Collision Detection between Circles

Ball.as

package  
{
	import flash.display.Sprite;
	/**
	 * Ball sprite
	 * @author Shiu
	 */
	public class Ball extends Sprite
	{
		private var _radius:Number;
		private var _col:Number;
		
		public function Ball(radius:Number = 10, col:Number = 0xFF6633) {
			this._radius = radius;
			this._col = col;
			this.draw();
		}
		
		//function to draw the sprite according to specifications
		private function draw():void {
			graphics.beginFill(_col);
			graphics.drawCircle(0, 0, _radius);
			graphics.endFill();
		}
		
		public function get radius():Number {
			return _radius;
		}
		
		public function set radius(value:Number):void {
			_radius = value;
			this.draw();
		}
		
		public function get col():Number {
			return _col;
		}
		
		public function set col(value:Number):void {
			_col = value;
			this.draw();
		}
	}
}

Main.as

package 
{
	import flash.display.Sprite;
	import flash.events.Event;
	
	/**
	 * Purpose: Demo on circle/ circle collision detection
	 * @author Shiu
	 */
	public class Main extends Sprite 
	{
		private var b:Ball;
		private var c:Ball;
		
		public function Main():void 
		{
			if (stage) init();
			else addEventListener(Event.ADDED_TO_STAGE, init);
		}
		
		private function init(e:Event = null):void 
		{
			removeEventListener(Event.ADDED_TO_STAGE, init);
			// entry point
			
			c = new Ball; addChild(c); c.x = 300; c.y = 500; c.radius = 30; c.col = 0x339911;
			b = new Ball; addChild(b); b.x = 200; b.y = 200;	//placing in the sprites
			
			b.addEventListener(Event.ENTER_FRAME, move); // adding in animation
		}
		
		//b will always move toward c
		private function move(e:Event):void 
		{
			var vec:Vector2D = new Vector2D(c.x - b.x, c.y - b.y);
			vec.setMagnitude(10);
			if (collide(b,c)) b.removeEventListener(Event.ENTER_FRAME, move);
			else {b.x += vec.x; b.y += vec.y;}
		}
		
		//collision detection
		private function collide (b1:Ball, b2:Ball) :Boolean {
			var isCollide:Boolean = false;
			var dist:Vector2D = new Vector2D(b1.x - b2.x, b1.y - b2.y);
			if (dist.getMagnitude() <= b1.radius +b2.radius) isCollide = true;
			return isCollide;
		}
	}
	
}