(parameter => OnBack());
}
return _backCommand;
}
}
// C#7 version
public void OnBack() => Back?.Invoke(this, EventArgs.Empty);
// C# <= 5
//protected void OnBack()
//{
// var handler = Back;
// if (handler != null)
// {
// handler(this, EventArgs.Empty);
// }
//}
}
// I don't know if you already have a DelegateCommand or RelayCommand class.
// Whatever you call it, if you don't have it, here's a quick and dirty one.
public class DelegateCommand : ICommand
{
public DelegateCommand(Action<object> exec, Func<object, bool> canExec = null)
{
_exec = exec;
_canExec = canExec;
}
Action<object> _exec;
Func<object, bool> _canExec;
public event EventHandler CanExecuteChanged;
public bool CanExecute(object parameter)
{
return _canExec == null || _canExec(parameter);
}
public void Execute(object parameter)
{
if (_exec != null)
{
_exec(parameter);
}
}
}
How to invoke BackCommand from child XAML:
<Button Content="Back" Command="{Binding BackCommand}" />
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.
After a major edit to this quesiton, I'm hoping it's now clear.
I'm very lost with binding in WPF when 1 change should affect multiple properties.
I regularly use VVM to bind my ViewModel to my View and I would say I'm OK with it.
I am trying to implement a state controller. This means that, what ever settings I made in part of my UI, the reflection is through out.
For example in my part of my UI, I can toggle a feature on or off, such as "show images"
When I make this change, I'd like everything in my application to be notified and act accordingly.
So, my StateController class will have a property
public bool ShowImages
And in my View, I'd likely have something like
<image Visible ="{Binding ShowImages", Converter={StaticConverter ConvertMe}}" />
The problem I have is how I go about making the StateController alert all of my ViewModels of this.
Currently, in each ViewModel I'm assuming I'd have to have the same property repeated
public bool ShowImages
EG
public class StateController : BaseViewModel
{
public bool ShowImages{get;set;}//imagine the implementation is here
}
public class ViewModelB : BaseViewModel
{
public bool ShowImages{}//imagine the implementation is here
}
public class ViewModelB : BaseViewModel
{
public bool ShowImages{}//imagine the implementation is here
}
So, my question is, if I updated ViewModelB.ShowImages, how would I first inform the StateController which in turn updates all ViewModels.
Is this something the INotifyPropertyChanged can do automatically for me since they all share the same propertyName, or do I have to implement the logic manually, eg
public static class StateController
{
public bool ShowImages{get;set;}//imagine the implementation is here
}
public class ViewModelA : BaseViewModel
{
public bool ShowImages
{
get { return StateController.ShowImages; }
set { StateControllerShowImages = value;
OnPropertyChanged("ShowImages"); }
}
}
public class ViewModelB : BaseViewModel
{
public bool ShowImages
{
get { return StateController.ShowImages; }
set { StateControllerShowImages = value;
OnPropertyChanged("ShowImages"); }
}
}
I hate the idea of the above implementation but it does show what I'm trying to achieve. I just hope there is a better way!
The PropertyChange notification is only raised for that one object model.
So raising a change notification of the "Name" property of ClassA will only update the UI in cases where it's bound to that specific ClassA.Name. It won't trigger a change notification for any ClassB.Name, or other instances of ClassA.Name.
I would suggest using a Singleton here for your StateModel, and having your other models subscribe to the StateModel.PropertyChanged event to know if it should update, like this answer.
public ViewModelA
{
public ViewModelA()
{
StateController.Instance.PropertyChanged += StateController_PropertyChanged;
}
void StateController_PropertyChanged(object sender, NotifyPropertyChangedEventArgs e)
{
// if singleton's ShowImages property changed, raise change
// notification for this class's ShowImages property too
if (e.PropertyName == "ShowImages")
OnPropertyChanged("ShowImages");
}
public bool ShowImages
{
get { return StateController.Instance.ShowImages; }
set { StateController.Instance.ShowImages = value; }
}
}
If I understood you correctly, you are looking for a mechanism that allows your different ViewModels to communicate between each other.
One possible way would be to implement the Observer Pattern (a code example can be found here: "Observer pattern with C# 4"). In this way your ViewModel subscribe each other to receive change notifications from a "publisher", i.e. the ViewModel that had its value changed. You have a good control over who receives which notification from which publisher. The downside of this approach is a tight coupling between your models.
My approach would be this:
Use a message dispatcher. Your ViewModels can subscribe to a certain type of message, e.g. ShowImagesChanged. If any of your ViewModels changed the ShowImages property, that ViewModel calls the dispatcher to send out such a ShowImagesChanged message with your current values.
This way you can keep you ViewModels decoupled from each other. Still, although the ViewModels do not know each other this gives a way to exchange data between them.
Personally, I have used the Caliburn Micro MVVM framework several times for this, but there should be enough other MVVM frameworks that provide the same functionality to fit your taste.
The Calibiurn Micro documentation and how easily the dispatcher can be used is here: Event Aggregator
To avoid code repetition you can create a class derived from BaseViewModel that implements your property and have ViewModelA, ViewModelB extend it. However, this does not solve the problem of keeping each instance updated.
In order to do so, you may:
Use a static class (your current solution) or a Singleton as suggested in one of the comments. This is simple but has potential problems such as race conditions and coupling.
Have your ShowImages binding property repeated in each ViewModel and update it by subscribing to a ShowImagesChanged event. This could be published through a Command executed from the UI. I'd say this is the WPF approach and has the benefit of decoupling the ShowImages state management from its consumption.
Assign the ShowImagesupdate responsibility to a single ViewModel and subscribe to the its PropertyChanged in the other ViewModels so that they update accordingly. Better than the first option, but still huge coupling.
Why repeat properties at all? Just bind to StateController itself.
Say we have singleton StateController:
public class StateController : INotifyPropertyChanged
{
private static StateController instance;
public static StateController Instance {
get { return instance ?? (instance = new StateController()); }
}
//here`s our flag
private bool isSomething;
public bool IsSomething
{
get { return isSomething; }
set
{
isSomething = value;
PropertyChanged(this, new PropertyChangedEventArgs("IsSomething"));
}
}
private StateController(){}
public event PropertyChangedEventHandler PropertyChanged = delegate { };
}
Then in base VM class just make a reference to this controller:
public StateController Controller { get { return StateController.Instance; } }
And where needed bind like this:
<CheckBox IsChecked="{Binding Controller.IsSomething}">
Test
</CheckBox>
This way every binding will work with one property and react to one property. If you need some custom code to work you can subscribe to PropertyChanged of StateController where needed and take action.
Suppose you have two classes:
public class A
{
int x = 0;
public void Increase()
{
x++;
}
}
public class B
{
A a;
private void DoSomething();
}
Is there a way for B to be "messaged" and execute DoSomething() when anything has changed in a (i.e. x has increased)? I know how I could make a subscribe to B, such that if B does RaiseSomeEvent(..), a reacts, but not the other way round.
Background: I'm trying to create a custom control
public class BattleshipCanvas : Canvas
{
public BSGrid BattleshipGrid {...}
...
}
that should redraw once anything inside the BattleshipGrid (BSGrid is a class encapsulating a two-dimensional array) changes, where BattleshipGrid will be bound to a certain BSGrid in the ViewModel. I thought about adding an event to BSGrid that is raised whenever I modify its data, but I don't know how to notify the BattleshipCanvas of that event.
I hope I could make it a little clear (it's hard for me to express what I want exactly here) and understandable, but if there arise any questions, feel free to comment and I'll try to answer them. Thanks!
You may be looking for events in c#. In your specific case you may like to make use of the INotifyPropertyChanged Interface. You can use it to inform other classes by events if a property inside the implementing class has changed.
This is also the base to use binding in your project later on.
public class A: INotifyPropertyChanged
{
//Event used to announce a change inside a property of your class
public event PropertyChangedEventHandler PropertyChanged;
int _x = 0;
public int X
{
get { return _x; }
set
{
if (_x != value)
{
_x = value;
OnPropertyChanged("X"); //invokes the event.
}
}
}
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) //make sure somebody subscribed to the event
handler.Invoke(this, new PropertyChangedEventArgs(propertyName)); //this calls all eventhandler methods which subscribed to your classes PropertyChanged event.
}
public void Increase()
{
X++; //use the property to invoke a property changed event.
}
}
public class B
{
A a;
public B()
{
a = new A();
a.PropertyChanged += new PropertyChangedEventHandler(a_PropertyChanged); //subscribe up to the event. (use -= to unsubscribe)
a.Increase()
}
//Catch the event
void a_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
//execute what you would like to do.
//you can use e.PropertyName to lookup which property has actually changed.
DoSomething();
}
private void DoSomething(){}
}
Events are probably the way to go. You can make any class in your project react to any event being raised in your program, no matter where the event is created/handled.
In your instance, it looks like you don't even need to send over any custom EventArgs.
The most simple example I could find of this is here:
http://timok.tumblr.com/post/57441520214/simplistic-event-example-in-c
I am having trouble with grasping the concept of a ObservableCollection inside MVVM. For start I would like to point out that I am doing this in a Windows 8/Metro App, not WPF or Silverlight.
According to microsoft documentation, this collection has the following usefulness:
"Represents a dynamic data collection that provides notifications when items get added, removed, or when the whole list is refreshed." From what I understand this helps you a lot when binding is involved. On the net I found a lot of simple examples, by creating a ObservableCollection on runtime and then working on it, but I didn't find out what is the proper way of using this collection with a repository.
Let' say I have the following repository interface that is an implementation for a ORM database backend, or a raw ADO.NET implementation
public interface IRepository<T>
{
ObservableCollection<T> GetAll();
void Create();
void Update();
void Delete();
T GetByKey(object key);
}
and a simple ViewModel that use the repository as a model
public class ViewModel
{
private ObservableCollection<Dummy> _obsListDummy;
private RelayCommand _addCommand,_deleteCommand,_updateCommand;
private IRepository<Dummy> _repositoryDummy;
public class ViewModel()
{
_repositoryDummy=Factory.GetRepository<Dummy>();
}
public ObservableCollection<Dummy> ObsListDummy
{
get
{
return _repositoryDummy.GetAll();
}
}
public RelayCommand AddCommand
{
get
{
if (_addCommand == null)
{
_addCommand = new RelayCommand(p => DoAdd();
//DoAdd method shows a popup for input dummy and then closes;
);
}
return _myCommand;
}
}
........
}
My view would be a simple XAML with a grid, also Dummy object has INotifyPropertyChanged implemented.
Right now with this implementation after adding or updating or deleting, the ObservableCollection isn't refreshing, I know I could have put IEnumerable instead, but I dont'see an elegant solution of how would make repository to sync with the ObservableCollection that is in the model, other than subscrbing to CollectionChanged and there you treat all the states, but to it seems that I would repeat myself along with the logic that I do in the repository. And to make matters even worse, let's say I would like to get some push notification from my repository, towards the ObservableCollection.
I hope I was understand about my problem.
Thanks in advance.
You should implement INotifyPropertyChanged on your ViewModel and your ObsListDummy property should inform the ViewModel about changes applied to the collection. So it should look like this:
public class ViewModel: INotifyPropertyChanged
{
// Declare the event
public event PropertyChangedEventHandler PropertyChanged;
// Create the OnPropertyChanged method to raise the event
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
private ObservableCollection<Dummy> _dummyCollection;
public ObservableCollection<Dummy> DummyCollection
{
get { return _dummyCollection; }
set
{
// Set the value and then inform the ViewModel about change with OnPropertyChanged
_dummyCollection = value;
OnPropertyChanged("DummyCollection");
}
}
}
This whole INotifyPropertyChanged interface and implementation includes some dirty work like declaring event and creating a helper method to raise the event so I would suggest you to use some libraries for that like MVVM Light.
You should use a member of type ObservableCollection to store your Dummy ViewModels. In your Initialize method you read the dummies from the repository, create Dummy ViewModels and put those in the ObservableCollection. Now your view will get updated, when you use Binding to ObsListDummy (and add / remove from that collection, also note that Binding only works with public properties).
Right now, you just have a new ObservableCollection on each read, no events involved, so your View will never know about a change.
Further your ViewModel shall implement INotifyPropertyChanged.