Flash/Flex学习笔记(25):摩擦力与屏幕环绕

摩擦力:

假如一个物体在某个方向上沿直线运行,摩擦力会使该方向上的速度越来越小,直到停止。
Flash/Flex学习笔记(25):摩擦力与屏幕环绕

上图示意了该过程,物体以moveAngle角度正向运动,最终的速度speed矢量为vx矢量与vy矢量的矢量和,在每个单位时间内的位移即Speed矢量的大小,分解到x,y轴后,即为vx与vy;加入摩擦力后,speed矢量每单位时间将减少Friction值,也就是视觉上的越来越慢。

var ball:Ball = new Ball(10);

ball.x = stage.stageWidth/2;

ball.y = stage.stageHeight/2;

addChild(ball);

Mouse.cursor = MouseCursor.BUTTON;

var Velocity:Number = 10;//速度最大值

var friction = 0.4;//摩擦力因子

stage.addEventListener(MouseEvent.MOUSE_DOWN,MouseDownHandler);

stage.addEventListener(MouseEvent.MOUSE_UP,MouseUpHandler);



function MouseDownHandler(e:MouseEvent):void{

	graphics.clear();

	//初始化小球位置以速度

	ball.x = stage.stageWidth/2;

	ball.y = stage.stageHeight/2;

	ball.vx = (Math.random()*2-1) * Velocity;

	ball.vy = (Math.random()*2-1) * Velocity;	

	graphics.moveTo(ball.x,ball.y);

	graphics.lineStyle(1,0xcccccc,1);

}



function MouseUpHandler(e:MouseEvent):void{	

	addEventListener(Event.ENTER_FRAME,EnterFrameHandler);

}



function EnterFrameHandler(e:Event):void{

	ball.x += ball.vx;

	ball.y += ball.vy;

	

	var speed:Number = Math.sqrt(ball.vx*ball.vx + ball.vy*ball.vy);

	var moveAngle = Math.atan2(ball.vy,ball.vx);

	

	speed -= friction;

	

	//减速后的新速度

	ball.vx = speed*Math.cos(moveAngle);

	ball.vy = speed*Math.sin(moveAngle);

	

	//防止减速过度,就成反向运动

	if (speed<=friction){

		ball.vx = 0;

		ball.vy = 0;

		removeEventListener(Event.ENTER_FRAME,EnterFrameHandler);

	}	

	graphics.lineTo(ball.x,ball.y);

	

}

上面这种方法从物理意义上讲最接近现实情况,不过有些复杂,在实际开发中还有一种更简单的办法,虽然不怎么严密,但从视觉效果上很难看出问题

var ball:Ball = new Ball(10);

ball.x = stage.stageWidth/2;

ball.y = stage.stageHeight/2;

addChild(ball);

Mouse.cursor = MouseCursor.BUTTON;

var Velocity:Number = 10;

var friction = 0.9;//摩擦力因子(小于1大于0即可)

stage.addEventListener(MouseEvent.MOUSE_DOWN,MouseDownHandler);

stage.addEventListener(MouseEvent.MOUSE_UP,MouseUpHandler);



function MouseDownHandler(e:MouseEvent):void{

	graphics.clear();	

	ball.x = stage.stageWidth/2;

	ball.y = stage.stageHeight/2;

	ball.vx = (Math.random()*2-1) * Velocity;

	ball.vy = (Math.random()*2-1) * Velocity;	

	graphics.moveTo(ball.x,ball.y);

	graphics.lineStyle(1,0xcccccc,1);

}



function MouseUpHandler(e:MouseEvent):void{	

	addEventListener(Event.ENTER_FRAME,EnterFrameHandler);

}



function EnterFrameHandler(e:Event):void{

	ball.x += ball.vx;

	ball.y += ball.vy;

	

	ball.vx = ball.vx * friction;//直接让x轴速度不断衰减

	ball.vy = ball.vy * friction;//直接让y轴速度不断衰减	

	

	if (Math.abs(ball.vx)<=0.0001 || Math.abs(ball.vy)<=0.0001){

		ball.vx = 0;

		ball.vy = 0;

		removeEventListener(Event.ENTER_FRAME,EnterFrameHandler);

	}	

	

	//trace(ball.vx);

	//trace(ball.vy);

	graphics.lineTo(ball.x,ball.y);

	

}

屏幕环绕:

这个叫法也许从字面上不太直观,说得更白一点就是:一个物体如果在运动过程中跑出了舞台边界,开发人员就要想办法让其从舞台的另一端出现,并继续运动,以保持连贯。前面的一篇文章 Flash/Flex学习笔记(23):运动学原理 中有一个飞船的示例,加入屏幕环绕处理后,代码如下:

package {

	import flash.display.Sprite;

	import flash.events.Event;

	import flash.events.KeyboardEvent;

	import flash.ui.Keyboard;

	import flash.display.StageAlign;

	import flash.display.StageScaleMode;

	import fl.controls.Label;

	

	public class ShipSim2 extends Sprite {

		

		private var ship:Ship;

		private var vr:Number=0;

		private var thrust:Number=0;

		private var vx:Number=0;

		private var vy:Number=0;		

		

		public function ShipSim2() {

			init();

		}

		

		private function init():void {

			stage.scaleMode=StageScaleMode.NO_SCALE;

			stage.align=StageAlign.TOP_LEFT;			

			

			ship = new Ship();

			addChild(ship);

			ship.x=stage.stageWidth/2;

			ship.y=stage.stageHeight/2;

			addEventListener(Event.ENTER_FRAME, EnterFrameHandler);

			stage.addEventListener(KeyboardEvent.KEY_DOWN, KeyDownHandler);

			stage.addEventListener(KeyboardEvent.KEY_UP, KeyUpHandler);

		}

		

		private function KeyDownHandler(event:KeyboardEvent):void {

			switch (event.keyCode) {

				case Keyboard.LEFT :

					vr=-5;

					break;

				case Keyboard.RIGHT :

					vr=5;

					break;

				case Keyboard.UP :

					thrust=0.2;

					ship.draw(true);

					break;

				default :

					break;

			}

		}

		

		private function KeyUpHandler(event:KeyboardEvent):void {

			vr=0;			

			thrust=0;

			ship.draw(false);

		}

		

		private function EnterFrameHandler(event:Event):void {

			ship.rotation+=vr;

			var angle:Number=ship.rotation*Math.PI/180;

			var ax:Number=Math.cos(angle)*thrust;

			var ay:Number=Math.sin(angle)*thrust;

			vx+=ax;

			vy+=ay;

			ship.x+=vx;

			ship.y+=vy;

			var left:Number=0;

			var right:Number=stage.stageWidth;

			var top:Number=0;

			var bottom:Number=stage.stageHeight;

			//屏幕环绕处理

			if (ship.x>right + ship.width/2) {

				ship.x=left-ship.width/2;

			} else if (ship.x < left - ship.width/2) {

				ship.x=right+ship.width/2;

			}

			if (ship.y-ship.height/2>bottom) {

				ship.y=top-ship.height/2;

			} else if (ship.y < top - ship.height / 2) {

				ship.y=bottom+ship.height/2;

			}

		}

	}

}

最后把这二者结合起来,看下效果:

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