// http://wonderfl.net/code/70fa822e65c4c2d9e9df9bf421cd35d7170ce9c2
// こちらの模写です。しかも動きちょっとおかしいです

package{

import flash.display.Sprite;
  import flash.geom.Point;
  import flash.events.MouseEvent;

[SWF(backgroundColor=0xffffff, frameRate=60)]

public class Splite extends Sprite{

private var _startPoint:Point;
    private var _endPoint:Point;
    private var _line:Sprite;
    private var _paper:Paper;
  
    public function Splite():void{
      _line = new Sprite();
      _paper = new Paper( [new Point(0,0), new Point(200,0), new Point(200,200), new Point(0,200)] );
      _paper.x = _paper.y = 100;
      stage.addEventListener( MouseEvent.MOUSE_DOWN, mouseDownHandler, false, 0, true);
      addChild(_paper);
      addChild(_line);
    }

private function mouseDownHandler( event:MouseEvent ):void{
      _startPoint = new Point( stage.mouseX, stage.mouseY);
      stage.removeEventListener( MouseEvent.MOUSE_DOWN, mouseDownHandler);
      stage.addEventListener( MouseEvent.MOUSE_UP, mouseUpHandler, false, 0, true);
      stage.addEventListener( MouseEvent.MOUSE_MOVE, mouseDragHandler, false, 0, true);
    }

private function mouseUpHandler( event:MouseEvent):void{
      _endPoint = new Point( stage.mouseX, stage.mouseY);
      _paper.cut(_startPoint, _endPoint);
      stage.removeEventListener(MouseEvent.MOUSE_UP, mouseUpHandler);
      stage.removeEventListener(MouseEvent.MOUSE_MOVE, mouseDragHandler);
      stage.addEventListener(MouseEvent.MOUSE_DOWN, mouseDownHandler, false, 0, true);
      _line.graphics.clear();
    }

private function mouseDragHandler( event:MouseEvent):void{
      _line.graphics.clear();
      _line.graphics.lineStyle( 1, 0x000000, 1);
      _line.graphics.moveTo( _startPoint.x, _startPoint.y);
      _line.graphics.lineTo( stage.mouseX, stage.mouseY);
      _line.graphics.endFill();
    }

}

import flash.display.Sprite;
  import flash.geom.Point;
  import flash.events.MouseEvent;
  class Paper extends Sprite{
    public function Paper( points:Array ):void{
      addChild( new PaperFragment(points));
    }
    public function cut(_startPoint:Point, _endPoint:Point):void{
    trace(String(this.numChildren));
    var num:int = this.numChildren;
      for(var i:uint = 0; i < num; i++){
        this.getChildAt(i).dispatchEvent(new MyEvent(MyEvent.CUT, _startPoint, _endPoint));
      }
    }
  }

class PaperFragment extends Sprite{
    private var _points:Array;

public function PaperFragment( points:Array ):void{
      trace("new PaperFragment");
      _points = points;
      drawRectFromPoint();
      addEventListener( MyEvent.CUT, cut, false, 0, true);
    }

private function cut(event:MyEvent):void{
      trace("cut PaperFragment");
      var _startPoint:Point = globalToLocal(event.startPoint);
      var _endPoint:Point = globalToLocal(event.endPoint);
      var _newPointArray:Array = [new Array(), new Array()];
      var _numCloss:int = 0;

for( var i:int = 0; i < _points.length; i++){
        var _target1:Point = _points[i];
        var _target2:Point = _points[i+1] ? _points[i+1] : _points[0];
        var _clossPt:Point = crossPoint( _startPoint, _endPoint, _target1, _target2);

_newPointArray[0].push(_target1);
	if(_clossPt){
	  _newPointArray[0].push(_clossPt);
	  _newPointArray[1].push(_clossPt);
	  _newPointArray.reverse();
	  _numCloss++;
	}
      }

if(_numCloss == 2){
        this.removeEventListener( MyEvent.CUT, cut);
        var _newObj1:PaperFragment = new PaperFragment(_newPointArray[0]);
        var _newObj2:PaperFragment = new PaperFragment(_newPointArray[1]);
	_newObj1.x = _newObj2.x = this.x;
	_newObj1.y = _newObj2.y = this.y;
	parent.addChild(_newObj1);
	parent.addChild(_newObj2);
	trace(parent);
	parent.removeChild(this);
      }
    
    }

/*
      * ------------------------------------------------------------
      * 交点を割り出す
      * ------------------------------------------------------------
     */
     private function crossPoint( _pt1:Point , _pt2:Point , _pt3:Point , _pt4:Point ):Point
     {    
         var _vector1:Point = _pt2.subtract( _pt1 );
         var _vector2:Point = _pt4.subtract( _pt3 );
         
         if ( cross( _vector1, _vector2 ) == 0.0) return null;
         
         var _s:Number = cross( _vector2 , _pt3.subtract( _pt1) ) / cross( _vector2 , _vector1 );
         var _t:Number = cross( _vector1, _pt1.subtract( _pt3 ) ) / cross( _vector1, _vector2 );

if ( isCross( _s ) && isCross( _t ) )
         {
             _vector1.x *= _s;
             _vector1.y *= _s;
             return _pt1.add( _vector1 );
         }
         else return null;
     }
         
     private function cross( _vector1:Point , _vector2:Point ):Number
     {
         return ( _vector1.x * _vector2.y - _vector1.y * _vector2.x );
     }
     
     public static function isCross( _n:Number ):Boolean
     {
         return ( ( 0 <= _n ) && ( _n <= 1) );
     }

private function drawRectFromPoint():void{
      this.graphics.beginFill(Math.random() * 0xffff00);
      this.graphics.moveTo(_points[0].x, _points[0].y);
      for( var i:int = 1; i < _points.length; i++){
	this.graphics.lineTo( _points[i].x, _points[i].y);
      }
      this.graphics.endFill();
    }
  
  }

import flash.events.Event;
  class MyEvent extends Event{
    public var startPoint:Point;
    public var endPoint:Point;
    public static const CUT:String = "cut";
    public function MyEvent(type:String, _startPoint:Point, _endPoint:Point ){
      trace("new MyEvent");
      super(type);
      this.startPoint = _startPoint;
      this.endPoint = _endPoint;
    }
  }

flash on 2009-12-28