Question

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 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.

Answer 1

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"); }
     }
}