How to let a parent class know about a change in its children?

Question

This is an example code:

public class MyParent : INotifyPropertyChanged
{
    List MyChildren;

    public bool IsChanged
    {
        get
               


        

Answer 1

INotifyPropertyChanged has already provided the mechanism for you: the PropertyChanged event. Just have the parent add a handler to its children's PropertyChanged, and then in that handler call RaiseChanged("IsChanged");

Also, you may want to put the INotifyPropertyChanged implementation in a base class, and have your (what appear to be) ViewModels inherit from that. Not required for this option, of course, but it will make the code a little cleaner.

Update: In the parent object:

// This list tracks the handlers, so you can
// remove them if you're no longer interested in receiving notifications.
//  It can be ommitted if you prefer.
List> changedHandlers =
    new List>();

// Call this method to add children to the parent
public void AddChild(MyChild newChild)
{
    // Omitted: error checking, and ensuring newChild isn't already in the list
    this.MyChildren.Add(newChild);
    EventHandler eh =
        new EventHandler(ChildChanged);
    newChild.PropertyChanged += eh;
    this.changedHandlers.Add(eh);
}

public void ChildChanged(object sender, PropertyChangedEventArgs e)
{
    MyChild child = sender as MyChild;
    if (this.MyChildren.Contains(child))
    {
        RaiseChanged("IsChanged");
    }
}

You don't actually have to add anything to the child class, since it is already raising the correct event when it changes.