I have following code and it is working fine.
public partial class MainWindow : Window
{
Person person;
public MainWindow()
{
InitializeComponent();
person = new Person { Name = "ABC" };
this.DataContext = person;
}
private void Button_Click(object sender, RoutedEventArgs e)
{
person.Name = "XYZ";
}
}
class Person: INotifyPropertyChanged
{
string name;
public string Name
{
get
{
return name;
}
set
{
name = value;
OnPropertyChanged("Name");
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string strPropertyName)
{
if(null != PropertyChanged)
{
PropertyChanged(this,
new PropertyChangedEventArgs(strPropertyName));
}
}
}
When I create the "person" object in the constructor of MainWindow, it will assign the value for "Name" property of person, that time PropertyChanged event is NULL.
If the same "person" class property "Name" assigned in Button_Click event, "PropertyChanged" event is NOT NULL and it is pointing to OnPropertyChanged.
My question is how "PropertyChanged" event is assigned to OnPropertyChanged method?
Thanks in advance.
The WPF data-binding infrastructure will add a PropertyChanged handler when you set the object as a DataContext, in order to detect changes to your properties.
You can watch this happen by setting a breakpoint.
The OnPropertyChanged method that it points to is an internal WPF method, as you can see by inspecting the Target property of the delegate.
The event will be null until something is subscribed to it. By the time the button click event has happened, it has a subscriber (via the databinding system).
Related
Here I added a model to my viewmodel:
public dal.UserAccount User {
get
{
return _user;
}
set
{
_user = value;
RaisePropertyChanged(String.Empty);
}
}
I handle property change event...
public event PropertyChangedEventHandler PropertyChanged;
private void RaisePropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
This is the binding I use:
<TextBox Text="{Binding User.firstname, Mode=TwoWay, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" />
Why the propertychange event is not triggered on updating view?
PropertyChanged is used to notify the UI that something has been changed in the Model.
Since you're changing an inner property of the User object - the User property itself is not changed and therefore the PropertyChanged event isn't raised.
Second - your Model should implement the INotifyPropertyChanged interface. - In other words make sure UserAccount implements INotifyPropertyChanged, otherwise changing the firstname will not affect the view either.
Another thing:
The parameter RaisePropertyChanged should receive is the Name of the property that has changed. So in your case:
Change:
RaisePropertyChanged(String.Empty);
To
RaisePropertyChanged("User");
From MSDN:
The PropertyChanged event can indicate all properties on the object have changed by using either null or String.Empty as the property name in the PropertyChangedEventArgs.
(No need to refresh all the Properties in this case)
You can read more on the concept of PropertyChanged here
You can invoke a property changed event from another class. Not particularly useful if you have all the sources. For closed source it might be. Though I consider it experimental and not production ready.
See this console copy paste example:
using System;
using System.ComponentModel;
using System.Runtime.InteropServices;
namespace ConsoleApp1
{
public class Program
{
static void Main(string[] args)
{
var a = new A();
a.PropertyChanged += A_PropertyChanged;
var excpl = new Excpl();
excpl.Victim = a;
excpl.Ghost.Do();
}
private static void A_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine("Event triggered");
}
}
[StructLayout(LayoutKind.Explicit)]
public struct Excpl
{
[FieldOffset(0)]
public A Victim;
[FieldOffset(0)]
public C Ghost;
}
public class A : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
}
public class C : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void Do()
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(""));
}
}
}
This is my ViewModel -
public class ViewModel
{
public ObservableCollection<Person> Persons { get; set; }
}
and this is Class Person:
public class Person : INotifyPropertyChanged
{
private string _firstName;
public string FirstName
{
get { return _firstName; }
set
{
_firstName = value;
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs("FirstName"));
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
Now, every time one of the persons's FirstName is changing I want to do some tasks,
lets say raise a messagebox.
How do I do that ?
You need to implement INotifyPropertyChanged
public class Person : INotifyPropertyChanged
{
private string firstName;
public string FirstName
{
get { return this.firstName;}
set
{
this.firstName = value;
this.RaisePropertyChanged("FirstName");
MessageBox.Show("Hello World");
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected void RaisePropertyChanged(string propertyName)
{
PropertyChangedEventHandler propertyChanged = this.PropertyChanged;
if ((propertyChanged != null))
{
propertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
Typically your person class will use the interface INotifyPropertyChanged, firing the PropertyChanged event whenever FirstName changes. This allows you to bind items in a view to your Person class and the view will be updated when the data changes.
To pop up a message box when any FirstName however, you will need some code behind in your view. One way to do it is to, as before, use INotifyProperty changed and subscribe to that on all Person objects in your view, using MessageBox.Show whenever an event changing FirstName is invoked. You can use the CollectionChanged event in the ObservableCollection to track Person objects in and out of the list to make sure that they are all connected to your Person FirstName changed event handler.
The best way to do it, in my opinion, is to have an event in the ViewModel rather than the Person class which fires whenever a change is made to any Person class (with the specific Person object as an argument). This will only work if the ViewModel is the only thing which can change Person.FirstName, and your View will have to bind to the ViewModel in an appropriate way to effect this.
You need to implement INotifyPropertyChanged on your viewmodel, and raise the property changed event when setting your persons collection. This will allow you listen for the fact that it has changed.
http://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged.aspx
I have a TextBlock, binded with an Object and when i update property of object its not refleting on UI, Why ?
Code:
In Windows1.xaml
<TextBlock Name="txtName" Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}" Width="100" Height="20" Margin="12,23,166,218" />
and In Windows.xaml.cs
public partial class Window1 : Window
{
Employee obj ;
public Window1()
{
InitializeComponent();
obj = new Employee();
obj.Name = "First";
txtName.DataContext = obj;
}
private void btnUpdate_Click(object sender, RoutedEventArgs e)
{
obj.Name = "changed";
}
}
public class Employee : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private string _name;
public string Name
{
set
{
this._name = value;
OnPropertyChanged(Name);
}
get { return this._name; }
}
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
}
OnPropertyChanged(Name);
should be:
OnPropertyChanged("Name");
otherwise if the name is set to "Kent", you're raising a property changed event saying that the "Kent" property has changed, which obviously doesn't exist.
As for UpdateSourceTrigger, that only applies to the source. The property you've bound is the target, not the source. And it doesn't make sense for a TextBlock to update its source, because there's no way for the user to modify the TextBlock text. A TextBox, on the other hand, would make sense. In that case, UpdateSourceTrigger determines the point at which the text in the TextBox is pushed back to the source property (e.g. as the user types each character, or when they move away from the TextBox).
Pass the name of the property as string, instead of the property value, like so:
OnPropertyChanged("Name");
UpdateSourceTrigger is for binding with the source property i.e normal .net property so to it requires to set on options like property change with mode twoway for dynamic change ex.text of textbox being change and the which updtaes a label to change. Where as if you want the change event to fire at the end i.e lost focus or click use explicit option with updatesourcetrigger.
I have a datagrid that is bound to a observableCollection of Employees
The user is allowed to do in line editing by double clicking the datagridRow.
When binding the property in question I also use UpdateSourceTrigger.
When I user presses the save button ,the saveCommand is triggered in my MVVM and I want to create a list of only the employees that I have had property modified.
All my ViewModels implements INotifyPropertyChanged.
Despite lots of links on google I cannot seem to find an example that takes you through or explain how to track the items that have changed in a observablecollection.
Can you help?
Create base class for your Employee, for example, EntityBase and enum describing its states:
public enum EntityState
{
NotChanged,
Changed,
Added,
Deleted
}
public abstract class EntityBase : INotifyPropertyChanging, INotifyPropertyChanged
{
public event PropertyChangingEventHandler PropertyChanging;
public event PropertyChangedEventHandler PropertyChanged;
private EntityState state = EntityState.NotChanged;
public EntityState State
{
get { return state; }
set { state = value; }
}
public EntityBase()
{
state = EntityState.NotChanged;
}
protected virtual void SendPropertyChanging(string propertyName)
{
if ((this.PropertyChanging != null))
{
this.PropertyChanging(this, new PropertyChangingEventArgs(propertyName));
}
}
protected virtual void SendPropertyChanged(string propertyName)
{
if ((this.PropertyChanged != null))
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
When one of your employes in collection changed - mark it with EntityState.Changed, and then you can request only changed entities from collection and process them as you want.
I'm not aware of any built in way to do what you want.
What I've done in the past is to implement a boolean IsDirty property on the objects in the collection. Then set the IsDirty property to true anytime you raise PropertyChanged.
What would be the cleanest way to have a Save state for an application so that whenever a property or content was updated the "Save" options would become enabled.
For example, there is a menu and toolbar "Save" buttons. When the WPF app first opens both buttons are disabled. When the user updates the properties or document, the buttons become enabled until a "Save" is done, at which point they go back to disabled.
Bind IsEnabled to a ViewModel that exposes an "IsDirty" or "HasBeenModified" boolean property, or something of similar ilk. The ViewModel would watch for changes to the Model and set IsDirty to true if the Model is modified for any reason. When saved, the ViewModel can be told to set IsDirty to false, disabling the button.
You are using the Model-View-ViewModel pattern, right? Here are a few links on the pattern to help you on your way, just in case:
http://en.wikipedia.org/wiki/Model_View_ViewModel
http://msdn.microsoft.com/en-us/magazine/dd419663.aspx
http://www.wintellect.com/CS/blogs/jlikness/archive/2010/04/14/model-view-viewmodel-mvvm-explained.aspx
Are all the properties you wish to watch for in the same class? If so, something like this will work:
Have the class derive from INotifyPropertyChanged;
Watch for property changes from the class and set an IsDirty flag when there are changes
Set IsEnabled for the Save button based on the IsDirty flag
When the Save command is executed, set IsDirty=false
Notifying Class Example
public class NotifyingClass : INotifyPropertyChanged
{
private string Property1Field;
public string Property1
{
get { return this.Property1Field; }
set { this.Property1Field = value; OnPropertyChanged("Property1"); }
}
private string Property2Field;
public string Property2
{
get { return this.Property2Field; }
set { this.Property2Field = value; OnPropertyChanged("Property2"); }
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
#endregion
}
Watching for Property Changes
public partial class MainWindow : Window
{
private bool isDirty;
NotifyingClass MyProperties = new NotifyingClass();
public MainWindow()
{
InitializeComponent();
this.MyProperties.PropertyChanged += (s, e) =>
{
this.isDirty = true;
};
}
}
How you set the disabled/enabled state depends on what type of button/command implementation you are doing. If you would like further help just let me know how you are doing it (event handler, RoutedCommand, RelayCommand, other) and I'll check it out.