Flash/Flex学习笔记(22)：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。 
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：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：