Flex: drawing a connector line between shapes

Here is an example doing what you want.

package
{
    import flash.display.Shape;
    import flash.display.Sprite;
    import flash.events.Event;
    import flash.geom.Matrix;
    import flash.geom.Point;
    import flash.ui.Mouse;

    /**
     * Sample class to draw squares and arrows between them.
     */
    public class SquareArrows extends Sprite
    {
        /**
         * Initialize the scene as soon as we can.
         */
        public function SquareArrows()
        {
            if(stage) {
                init();
            }
            else {
                addEventListener(Event.ADDED_TO_STAGE, init);
            }
        }

        /**
         * Draw two squares and an arrow between them.
         */
        private function init(e : Event = null) : void
        {
            if(hasEventListener(Event.ADDED_TO_STAGE)) {
                removeEventListener(Event.ADDED_TO_STAGE, init);
            }

            // Drawing random-sized squares.
            var squareOne : Shape =
                    getSquareShape((Math.random() * 50) + 20, 0xBBBBBB);
            var squareTwo : Shape =
                    getSquareShape((Math.random() * 50) + 20, 0xDDDDDD);
            addChild(squareOne);
            addChild(squareTwo);

            // Draw the connector.
            var connector : Shape = getConnectorShape(squareOne, squareTwo);
            addChild(connector);
        }

        /**
         * Draw a connector arrow between two square shapes.
         */
        private function getConnectorShape(connectFrom : Shape, connectTo : Shape) : Shape
        {
            // Getting the center of the first square.
            var centerFrom : Point = new Point();
            centerFrom.x = connectFrom.x + (connectFrom.width / 2);
            centerFrom.y = connectFrom.y + (connectFrom.height / 2);

            // Getting the center of the second square.
            var centerTo : Point = new Point();
            centerTo.x = connectTo.x + (connectTo.width / 2);
            centerTo.y = connectTo.y + (connectTo.height / 2);

            // Getting the angle between those two.
            var angleTo : Number =
                Math.atan2(centerTo.x - centerFrom.x, centerTo.y - centerFrom.y);
            var angleFrom : Number =
                Math.atan2(centerFrom.x - centerTo.x, centerFrom.y - centerTo.y);

            // Getting the points on both borders.
            var pointFrom : Point = getSquareBorderPointAtAngle(connectFrom, angleTo);
            var pointTo : Point = getSquareBorderPointAtAngle(connectTo, angleFrom);

            // Calculating arrow edges.
            var arrowSlope : Number = 30;
            var arrowHeadLength : Number = 10;
            var vector : Point =
                new Point(-(pointTo.x - pointFrom.x), -(pointTo.y - pointFrom.y));

            // First edge of the head...
            var edgeOneMatrix : Matrix = new Matrix();
            edgeOneMatrix.rotate(arrowSlope * Math.PI / 180);
            var edgeOneVector : Point = edgeOneMatrix.transformPoint(vector);
            edgeOneVector.normalize(arrowHeadLength);
            var edgeOne : Point = new Point();
            edgeOne.x = pointTo.x + edgeOneVector.x;
            edgeOne.y = pointTo.y + edgeOneVector.y;

            // And second edge of the head.
            var edgeTwoMatrix : Matrix = new Matrix();
            edgeTwoMatrix.rotate((0 - arrowSlope) * Math.PI / 180);
            var edgeTwoVector : Point = edgeTwoMatrix.transformPoint(vector);
            edgeTwoVector.normalize(arrowHeadLength);
            var edgeTwo : Point = new Point();
            edgeTwo.x = pointTo.x + edgeTwoVector.x;
            edgeTwo.y = pointTo.y + edgeTwoVector.y;

            // Drawing the arrow.
            var arrow : Shape = new Shape();
            with(arrow.graphics) {
                lineStyle(2);
                // Drawing the line.
                moveTo(pointFrom.x, pointFrom.y);
                lineTo(pointTo.x, pointTo.y);

                // Drawing the arrow head.
                lineTo(edgeOne.x, edgeOne.y);
                moveTo(pointTo.x, pointTo.y);
                lineTo(edgeTwo.x, edgeTwo.y);
            }
            return arrow;
        }

        /**
         * Utility method to get a point on a square border at a certain angle.
         */
        private function getSquareBorderPointAtAngle(square : Shape, angle : Number) : Point
        {
            // Calculating rays of inner and outer circles.
            var minRay : Number = Math.SQRT2 * square.width / 2;
            var maxRay : Number = square.width / 2;

            // Calculating the weight of each rays depending on the angle.
            var rayAtAngle : Number = ((maxRay - minRay) * Math.abs(Math.cos(angle * 2))) + minRay;

            // We have our point.
            var point : Point = new Point();
            point.x = rayAtAngle * Math.sin(angle) + square.x + (square.width / 2);
            point.y = rayAtAngle * Math.cos(angle) + square.y + (square.height / 2);
            return point;
        }

        /**
         * Utility method to draw a square of a given size in a new shape.
         */
        private function getSquareShape(edgeSize : Number, fillColor : Number) : Shape
        {
            // Draw the square.
            var square : Shape = new Shape();
            with(square.graphics) {
                lineStyle(1);
                beginFill(fillColor);
                drawRect(0, 0, edgeSize, edgeSize);
                endFill();
            }

            // Set a random position.
            square.x = Math.random() * (stage.stageWidth - square.width);
            square.y = Math.random() * (stage.stageHeight - square.height);

            return square;
        }
    }
}

This code isn't totally optimized. The idea is more to explain how it works. Basically, we are defining two (random) squares, and tracing a line between them. To trace the line, we calculate an angle from the center of the first square to the center of the second one, and we use a special method (getSquareBorderPointAtAngle) to extract a point on the square border in the right direction.

This method is the first key point of this snippet. We calculate that using simple circle geometry, with a little complexification on how we make the point match the border instead of matching a circle around or inside the square.

Then, we draw an arrow head. For that, we're making use of the Flash Matrix class, because it's much easier this way than to calculate it from the scratch.

And here we're done.