I am very new to WPF MVVM.
After implementing an INotifyPropertyChanged interface, there is one event which gets added:
public event PropertyChangedEventHandler PropertyChanged;
as per my understanding, "PropertyChangedEventHandler" is a delegate, so I want to understand what methods are subscribing to this delegates?
INotifyPropertyChanged gives you:
public event PropertyChangedEventHandler PropertyChanged;
then you can add an OnPropertyChanged method to process when a property changes:
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
var e = new PropertyChangedEventArgs(propertyName);
handler(this, e);
}
}
So then, when you want to inform that a property has changed, you can call OnPropertyChanged such as from the property's set method itself. In this example, ShowRock is a property and when it changes (via the setter) it lets the world know that it changed:
public bool ShowRock
{
get { return _showRock; }
set
{
_showRock = value;
OnPropertyChanged("ShowRock");
}
}
Related
I'm trying to add properties to my Model playing with my first MVVM app.
Now I want to add a place to save specific data in a clean way, so I used a struct.
But I am having issues to notify property changed, it does not have access to the method (An object reference is required for the non-static field)
Can someone explain to me why this happens and inform me on a strategy that fit my needs?
Thanks!
public ObservableCollection<UserControl> TimerBars
{
get { return _TimerBars; }
set
{
_TimerBars = value;
OnPropertyChanged("TimerBars");
}
}
public struct FBarWidth
{
private int _Stopped;
public int Stopped
{
get { return _Stopped; }
set
{
_Stopped = value;
OnPropertyChanged("Name"); //ERROR: An object reference is required for the non-static field
}
}
private int _Running;
//And more variables
}
private void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
public event PropertyChangedEventHandler PropertyChanged;
OnPropertyChanged needs to be defined in the scope that you wish to update properties on.
For that to work you'll have to implement the interface INotifyPropertyChanged.
And finally you have to provide the correct argument to the OnPropertyChanged method. In this example "Stopped"
public struct FBarWidth : INotifyPropertyChanged
{
private int _Stopped;
public int Stopped
{
get { return _Stopped; }
set
{
_Stopped = value;
OnPropertyChanged("Stopped");
}
}
private int _Running;
//And more variables
private void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
Edit: In your comment you mentioned that you've got a class sorounding the code you provided in your example.
That means you've nested a struct inside a class.
Just because you've nested your struct, doesn't mean it inherits properties and methods from the outer class. You still need to implement INotifyPropertyChanged inside your struct and define the OnPropertyChanged method inside it.
I have implemented WPF data binding with INotifyPropertyChanged.
public class ExportNode : INotifyPropertyChanged
{
public uint Handle { get; set; }
public String Text { get; set; }
private bool _ischecked;
public bool IsChecked
{
get
{
return _ischecked;
}
set
{
_ischecked = value;
OnPropertyChanged("IsChecked");
}
}
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));
}
}
}
And than subscribing to event form my code, so whenever I change property in UI, it fires callback.
But now I'm trying to figure out the best way to change property from code, and than not fire callback, just update UI.
void newNode_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName == "IsChecked")
{
}
}
For now I just thought about implementing some "blocker" member property in ExportNode
protected void OnPropertyChanged(string name)
{
if (Blocked)
return;
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
or delete event form instance before change.
newNode.PropertyChanged -= newNode_PropertyChanged;
newNode.IsChecked = true;
newNode.PropertyChanged += newNode_PropertyChanged;
But is there any better way? I just don't understand some basics? :-)
Thank you very much
Roman
You've got this a little backwards.
INotifyPropertyChanged, and thus the PropertyChanged event, is what makes the UI update, in fact, its what makes the whole binding system work.
So to update the UI, you have to raise that event. Now, from the code side, you almost never subscribe to that event, because you could just invoke a method from the setter. Something like:
set
{
_ischecked = value;
OnPropertyChanged("IsChecked");
if (!Blocked)
MyOtherMethod();
}
Note that if you are dealing with threads, that Blocked condition is a major synchronization hazard.
If you really need to register for PropertyChanged from code, then your best bet is to just unregister with -=. That way the UI still gets its event, but you don't.
I need to implement INotifyPropertyChanged into my class (my goal is, to update a ListView every time an Item from 'JobDataGroup.Items' gets deleted), but in every tutorial, OnPropertyChanged gets called from the setter. Since I have no setter, how do I procede?
My Class:
public class JobDataGroup : repVReportsDataCommon, INotifyPropertyChanged
{
public ObservableCollection<ServiceJobItem> Items
{
get { return new ObservableCollection<ServiceJobItem>(repVReportsDataSource.GetJobItems().Where(_predicate)); }
}
#region PropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string info)
{
PropertyChangedEventHandler handler = PropertyChanged;
if(handler != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}
#endregion
}
It doesn't matter from where you call your OnPropertyChanged method, call it when the property needs to be reevaluated.
Since you are recreating your collection every time the getter is accessed, there is no point in using an ObservableCollection.
You might as well just use a List instead and raise OnPropertyChanged manually:
public List<ServiceJobItem> Items { get; private set; }
void UpdateItems() {
Items = new List<ServiceJobItem>(repVReportsDataSource.GetJobItems().Where(_predicate));
OnPropertyChanged("Items");
}
i think both are same,but i found use of them in only one file such as below code.here code for raisepropertychanged .
public decimal Amount
{
get
{
return _amount;
}
set
{
_amount = value;
RaisePropertyChanged("Amount");
}
}
here code for PropertyChanged:
public event PropertyChangedEventHandler PropertyChanged;
private void RaisePropertyChanged(string propertyName)
{
// take a copy to prevent thread issues
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
plz explain difference between them:
PropertyChanged is an event. RaisePropertyChanged is the method used to raise the event.
Of course, you could invoke the event directly from your property setter, but then you would have to check every time if the handler is not null... better to do it in one place.
I am writing a WPF application using Self-Tracking Entities. I'm having trouble Disabling/Enabling my Save button as my Model's values are changed. Normally with the regular Entity Framework Model I am able to simply subscribe to the Model.PropertyChanged event in my ViewModel, then RaisePropertyChanged for my Save Button, which checks validation and Disabled or Enables my save button.
For some reason, with Self-Tracking Entities I noticed that the Model.PropertyChanged event is marked as protected, so I am unable to subscribe to it directly in my ViewModel. Is there any way to subscribe to this event without modifying the T4 Template??
protected virtual void OnPropertyChanged(String propertyName)
{
if (ChangeTracker.State != ObjectState.Added && ChangeTracker.State != ObjectState.Deleted)
{
ChangeTracker.State = ObjectState.Modified;
}
if (_propertyChanged != null)
{
_propertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
protected virtual void OnNavigationPropertyChanged(String propertyName)
{
if (_propertyChanged != null)
{
_propertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
event PropertyChangedEventHandler INotifyPropertyChanged.PropertyChanged{ add { _propertyChanged += value; } remove { _propertyChanged -= value; } }
private event PropertyChangedEventHandler _propertyChanged;
private ObjectChangeTracker _changeTracker;
I've always just modified the template to make it not protected. It's a template for a reason :)
I suppose an alternative would be to create a public method on your class which raises the PropertyChanged notification internally.
partial class MyModel
{
public RaiseEFPropertyChanged(string propertyName)
{
RaisePropertyChanged(propertyName);
}
}