I'd like to implent the System.ComponentModel.INotifyPropertyChanged interface for a property on a base class, but I'm not quite sure how to hook it up.
Here's the signature for the property I'd like to get notifications for:
public abstract bool HasChanged();
And my code in the base class for handling the change:
public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(String info)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(info));
}
}
How do I handle the hookup of the event in the base class without having to call OnPropertyChanged() in each child class?
Thanks,
Sonny
EDIT:
OK... so I think that when the value for HasChanged() changes, I'm supposed to call OnPropertyChanged("HasChanged"), but I'm not sure how to get that into the base class. Any ideas?
Is this what you are after?
public abstract class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
//make it protected, so it is accessible from Child classes
protected void OnPropertyChanged(String info)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(info));
}
}
}
Notice the OnPropertyChanged accessible level is protected. And then in your concrete class or child classes, you do:
public class PersonViewModel : ViewModelBase
{
public PersonViewModel(Person person)
{
this.person = person;
}
public string Name
{
get
{
return this.person.Name;
}
set
{
this.person.Name = value;
OnPropertyChanged("Name");
}
}
}
EDIT: after reading the OP question again, I realize that he does not want to call the OnPropertyChanged in the child class, so I am pretty sure this will work:
public abstract class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private bool hasChanged = false;
public bool HasChanged
{
get
{
return this.hasChanged;
}
set
{
this.hasChanged = value;
OnPropertyChanged("HasChanged");
}
}
//make it protected, so it is accessible from Child classes
protected void OnPropertyChanged(String info)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(info));
}
}
}
and in child class:
public class PersonViewModel : ViewModelBase
{
public PersonViewModel()
{
base.HasChanged = true;
}
}
Related
I am implementing a cart in Xamarin.Forms. In my cart page there is a ListView with data. Each of the cell contains a button to select the count of item and amount. In the cart view there is a grand total label.
My problem is the grand total is not updating while the number picker changes. The calculation method is called upon item adding view cell. I know that i need to implement INotifyProperty for this, but I'm unsure of how to do it.
I have a base view model which inherits INotifyProperty that contains an event.
public class BaseViewModel : INotifyPropertyChanged
{
private double _price;
public double Price
{
get
{
return _price;
}
set
{
_price = value;
OnPropertyChanged("Price");}
}
protected virtual void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
View model
public BaseViewModel()
{
App.Instance.ViewModel = this;
TempList = TempList ?? new ObservableCollection<cm_items>();
this.Title = AppResources.AppResource.Cart_menu_title;
this.Price = CartCell.price;
}
As a design methodology, its better to implement MVVM as a subclass and implement it to your ViewModel.
Sample Implementation:
public class ObservableProperty : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
var handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
I also strongly suggest implementing ICommand as a Dictionary structure like:
public abstract class ViewModelBase : ObservableProperty
{
public Dictionary<string,ICommand> Commands { get; protected set; }
public ViewModelBase()
{
Commands = new Dictionary<string,ICommand>();
}
}
So all todo in your ViewModel is just inherit the ViewModelBase class and use it
class LoginViewModel : ViewModelBase
{
#region fields
string userName;
string password;
#endregion
#region properties
public string UserName
{
get {return userName;}
set
{
userName = value;
OnPropertyChanged("UserName");
}
}
public string Password
{
get{return password;}
set
{
password = value;
OnPropertyChanged("Password");
}
}
#endregion
#region ctor
public LoginViewModel()
{
//Add Commands
Commands.Add("Login", new Command(CmdLogin));
}
#endregion
#region UI methods
private void CmdLogin()
{
// do your login jobs here
}
#endregion
}
Finally: Xaml Usage:
<Entry Placeholder="Username" Text="{Binding UserName}"/>
<Entry Placeholder="Password" Text="{Binding Password}" IsPassword="True"/>
<Button Text="Login" Command="{Binding Commands[Login]}"/>
For example try this view model:
public abstract class BaseViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected bool SetPropertyValue<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
{
if (value == null ? field != null : !value.Equals(field))
{
field = value;
var handler = this.PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
return true;
}
return false;
}
}
and in inherited classes use it like this:
private int myProperty;
public int MyProperty
{
get { return this.myProperty; }
set { this.SetPropertyValue(ref this.myProperty, value); }
}
When I started Xamarin coding, the MVVM was a bit confusing until I discovered that the PropertyChangedEvent on the ViewModel fired off a signal to the View (ContentPage), and updated the Label/textbox/etc.
For those looking for the 'latest and greatest'... Here's some revised code:
private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
and on your property Setter:
public string SomeProperty
{
get { return _somProperty; }
set
{
_someProperty= value;
OnPropertyChanged();
}
}
}
Nice? No? Saves having to pass the property name each time!
How can I use a dotted path as a property name of a PropertyChangedEventHandler?
public class Person
{
private int _age;
public int Age
{
get { return _age;}
set
{
_age = value;
OnPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
}
public partial class MyControl : UserControl, INotifyPropertyChanged
{
public Person Person
{
get { return (Person)GetValue(PersonProperty); }
set { SetValue(PersonProperty, value); }
}
public static DependencyProperty PersonProperty =
DependencyProperty.Register("Person", typeof (Person), typeof (MyControl), null);
private void someMethod()
{
OnPropertyChanged("Person.Age");
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
}
<TextBox Text="{Binding Person.Age, Mode=TwoWay}"/>
But OnPropertyChanged("Person.Age") cannot resolve the symbol.
Is it possible to use a dotted path as a propertyName of OnPropertyChanged()?
The Age setter, you should always call OnPropertyChanged("Age").
INotifyPropertyChanged isn't meant to be used for sub-properties. You also don't need it on a UserControl, since dependency properties already provide notification. Once you fix your OnPropertyChanged call in the Person class you should be fine.
You have a couple of options to fix the Person.Age setter:
Call OnPropertyChanged("Age") (and remove the = null in the OnPropertyChanged signature.
If you are targeting .NET 4.5 or later, the preferred solution is to change the Person.OnPropertyChanged signature to be OnPropertyChanged(string [CallerMemberName] propertyName = null). Calling OnPropertyChanged() from the Age setter will then fill set propertyName to Age. See the this blog post or the MSDN documentation for more details.
Cant seem to see where I am going wrong? the OnPropertyChange is not being recondnised any suggestions?
public class MedicationList : INotifyPropertyChanged
{
public int MedicationID { get; set; }
public string Description
{
get
{
return Description;
}
set
{
OnPropertyChanged( "Description" );
Description = value;
}
}
}
}
EDIT I have added public class MedicationList : INotifyPropertyChanged
You should implement INotifyPropertyChanged interface, which has single PropertyChanged event declared. You should raise this event if some of object's properties changed. Correct implementation:
public class MedicationList : INotifyPropertyChanged
{
private string _description; // storage for property value
public event PropertyChangedEventHandler PropertyChanged;
public string Description
{
get { return _description; }
set
{
if (_description == value) // check if value changed
return; // do nothing if value same
_description = value; // change value
OnPropertyChanged("Description"); // pass changed property name
}
}
// this method raises PropertyChanged event
protected void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null) // if there is any subscribers
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
I bet you want to do something like this:
public class MedicationList : INotifyPropertyChanged {
public int MedicationID { get; set; }
private string m_Description;
public string Description {
get { return m_Description; }
set {
m_Description = value;
OnPropertyChanged("Description");
}
}
private void OnPropertyChanged(string propertyName) {
if (string.IsNullOrEmpty(propertyName))
throw new ArgumentNullException("propertyName");
var changed = PropertyChanged;
if (changed != null) {
changed(this, new PropertyChangedEventArgs(propertyName));
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
You need the actual code the interface implements inside of your class.
/// <summary>
/// Public event for notifying the view when a property changes.
/// </summary>
public event PropertyChangedEventHandler PropertyChanged;
/// <summary>
/// Raises the PropertyChanged event for the supplied property.
/// </summary>
/// <param name="name">The property name.</param>
internal void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
This method needs to be defined by your type to raise the INotifyPropertyChanged::PropertyChanged event
public PropertyChangedEventHandler PropertyChanged;
...
private void OnPropertyChanged(string propertyName) {
var saved = PropertyChanged;
if (saved != null) {
var e = new PropertyChangedEventArgs(propertyName);
saved(this, e);
}
}
There's a difference between a base class and an interface.
With a base class, the members are automatically inherited and one needs to do nothing (except if some members need an override). With an interface, the class does not inherit the interface members automatically; you have to introduce them in your class. If you don't the compiler will complain.
INotifyPropertyChanged is an interface.
You need to inherent BaseViewModel.
public class MedicationList : BaseViewModel
I use NotifyPropertyWeaverMsBuildTask to handle NotifyPropertyChanged for automatic properties. I know OnPropertyChanged() method rise when Property value is changed. But when this method is called value of property is changed and old value is lost. Is there any way to get old value?
tanx.
If you want to use the old value inside the OnPropertyChanged then write it like this
public void OnPropertyChanged(string propertyName, object before, object after)
Then if your code looks like this
public class Person : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public string Name { get; set; }
public void OnPropertyChanged(string propertyName, object before, object after)
{
// do something with before/after
var propertyChanged = PropertyChanged;
if (propertyChanged != null)
{
propertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
This will be injected
public class Person : INotifyPropertyChanged
{
private string name;
public event PropertyChangedEventHandler PropertyChanged;
public string Name
{
get { return name; }
set
{
object before = Name;
name = value;
OnPropertyChanged("Name", before, Name);
}
}
public void OnPropertyChanged(string propertyName, object before, object after)
{
// do something with before/after
var propertyChanged = PropertyChanged;
if (propertyChanged != null)
{
propertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
More information is available here https://github.com/SimonCropp/NotifyPropertyWeaver/wiki/BeforeAfter
Does this meet your requirements?
INotifyPropertyChanged doesn't provide a means to capture the previous value of a property; you'd have to implement your own.
I apologize for the newbie question, but I am struggling with this problem. I have the following TextBlock defined:
<TextBlock Text="{Binding Source={x:Static local:DeviceManager.Instance},
Path=Player.CurrentArtist}"></TextBlock>
The DeviceManager is a singleton that functions as a facade for other classes. For example, Player is a property of type IPlayer which represents an music-playing application. I would like the TextBlock to display the artist that is currently playing, which is periodically updated in the Player.CurrentArtist property.
Unfortunately, I cannot get the TextBlock to update when the CurrentArtist property updates. Both the DeviceManager and the IPlayer implement INotifyPropertyChanged, but when I step through the application, the DeviceManager does not have an event handler attached to it.
Does anyone have a suggestion for how to update the text block while preserving the singleton-facade?
Here is the code for the INotifyPropertyChanged members in both the DeviceManager and the IPlayer subclass:
public sealed class DeviceManager : INotifyPropertyChanged
{
// Singleton members omitted
public IPlayer Player
{
get { return player; }
set
{
this.player = value;
player.PropertyChanged += new PropertyChangedEventHandler(device_PropertyChanged);
}
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
private void device_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(sender, e);
}
}
#endregion
}
class MediaPlayer : IPlayer
{
private string artist;
private string title;
public event PropertyChangedEventHandler PropertyChanged;
public void Play(string artist, string title)
{
this.artist = artist;
this.title = title;
OnPropertyChanged("Player:Song");
}
private void OnPropertyChanged(string p)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(p));
}
}
public string CurrentTitle
{
get { return title; }
}
public string CurrentArtist
{
get { return artist; }
}
}
The problem is that WPF is never notified of the value of the CurrentArtist property changing. You can either implement a private setter for the CurrentArtist property, which will trigger the PropertyChanged event, or trigger a PropertyChanged event for the CurrentArtist property in MediaPlayer.Play().
WPF only responds to PropertyChanged if the name you pass in (i.e. right now "Player:Song") is the same as the property you're bound to - change the PropertyChanged to "CurrentArtist" and you'll see it update properly.
You are not raising the PropertyChanged event, what you need is:
public sealed class DeviceManager : INotifyPropertyChanged
{
// Singleton members omitted
public IPlayer Player
{
get { return player; }
set
{
this.player = value;
OnPropertyChanged(this, new PropertyChangedEventArgs("Player"));
}
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(sender, e);
}
}
#endregion
}
How does the UI know when you change the Player property? From that code it does not look like it raises PropertyChanged to me. Can you post a complete working sample of the problem? Otherwise we're forced to just guess.