I write the following code to bind the data from a background object to a WinForm UI. I use the INotifyPropertyChanged interface to notify the UI of the property change. But I DIDN'T see any event handler been explicityly assigned to the event PropertyChanged. And I checked my assembly with .NET Reflector and still found no corresponding event-handler? Where is the event handler for PropertyChanged event? Is this yet another compiler trick of Microsoft?
Here is the code of the background object:
public class Calculation :INotifyPropertyChanged
{
private int _quantity, _price, _total;
public Calculation(int quantity, int price)
{
_quantity = quantity;
_price = price;
_total = price * price;
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String info)
{
if (PropertyChanged != null)// I DIDN'T assign an event handler to it, how could
// it NOT be null??
{
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}
public int Quantity
{
get { return _quantity; }
set
{
_quantity = value;
//Raise the PropertyChanged event
NotifyPropertyChanged("Quantity");
}
}
public int Price
{
get { return _price; }
set
{
_price = value;
//Raise the PropertyChanged event
NotifyPropertyChanged("Price");
}
}
public int Total
{
get { return _quantity * _price; }
}
}
Many Thanks!
I'm not sure I understand the question, but if you are using data-binding it is the bound control / form that binds to your class - either via the INotifyPropertyChanged interface (as in this case) or as reflection against the *Changed pattern (via PropertyDescriptor). If you really want you could intercept the add/remove parts of the event and look at the stack trace to see who is adding/removing handlers:
private PropertyChangedEventHandler propertyChanged;
public event PropertyChangedEventHandler PropertyChanged {
add { propertyChanged += value; } // <<======== breakpoint here
remove { propertyChanged -= value; } // <<===== breakpoint here
}
Related
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've used INotifyPropertyChanged within a custom class to fire an event when a variable has changed, but I was wondering if there was a simple way to fire an event on a single variable's change, such as a double.
For instance, in a WPF app, if I have
private double a;
in MainWindow.xaml.cs, is there a simple way to fire the event any time a is assigned to?
A field doesn't have any means of tracking changes. In order to make it work, it would need to be a property, and something would need to handle the tracking. That is the purpose of the INotifyPropertyChanged interface.
The normal means of tracking changes to this value would be to implement INotifyPropertyChanged in your class.
If I understand you correctly, you need to create a Setter for a, which then fires the properychange event/custom event instead of encapsulate a into a class.
Something like this:
private double a;
public double A
{
get { return a; }
set { a = value;
firepropertyChange(a);
}
}
Yes (depends), if you wrap your variable access through a property and fire your event on change, and make sure all access to that variable is through the property, like
private double a;
public double PropertyA
{
get
{
return a;
}
set
{
// set value and fire event only when value is changed
// if we want to know when value set is the same then remove if condition
if (a != value)
{
a = value;
PropertyChanged("PropertyA");
}
}
}
// when changing a, make sure to use the property instead...
PropertyA = 5.2;
...otherwise, no
If you are using C# 5.0, you can employ CallerMemberName attribute this way:
class MyData : INotifyPropertyChanged
{
private string _name;
public string Name
{
get { return _name; }
set
{
_name = value;
RaisePropertyChanged();
}
}
private string _anotherProperty;
public string AnotherProperty
{
get { return _anotherProperty; }
set
{
_anotherProperty = value;
RaisePropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void RaisePropertyChanged([CallerMemberName] string caller = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(caller));
}
}
}
As you see you just have to call RaisePropertyChanged(); in set for each property, without typing the property names over and over.
Another approach would be to define a ModelBase class:
class ModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void Set<T>(ref T field, T value, [CallerMemberName] string propertyName = "")
{
if (!Object.Equals(field, value))
{
field = value;
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
And then derive your model from ModelBase:
class Conf : ModelBase
{
NodeType _nodeType = NodeType.Type1;
public NodeType NodeType
{
get { return _nodeType; }
set { Set(ref _nodeType, value); }
}
}
I am implementing INotifyPropertyChanged and as part of that interface I have the member
public event PropertyChangedEventHandler PropertyChanged;
I have the following code for when some property gets changed -
public string FavoriteColor
{
get { return this.favoriteColor; }
set
{
if (value != this.favoriteColor)
{
this.favoriteColor = value;
**if (this.PropertyChanged != null)**
{
this.PropertyChanged(this, new PropertyChangedEventArgs("FavoriteColor"));
}
}
}
}
Now, I have never set the PropertyChanged variable anywhere in my code, yet if I put a breakpoint at this line it shows that PropertyChanged does have a value. So how is it getting set?
If you bind to the property the binding system subscribes to the event.
What is the best way to bind a property to a control so that when the property value is changed, the control's bound property changes with it.
So if I have a property FirstName which I want to bind to a textbox's txtFirstName text value. So if I change FirstName to value "Stack" then the property txtFirstName.Text also changes to value "Stack".
I know this may sound a stupid question but I'll appreciate the help.
You must implement INotifyPropertyChanged And add binding to textbox.
I will provide C# code snippet. Hope it helps
class Sample : INotifyPropertyChanged
{
private string firstName;
public string FirstName
{
get { return firstName; }
set
{
firstName = value;
InvokePropertyChanged(new PropertyChangedEventArgs("FirstName"));
}
}
#region Implementation of INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged;
public void InvokePropertyChanged(PropertyChangedEventArgs e)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, e);
}
#endregion
}
Usage :
Sample sourceObject = new Sample();
textbox.DataBindings.Add("Text",sourceObject,"FirstName");
sourceObject.FirstName = "Stack";
A simplified version of the accepted answer that does NOT require you to type names of properties manually in every property setter like OnPropertyChanged("some-property-name"). Instead you just call OnPropertyChanged() without parameters:
You need .NET 4.5 minimum.
CallerMemberName is in the System.Runtime.CompilerServices namespace
public class Sample : INotifyPropertyChanged
{
private string _propString;
private int _propInt;
//======================================
// Actual implementation
//======================================
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
//======================================
// END: actual implementation
//======================================
public string PropString
{
get { return _propString; }
set
{
// do not trigger change event if values are the same
if (Equals(value, _propString)) return;
_propString = value;
//===================
// Usage in the Source
//===================
OnPropertyChanged();
}
}
public int PropInt
{
get { return _propInt; }
set
{
// do not allow negative numbers, but always trigger a change event
_propInt = value < 0 ? 0 : value;
OnPropertyChanged();
}
}
}
Usage stays the same:
var source = new Sample();
textbox.DataBindings.Add("Text", source, "PropString");
source.PropString = "Some new string";
Hope this helps someone.
I am creating a custom control and i want to add some properties in it.
On few of the properties i want to create some events.
Say
if i have a property
public int Date {get; set;}
now if its value is changing i want to trigger a change event. SO how can i add event on this
Use a "normal" property rather than an automatic property, and raise the change event in the setter:
private int _date;
public int Date
{
get { return _date; }
set
{
if (value != _date)
{
_date = value;
// raise change event here
}
}
}
To raise the change event, if this is a standard INotifyPropertyChanged.PropertyChanged event:
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs("Date");
}
It's recommended practice to isolate this into an OnPropertyChanged method.
If you're raising a custom DateChanged event, the logic will be similar but with different names and event args.
The typical pattern to do this would be like so:
// declare the event
public event EventHandler DateChanged;
// declare backing field for the property
private int _date;
public int Date
{
get { return _date; }
set
{
// bool indicating whether the new value is indeed
// different from the old one
bool raiseEvent = _date != value;
// assign the value to the backing field
_date = value;
// raise the event if the value has changed
if (raiseEvent)
{
OnDateChanged(EventArgs.Empty);
}
}
}
protected virtual void OnDateChanged(EventArgs e)
{
EventHandler temp = this.DateChanged;
// make sure that there is an event handler attached
if (temp != null)
{
temp(this, e);
}
}
This example shows the implementation of an PropertyChanged event. For a PropertyChanging event, it's the same thing, but you raise the event before assigning the value in the property set accessor.
Well, you will need to define your event first of all, and a method to raise it.
Then you will need to switch away from an auto implemented property
private int _date;
public int Date
{
get {return _date;}
set
{
if(!_date.Equals(value))
//Raise event here
_date = value;
}
}
If you need some help with the events part, here is a tutorial that I wrote to give you the detail.
Also you can implement INotifyPropertyChanged interface and just raise an event in you property setter, here is full code sample that you can use and play with:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
namespace ConsoleApplication1
{
class Foo : INotifyPropertyChanged
{
private object myProperty;
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (PropertyChanged != null)
{
PropertyChanged(sender, e);
}
}
public object MyProperty
{
get { return this.myProperty;}
set
{
if (this.myProperty != value)
{
this.myProperty = value;
this.OnPropertyChanged(this, new PropertyChangedEventArgs("MyPropery"));
}
}
}
}
class Program
{
static void Main(string[] args)
{
Foo foo = new Foo();
foo.PropertyChanged += new PropertyChangedEventHandler(foo_PropertyChanged);
foo.MyProperty = "test";
}
static void foo_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine("raised");
}
}
}