cocos2dhtml5背景循环移动

var mainLayer = cc.Layer.extend({   ctor : function () {    this._super();    cc.associateWithNative(this, cc.Layer);

   // 中间背景    var mainBG1 = cc.Sprite.create(Res.Images.Main.main_bg);    var scale = WIN_HEIGHT / mainBG1.getContentSize().height;    mainBG1.setScale(scale);    mainBG1.setAnchorPoint(cc.p(0.5, 0.5));    mainBG1.setPosition(cc.p(WIN_WIDTH / 2, WIN_HEIGHT / 2));    this.addChild(mainBG1);    this.mainBG1 = mainBG1;

   // 粒子 火焰    this.ParticleSnow = cc.ParticleSystem.create(Res.Plists.Splash.splash_plist);    mainBG1.addChild(this.ParticleSnow);    this.ParticleSnow.setPosition(cc.p(260, 120));    if (this.ParticleSnow.setShapeType)     this.ParticleSnow.setShapeType(cc.PARTICLE_STAR_SHAPE);

   // 粒子 雪花    this._emitter = cc.ParticleRain.create();    mainBG1.addChild(this._emitter);    this._emitter.setPosition(cc.p(320, 960));    this._emitter.setLife(5);    this._emitter.setLifeVar(1);    this._emitter.setGravity(cc.p(0, -10));    this._emitter.setSpeed(10);    this._emitter.setSpeedVar(200);

   var startColor = this._emitter.getStartColor();    startColor.r = 0.9;    startColor.g = 0.9;    startColor.b = 0.9;    this._emitter.setStartColor(startColor);

   var startColorVar = this._emitter.getStartColorVar();    startColorVar.b = 0.1;    this._emitter.setStartColorVar(startColorVar);    this._emitter.setEmissionRate(this._emitter.getTotalParticles() / this._emitter.getLife());    this._emitter.setTexture(cc.TextureCache.getInstance().addImage(Res.Images.Main.menu_snow));    if (this._emitter.setShapeType)     this._emitter.setShapeType(cc.PARTICLE_STAR_SHAPE);

   // var mainBG2 = cc.Sprite.create(Res.Images.Main.main_bg);    // mainBG2.setScale(scale);    // mainBG2.setAnchorPoint(cc.p(0, 0));    // mainBG2.setPosition(cc.p(mainBG1.getContentSize().width, 0));    // this.addChild(mainBG2);    // this.mainBG2 = mainBG2;    // this.schedule(this.update);

   // 上中下三层    this.topLayer = Global.topLayer = new topLayer();    this.addChild(this.topLayer, 1, 1);

   this.botLayer = Global.botLayer = new botLayer();    this.addChild(this.botLayer, 1, 2);

   this.midLayer = Global.midLayer = new midLayer();    this.midLayer.setScale(WIN_SCALE_WIDTH);    this.addChild(this.midLayer, 0, 3);   },   update : function () {    //创建两个点对象..分别保存两张地图的位置    var map_1p = this.mainBG1.getPosition();    var map_2p = this.mainBG2.getPosition();

   //每次调用这个函数的时候就设置两张地图的位置..这里的-3值越大也就越快.    this.mainBG1.setPosition(cc.p(map_1p.x - 0.2, map_1p.y));    this.mainBG2.setPosition(cc.p(map_2p.x - 0.2, map_2p.y));

   //分别判断地图.如果完全出场景了..马上调整位置接上    var scale = WIN_HEIGHT / this.mainBG1.getContentSize().height;    if (map_1p.x < 0) {     this.mainBG2.setPosition(cc.p(map_1p.x + this.mainBG1.getContentSize().width * scale - 2, map_1p.y));    }

   if (map_2p.x < 0) {     this.mainBG1.setPosition(cc.p(map_2p.x + this.mainBG2.getContentSize().width * scale - 2, map_2p.y));    }   }  });