Reactive Extensions(RX) Rotation angle - system.reactive

Question

We have all seen the nice RX handling of mouse drag n drop.
I want something similar but outputting the change in angle to the centre of the screen.
Like if the user hits the mouse button and circles the screen twice I would get values from 0 to 720 degrees.
Essentially rotating the thing on screen.
How would one do that?

Answer 1

This will give you the angle in radians from the center of the object. It will only operate in the range (-Pi, Pi] however. If you rotate three times, you won't get a value higher than a single rotation.
var mousedown = from evt in Observable.FromEventPattern<MouseButtonEventArgs>(this, "MouseDown")
select evt.EventArgs.GetPosition(this);
var mouseup = from evt in Observable.FromEventPattern<MouseEventArgs>(this, "MouseUp")
select evt.EventArgs.GetPosition(this);
var mousemove = from evt in Observable.FromEventPattern<MouseEventArgs>(this, "MouseMove")
select evt.EventArgs.GetPosition(this);
Vector center = new Vector(this.Width / 2, this.Height / 2);
var radian = from start in mousedown
from pos in mousemove.StartWith(start).TakeUntil(mouseup)
select Math.Atan2((pos - center).Y, (pos -center).X);
EDIT
If you are after the change in angle, the following should work:
var angle = from start in mousedown
from pos in mousemove.StartWith(start).TakeUntil(mouseup)
select Vector.AngleBetween(pos - center, start - center);

Answer 2

Thanks for your help. Ive figured out the last bit myself:
var down = Observable.FromEventPattern<MouseButtonEventArgs>(this, "MouseDown").Select(e=>e.EventArgs.GetPosition(this));
var move = Observable.FromEventPattern<MouseEventArgs>(this, "MouseMove").Select(e => e.EventArgs.GetPosition(this));
var up = Observable.FromEventPattern<MouseEventArgs>(this, "MouseUp").Select(e => e.EventArgs.GetPosition(this));
Vector center = new Vector(this.Width / 2, this.Height / 2);
var f = from start in down
from pos in move.StartWith(start).TakeUntil(up).Buffer(2, 1)
where pos.Count == 2
select Vector.AngleBetween(new Vector(pos[0].X, pos[0].Y) - center, new Vector(pos[1].X, pos[1].Y) - center);
f.ObserveOnDispatcher().Subscribe(p => {
game.Player.Angle += p;
});