Notification object about setting property's member

Assume we have following pseudocode:

class XY
{
    int X { get; set; }
    int Y { get; set; }
}
class Foo
{
    XY _xy;
    XY xy
    {
        get 
        {
            return _xy;
        }
        set
        {
            Write("Foo's XY is set!");
            _xy = value;
        }
    }
}

This works fine as long as I'm doing

Foo foo;
foo.xy = XY(1, 3);
XY temp = foo.xy;
temp.y = 5;
foo.xy = temp;

but doesn't work for:

Foo foo;
foo.xy = XY(1, 3);
foo.xy.y = 5;      // no "Foo's XY is set!" here

How the latter can be开发者_如何学编程 achieved? Specifically I mean Lua (with _index/_newindex) but I'm writing example code in C#ish language because I think most people know it well and I believe this is more generic programming problem.

Why would it? You didn't set Foo.xy. You should implement some kind of notification to the base object if you want to achieve that.

In C# the common pattern is implementing INotifyPropertyChanged interface on XY and subscribing Foo to it.

Example:

class XY: INotifyPropertyChanged
{
    public X { get {...} set { _x = value; PropertyChanged("X"); }}
    // implementation of interface...
}

class Foo
{
     public Foo(XY xy)
     {
          this._xy = xy;
          this._xy.PropertyChanged += delegate { Console.WriteLine("changed"); }
     }
}

You are actually calling the "getter" in your example. That's why do you don't see your output.

foo.xy.y = 5

You are not replacing the reference to the variable, you are retrieving it.