HTML5游戏开发进阶 4 ：物理引擎集成

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原创最新推荐文章于 2026-10-02 13:34:26 发布 1.2k 阅读 AI 写同款· GEO 优化 ›

HTML5 前端物理引擎：游戏开发必备本文旨在全面介绍HTML5前端物理引擎的技术原理和实现方法，帮助开发者理解如何在网页中实现真实的物理效果。我们将涵盖从基础概念到实际应用的完整知识体系。核心概念与联系：介绍物理引擎的基本原理核心算法原理：解析物理引擎的关键算法项目实战：通过实际案例展示物理引擎的应用工具和资源推荐：分享实用的开发工具未来发展趋势：展望物理引擎的发展方向物理引擎：模拟物理规律（如重力、碰撞、摩擦等）的软件组件刚体：形状和大小不会因外力作用而改变的理想化物体碰撞检测：判断两个或多个物体是否发生接触的过程。阅读详情

首先向关卡中添加物体，对这些物体进行Box2D物理仿真，然后让它们在游戏中动起来。我们将使用这些物体创建测试关卡，加入鼠标交互，使游戏真正具有可玩性。最后，再向测试关卡加入音效和背景音乐，制作出一个完整的游戏。

4.1 定义物体

装载的物体：英雄、坏蛋、地面和环境中的障碍物。

属性type包含："hero", "villain", "ground", "block"这样的值。

entities 对象

4.2 添加Box2D

创建box2d对象：

4.3 创建物体

定义entities.create方法：

4.4 向关卡加入物体

levels.array数组

levels.load

4.5 设置Box2D调试绘图

仅仅是设计和测试关卡

4.6 绘制物体

entities对象内部定义draw()方法

4.7 Box2D动画

时间步长

box2d.step()

4.8 加载英雄

第一个阶段load-next-hero

game.countHeroesAndVillains();

4.9 发射英雄

game.mouseOnCurrentHero()

使用以下三个阶段实现发射英雄：

wait-for-firing：游戏画面平移至弹弓，等待鼠标单击并拖动英雄，然后切换至firing阶段
firing：游戏随着鼠标移动英雄，直到鼠标按键松开，此时以特定的速度将英雄抛射出去，并切换到fired阶段。抛射的速度基于英雄与弹弓的距离。
fired：游戏画面跟随英雄移动，直至英雄静止下来或者飞出关卡边界之外，然后游戏将英雄从游戏世界中移除，并切换至load-next-hero阶段。

4.10 结束关卡

显示关卡结束画面

4.11 碰撞损坏

添加listener事件

4.12 绘制弹弓橡胶带

game.drawSlingshotBand()

4.13 切换关卡

重新开始关卡和开始下一个关卡的按钮

4.14 添加声音

首先，添加一些音效，如弹弓被释放、英雄或坏蛋弹跳、障碍物被摧毁的音效。

小技巧：在http://www.ccMixter.com中可以为自己的游戏找到一些免费又优质的音乐

设置断裂和反弹的音效

碰撞时播放弹跳声：listener.PostSolve

别摧毁时播放破坏声：

被发射时播放弹弓释放的声音

添加背景音乐：

index.html

<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-type" content="text/html; charset=utf-8">
    <title>Froot Wars</title>
    <script src="js/jquery.min.js" type="text/javascript" charset="utf-8"></script>
    <script src="js/Box2dWeb-2.1.a.3.min.js" type="text/javascript" charset="utf-8"></script>
    <script src="js/game.js" type="text/javascript" charset="utf-8"></script>
    <link rel="stylesheet" href="style.css" type="text/css" media="screen" charset="utf-8">
  </head>
  <body>
    <div id="gamecontainer">
      <canvas id="gamecanvas" width="640" height="480" class="gamelayer"></canvas>
      <div id="scorescreen" class="gamelayer">
        <img id="togglemusic" src="images/icons/sound.png" onclick="game.toggleBackgroundMusic()">
        <img src="images/icons/prev.png" onclick="game.restartLevel();">
        <span id="score">Score: 0</span>
      </div>
      <div id="gamestartscreen" class="gamelayer">
        <img src="images/icons/play.png" alt="Play Game" onclick="game.showLevelScreen();"><br>
        <img src="images/icons/settings.png" alt="Settings">
      </div>
      <div id="levelselectscreen" class="gamelayer">
      </div>
      <div id="loadingscreen" class="gamelayer">
        <div id="loadingmessage"></div>
      </div>
      <div id="endingscreen" class="gamelayer">
        <div>
          <p id="endingmessage"> The Level Is Over Message<p>
          <p id="playcurrentlevel" onclick="game.restartLevel();"><img src="images/icons/prev.png" >Replay Current Level</p>
          <p id="playnextlevel" onclick="game.startNextLevel();"><img src="images/icons/next.png"> Play Next Level </p>
          <p id="showLevelScreen" onclick="game.showLevelScreen();"><img src="images/icons/return.png"> Return to Level Screen</p>
        </div>
      </div>
    </div>
    <canvas id="debugcanvas" width="1000" height="480" style="border:1px solid block;"></canvas>
  </body>
</html>

style.css

#gamecontainer {
	width: 640px;
	height: 480px;
	background: url(images/splashscreen.png);
	border: 1px solid block;
}
.gamelayer {
	width: 640px;
	height: 480px;
	position: absolute;
	display: none;
}
/* 开始菜单画面 */
#gamestartscreen {
	padding-top:250px;
	text-align:center;
}
#gamestartscreen img{
	margin:10px;
	cursor:pointer;
}
/* 关卡选择画面 */
#levelselectscreen {
	padding-top: 150px;
	padding-left: 50px;
}
#levelselectscreen input {
	margin: 20px;
	cursor: pointer;
	background: url(images/icons/level.png) no-repeat;
	color: yellow;
	font-size: 20px;
	width: 64px;
	height: 64px;
	border: 0;
}
/* 加载画面 */
#loadingscreen {
	background: rgba(100,100,100,0.3);
}
#loadingmessage {
	margin-top:400px;
	text-align:center;
	height: 48px;
	color:white;
	background:url(images/loader.gif) no-repeat center;
	font:12px Arial;
}
/* 计分板 */
#scorescreen {
	height: 60px;
	font: 32px Comic Sans MS;
	text-shadow: 0 0 2px #000;
	color: white;
}
#scorescreen img{
	opacity: 0.6;
	top: 10px;
	position: relative;
	padding-left: 10px;
	cursor: pointer;
}
#scorescreen #score {
	position: absolute;
	top: 5px;
	right: 20px;
}
/* 结束画面 */
endingscreen {
	text-align: center;
}
#endingscreen div {
	height: 430px;
	padding-top: 50px;
	border: 1px;
	background: rgba(1,1,1,0.5);
	text-align: left;
	padding-left: 100px;
}
#endingscreen p {
	font: 20px Comic Sans MS;
	text-shadow: 0 0 2px #000;
	color: white;
}
#endingscreen p img {
	top: 10px;
	position: relative;
	cursor: pointer;
}
#endingscreen #endingmessage {
	font: 32px Comic Sans MS;
	text-shadow: 0 0 2px #000;
	color: white;
}

game.js

// Declare all the commonly used objects as variables for convenience
var b2Vec2 = Box2D.Common.Math.b2Vec2;
var b2BodyDef = Box2D.Dynamics.b2BodyDef;
var b2Body = Box2D.Dynamics.b2Body;
var b2FixtureDef = Box2D.Dynamics.b2FixtureDef;
var b2Fixture = Box2D.Dynamics.b2Fixture;
var b2World = Box2D.Dynamics.b2World;
var b2PolygonShape = Box2D.Collision.Shapes.b2PolygonShape;
var b2CircleShape = Box2D.Collision.Shapes.b2CircleShape;
var b2DebugDraw = Box2D.Dynamics.b2DebugDraw;

// 建立requestAnimationFrame和cancelAnimationFrame以在游戏代码中使用
(function() {
    var lastTime = 0;
    var vendors = ['ms', 'moz', 'webkit', 'o'];
    for (var x=0; x<vendors.length && !window.requestAnimationFrame; ++x) {
        window.requestAnimationFrame = window[vendors[x] + 'RequestAnimationFrame'];
        window.cancelAnimationFrame = window[vendors[x] + 'CancelAnimationFrame'] ||
           window[vendors[x] + 'CancelAnimationFrame'];
    }
    if (!window.requestAnimationFrame) {
        window.requestAnimationFrame = function(callback, element) {
            var currTime = new Date().getTime();
            var timeToCall = Math.max(0, 16-(currTime-lastTime));
            var id = window.setTimeout(function() { 
                callback(currTime + timeToCall);},
                timeToCall);
            lastTime = currTime + timeToCall;
            return id;
        };
    }
    if (!window.cancelAnimationFrame){
        window.cancelAnimationFrame = function(id) {
            clearTimeout(id);
        };
    }
}());
//
var game = {
    //开始初始化对象，预加载资源，并显示开始画面
    init: function(){
        // 初始化对象
        levels.init();
        loader.init();
        mouse.init();

        //加载所有的音效及背景音乐
        //由Gurdonark创作的"Kindergarten"
        game.backgroundMusic = loader.loadSound('audio/gurdonark-kindergarten');
        game.slingshotReleaseSound = loader.loadSound('audio/released');
        game.bounceSound = loader.loadSound('audio/bounce');
        game.breakSound = {
            "glass": loader.loadSound('audio/glassbreak'),
            "wood": loader.loadSound('audio/woodbreak')
        };

        //隐藏所有的游戏图层，显示开始画面
        $('.gamelayer').hide();
        $('#gamestartscreen').show();
        //获取游戏画布及其绘图环境的引用
        game.canvas = $('#gamecanvas')[0];
        game.context = game.canvas.getContext('2d');
    },
    showLevelScreen: function(){
        $('.gamelayer').hide();
        $('#levelselectscreen').show('slow');
    },
    //控制背景音乐的方法
    startBackgroundMusic: function() {
        var toggleImage = $("#togglemusic")[0];
        game.backgroundMusic.play();
        toggleImage.src = "images/icons/sound.png";
    },
    stopBackgroundMusic: function() {
        var toggleImage = $("#togglemusic")[0];
        toggleImage.src = "images/icons/nosound.png";
        game.backgroundMusic.pause();
        game.backgroundMusic.currentTime = 0;
    },
    toggleBackgroundMusic: function() {
        var toggleImage = $("#togglemusic")[0];
        if (game.backgroundMusic.paused) {
            game.backgroundMusic.play();
            toggleImage.src = "images/icons/sound.png";
        } else {
            toggleImage.src = "images/icons/nosound.png";
            game.backgroundMusic.pause();
        }
    },

    // 游戏阶段
    mode: "intro", //游戏状态（intro,waiting for firing, firing, fired）
    // 弹弓的x和y坐标
    slingshotX: 140,
    slingshotY: 280,
    start: function() {
        //隐藏其他所有的图层
        $('.gamelayer').hide();
        //显示游戏画布和得分
        $('#gamecanvas').show();
        $('#scorescreen').show();

        game.startBackgroundMusic();

        game.mode = "intro";
        game.offsetLeft = 0; //记录游戏画面在关卡中平移的距离
        game.ended = false;
        game.animationFrame = window.requestAnimationFrame(game.animate, game.canvas);
    },
    // 画面最大平移速度，单位为像素每帧
    maxSpeed: 3,
    // 画面最大和最小平移范围
    minOffset: 0,
    maxOffset: 300,
    // 画面当前平移位置
    offsetLeft: 0,
    // 游戏得分
    score: 0,
    // 画面中心移动到newCenter
    panTo: function(newCenter) {
        if (Math.abs(newCenter - game.offsetLeft - game.canvas.width/4) > 0
            && game.offsetLeft <= game.maxOffset && game.offsetLeft >= game.minOffset) {
            var deltaX = Math.round((newCenter - game.offsetLeft - game.canvas.width/4)/2);
            if (deltaX && Math.abs(deltaX) > game.maxSpeed) {
                deltaX = game.maxSpeed*Math.abs(deltaX)/(deltaX);
            }
            game.offsetLeft += deltaX;
        } else {
            return true;
        }
        if (game.offsetLeft < game.minOffset) {
            game.offsetLeft = game.minOffset;
            return true;
        } else if (game.offsetLeft > game.maxOffset) {
            game.offsetLeft = game.maxOffset;
            return true;
        }
        return false;
    },
    countHeroesAndVillains:function(){
        game.heroes = [];
        game.villains = [];
        for (var body = box2d.world.GetBodyList(); body; body = body.GetNext()) {
            var entity = body.GetUserData();
            if(entity){
                if(entity.type == "hero"){              
                    game.heroes.push(body);         
                } else if (entity.type =="villain"){
                    game.villains.push(body);
                }
            }
        }
    },
    //鼠标是否放置在当前英雄上
    mouseOnCurrentHero:function(){
        if(!game.currentHero){
            return false;
        }
        var position = game.currentHero.GetPosition();
        var distanceSquared = Math.pow(position.x*box2d.scale - mouse.x-game.offsetLeft,2) + Math.pow(position.y*box2d.scale-mouse.y,2);
        var radiusSquared = Math.pow(game.currentHero.GetUserData().radius,2);      
        return (distanceSquared<= radiusSquared);   
    },
    showEndingScreen: function(){
        game.stopBackgroundMusic();
        if (game.mode == "level-success"){
            if (game.currentLevel.number < levels.data.length-1){
                $('#endingmessage').html('Level Complete. Well Done!!!');
                $('#playnextlevel').show();
            } else {
                $('#endingmessage').html('All Levels Complete. Well Done!!!');
                $('#playnextlevel').show();
            }
        } else if (game.mode == "level-failure"){
            $('#endingmessage').html('Failed. Play Again?');
            $('#playnextlevel').show();
        } 
        $('#endingscreen').show();
    },
    handlePanning: function() {
        if (game.mode == "intro"){
            if (game.panTo(700)) {
                game.mode = "load-next-hero";
            }
        }
        if (game.mode == "load-next-hero"){
            game.countHeroesAndVillains();
            // 检查是否有坏蛋还活着，如果没有，结束关卡
            if (game.villains.length == 0){
                game.mode = "level-success";
                return;
            }
            // 检查是否还有可装填英雄，如果没有，结束关卡
            if (game.heroes.length == 0){
                game.mode = "level-failure";
                return;
            }
            // 装填英雄
            if (!game.currentHero){
                game.currentHero = game.heroes[game.heroes.length-1];
                game.currentHero.SetPosition({x:180/box2d.scale,y:200/box2d.scale});
                game.currentHero.SetLinearVelocity({x:0,y:0});
                game.currentHero.SetAngularVelocity(0);
                game.currentHero.SetAwake(true);    
            } else {
                //等待英雄结束弹跳并进入休眠
                game.panTo(game.slingshotX);
                if (!game.currentHero.IsAwake()){
                    game.mode = "wait-for-firing";
                }
            }
        }
        if (game.mode == "wait-for-firing"){
            if (mouse.dragging) {
                if (game.mouseOnCurrentHero()){
                    game.mode = "firing";
                } else {
                    game.panTo(mouse.x + game.offsetLeft);
                }
            } else {
                game.panTo(game.slingshotX);
            }
        }
        if (game.mode == "firing"){
            if (mouse.down) {
               game.panTo(game.slingshotX); 
               game.currentHero.SetPosition({x:(mouse.x + game.offsetLeft)/box2d.scale,
                 y:mouse.y/box2d.scale});
            } else {
                game.mode = "fired";
                game.slingshotReleaseSound.play();
                var impulseScaleFactor = 0.75;
                var impulse = new b2Vec2((game.slingshotX+35-mouse.x-game.offsetLeft)*
                    impulseScaleFactor, (game.slingshotY+25-mouse.y)*impulseScaleFactor);
                game.currentHero.ApplyImpulse(impulse, game.currentHero.GetWorldCenter());
            }
        }
        if (game.mode == "fired"){
            // 跟随当前英雄移动画面
            var heroX = game.currentHero.GetPosition().x*box2d.scale;
            game.panTo(heroX);
            //直到该英雄停止移动或移除边界
            if (!game.currentHero.IsAwake() || heroX<0 
                || heroX>game.currentLevel.foregroundImage.width){
                //然后删除旧的英雄
                box2d.world.DestroyBody(game.currentHero);
                game.currentHero = undefined;
                //加载下一个英雄
                game.mode = "load-next-hero";
            }
        }
        if (game.mode == "level-success" || game.mode == "level-failure"){
            if (game.panTo(0)) {
                game.ended = true;
                game.showEndingScreen();
            }
        }
    },
    animate: function() {
        //移动背景
        game.handlePanning();
        //使角色运动
        var currentTime = new Date().getTime();
        var timeStep;
        if (game.lastUpdateTime){
            timeStep = (currentTime - game.lastUpdateTime)/1000;
            box2d.step(timeStep);
        }
        game.lastUpdateTime = currentTime;

        //使用视差滚动绘制背景，背景图像和前景图像以不同的速度移动，
        //这个差异会造成一种错觉：背景上的云彩离我们更远
        game.context.drawImage(game.currentLevel.backgroundImage, 
            game.offsetLeft/4, 0, 640, 480, 0, 0, 640, 480);
        game.context.drawImage(game.currentLevel.foregroundImage, 
            game.offsetLeft, 0, 640, 480, 0, 0, 640, 480);
        // 绘制弹弓
        game.context.drawImage(game.slingshotImage, game.slingshotX- 
            game.offsetLeft, game.slingshotY);
        // 绘制所有的物体
        game.drawAllBodies();
        //发射英雄时绘制橡胶带
        if (game.mode == "firing"){
            game.drawSlingshotBand();
        }
        // 再次绘制弹弓的外侧支架
        game.context.drawImage(game.slingshotFrontImage, game.slingshotX-
            game.offsetLeft, game.slingshotY);
        if (!game.ended) {
            game.animationFrame = window.requestAnimationFrame(game.animate, game.canvas);
        }
    },
    //绘制弹弓橡胶带
    drawSlingshotBand: function() {
        game.context.strokeStyle = "rgb(68, 31, 11)"; //暗棕色
        game.context.lineWidth = 6; //
        //用英雄被拖拽的角度和半径计算英雄的末端，相对于英雄的中心
        var radius = game.currentHero.GetUserData().radius;
        var heroX = game.currentHero.GetPosition().x * box2d.scale;
        var heroY = game.currentHero.GetPosition().y * box2d.scale;
        var angle = Math.atan2(game.slingshotY+25-heroY, game.slingshotX+50-heroX);

        var heroFarEdgeX = heroX - radius * Math.cos(angle);
        var heroFarEdgeY = heroY - radius * Math.cos(angle);
        game.context.beginPath();
        //从弹弓顶端开始绘制（背面）
        game.context.moveTo(game.slingshotX+50-game.offsetLeft, game.slingshotY+25);
        //画到英雄的中心
        game.context.lineTo(heroX-game.offsetLeft, heroY);
        game.context.stroke();
        //再次绘制英雄
        entities.draw(game.currentHero.GetUserData(), game.currentHero.GetPosition(),
            game.currentHero.GetAngle());
        game.context.beginPath();
        //移动到英雄离弹弓顶部最远的边缘
        game.context.moveTo(heroFarEdgeX-game.offsetLeft, heroFarEdgeY+25);
        //将线画回弹弓（正面）
        game.context.lineTo(game.slingshotX-game.offsetLeft+10,game.slingshotY+30);
        game.context.stroke();
    },
    //重新开始和下一关
    restartLevel: function() {
        window.cancelAnimationFrame(game.animationFrame);
        game.lastUpdateTime = undefined;
        levels.load(game.currentLevel.number);
    },
    startNextLevel: function() {
        window.cancelAnimationFrame(game.animationFrame);
        game.lastUpdateTime = undefined;
        levels.load(game.currentLevel.number+1);
    },
    drawAllBodies: function() {
        box2d.world.DrawDebugData();
        // 遍历所有的物体，并在游戏canvas上绘制出来
        for (var body=box2d.world.GetBodyList(); body; body=body.GetNext()){
            var entity = body.GetUserData();
            if (entity){
                //判断生命值，是否要显示
                var entityX = body.GetPosition().x * box2d.scale;
                if (entityX<0 || entityX>game.currentLevel.foregroundImage.width ||
                    (entity.health && entity.health<0)) {
                    box2d.world.DestroyBody(body);
                    if (entity.type == "villain"){
                        game.score += entity.calories;
                        $('#score').html('Score: ' + game.score);
                    }
                    if (entity.breakSound){
                        entity.breakSound.play();
                    }
                } else {
                    entities.draw(entity, body.GetPosition(), body.GetAngle());
                }
            }
        }
    },
}
$(window).load(function() {
    game.init();
});
// 关卡
var levels = {
    //关卡数据
    data: [
    {   //第一关
        foreground: 'desert-foreground',
        background: 'clouds-background',
        entities:[
            {type:"ground", name:"dirt", x:500,y:440,width:1000,height:20,isStatic:true},
            {type:"ground", name:"wood", x:185,y:390,width:30,height:80,isStatic:true},

            {type:"block", name:"wood", x:520,y:380,angle:90,width:100,height:25},
            {type:"block", name:"glass", x:520,y:280,angle:90,width:100,height:25},                             
            {type:"villain", name:"burger",x:520,y:205,calories:590},

            {type:"block", name:"wood", x:620,y:380,angle:90,width:100,height:25},
            {type:"block", name:"glass", x:620,y:280,angle:90,width:100,height:25},                             
            {type:"villain", name:"fries", x:620,y:205,calories:420},               

            {type:"hero", name:"orange",x:80,y:405},
            {type:"hero", name:"apple",x:140,y:405},
        ]
    },
    {   //第二关
        foreground: 'desert-foreground',
        background: 'clouds-background',
        entities:[
                {type:"ground", name:"dirt", x:500,y:440,width:1000,height:20,isStatic:true},
                {type:"ground", name:"wood", x:185,y:390,width:30,height:80,isStatic:true},
    
                {type:"block", name:"wood", x:820,y:380,angle:90,width:100,height:25},
                {type:"block", name:"wood", x:720,y:380,angle:90,width:100,height:25},
                {type:"block", name:"wood", x:620,y:380,angle:90,width:100,height:25},
                {type:"block", name:"glass", x:670,y:317.5,width:100,height:25},
                {type:"block", name:"glass", x:770,y:317.5,width:100,height:25},                

                {type:"block", name:"glass", x:670,y:255,angle:90,width:100,height:25},
                {type:"block", name:"glass", x:770,y:255,angle:90,width:100,height:25},
                {type:"block", name:"wood", x:720,y:192.5,width:100,height:25}, 

                {type:"villain", name:"burger",x:715,y:155,calories:590},
                {type:"villain", name:"fries",x:670,y:405,calories:420},
                {type:"villain", name:"sodacan",x:765,y:400,calories:150},

                {type:"hero", name:"strawberry",x:30,y:415},
                {type:"hero", name:"orange",x:80,y:405},
                {type:"hero", name:"apple",x:140,y:405},
        ]
    }
    ],
    // 初始化关卡选择画面
    init: function() {
        var html = "";
        for (var i=0; i<levels.data.length; i++) {
            var level = levels.data[i];
            html += '<input type="button" value="' + (i+1) + '">';
        };
        $('#levelselectscreen').html(html);
        //单击按钮时加载关卡
        $('#levelselectscreen input').click(function(){
            levels.load(this.value - 1);
            $('#levelselectscreen').hide();
        });
    },
    // 为某一关加载所有的数据和图像
    load: function(number){
        box2d.init();//
        //声明一个新的当前关卡对象
        game.currentLevel = {number:number, hero:[]};
        game.score = 0;
        $('#score').html('Score: ' + game.score);
        game.currentHero = undefined;
        var level = levels.data[number];
        //加载背景、前景和弹弓图像
        game.currentLevel.backgroundImage = loader.loadImage("images/backgrounds/" + level.background + ".png");
        game.currentLevel.foregroundImage = loader.loadImage("images/backgrounds/" + level.foreground + ".png");
        game.slingshotImage = loader.loadImage("images/slingshot.png");
        game.slingshotFrontImage = loader.loadImage("images/slingshot-front.png");
        // 加载所有的物体
        for (var i=level.entities.length-1; i>=0; i--){
            var entity = level.entities[i];
            entities.create(entity);
        };
        // 一旦所有的图像加载完成，就调用game.start()函数
        if (loader.loaded){
            game.start();
        } else {
            loader.onload = game.start;
        }
    }
}
//
var entities = {
    //定义物体类型（玻璃、木材和地面），以及英雄和坏蛋（橙子、苹果和汉堡）
    definitions:{
        "glass":{
            fullHealth:100,
            density:2.4,
            friction:0.4,
            restitution:0.15,
        },
        "wood":{
            fullHealth:500,
            density:0.7,
            friction:0.4,
            restitution:0.4,
        },
        "dirt":{
            density:3.0,
            friction:1.5,
            restitution:0.2,    
        },
        "burger":{
            shape:"circle",
            fullHealth:40,
            radius:25,
            density:1,
            friction:0.5,
            restitution:0.4,    
        },
        "sodacan":{
            shape:"rectangle",
            fullHealth:80,
            width:40,
            height:60,
            density:1,
            friction:0.5,
            restitution:0.7,    
        },
        "fries":{
            shape:"rectangle",
            fullHealth:50,
            width:40,
            height:50,
            density:1,
            friction:0.5,
            restitution:0.6,    
        },
        "apple":{
            shape:"circle",
            radius:25,
            density:1.5,
            friction:0.5,
            restitution:0.4,    
        },
        "orange":{
            shape:"circle",
            radius:25,
            density:1.5,
            friction:0.5,
            restitution:0.4,    
        },
        "strawberry":{
            shape:"circle",
            radius:15,
            density:2.0,
            friction:0.5,
            restitution:0.4,    
        },
    },
       //根据物体创建Box2D物体，并将其加入世界
    create:function(entity){
        var definition = entities.definitions[entity.name];
        if (!definition){
            console.log("undefined entity name", entity.name);
            return;
        }
        //console.log(entity);
        switch (entity.type){
            case "block":  //简单的矩形
              entity.health = definition.fullHealth;
              entity.fullHealth = definition.fullHealth;
              entity.shape = "rectangle";
              entity.sprite = loader.loadImage("images/entities/"
                +entity.name+".png");
              entity.breakSound = game.breakSound[entity.name];
              box2d.createRectangle(entity, definition);
              break;
            case "ground":  //简单的矩形
              entity.shape = "rectangle";
              box2d.createRectangle(entity, definition);
              break;
            case "hero":
            case "villain":
              entity.health = definition.fullHealth;
              entity.fullHealth = definition.fullHealth;
              entity.sprite = loader.loadImage("images/entities/"
                +entity.name+".png");
              entity.shape = definition.shape;
              entity.bounceSound = game.bounceSound;
              if (definition.shape == "circle"){
                entity.radius = definition.radius;
                box2d.createCircle(entity, definition);
              } else if (definition.shape == "rectangle") {
                entity.width = definition.width;
                entity.height = definition.height;
                box2d.createRectangle(entity, definition);
              }
              break;
            default:
              console.log("Undefined entity type", entity.type);
              break;
        }
    },
    //以物体、物体的位置和角度为参数，在游戏画面中绘制物体
    draw: function(entity, position, angle){
        game.context.translate(position.x*box2d.scale-game.offsetLeft,
            position.y*box2d.scale);
        game.context.rotate(angle);
        switch(entity.type){
            case "block":
              game.context.drawImage(entity.sprite,0,0,entity.sprite.width,
                 entity.sprite.height, -entity.width/2-1, -entity.height/2-1,
                 entity.width+2, entity.height+2);
              break;
            case "villain":
            case "hero":
              if (entity.shape == "circle"){
                game.context.drawImage(entity.sprite,0,0,entity.sprite.width,
                 entity.sprite.height, -entity.radius-1, -entity.radius-1,
                 entity.radius*2+2, entity.radius*2+2);
              } else if (entity.shape == "rectangle") {
                game.context.drawImage(entity.sprite,0,0,entity.sprite.width,
                 entity.sprite.height, -entity.width/2-1, -entity.height/2-1,
                 entity.width+2, entity.height+2);
              }
              break;
            case "ground":
              break;
        }
        game.context.rotate(-angle);
        game.context.translate(-position.x*box2d.scale + game.offsetLeft, -position.y*box2d.scale);
    },
}
//创建box2d对象
var box2d = {
    scale: 30,
    init: function() {
        //创建Box2D世界，大部分物理运算将在其中完成
        var gravity = new b2Vec2(0, 9.8); //重力加速度9.8m/s^2
        var allowSleep = true;  //允许静止的物体进入休眠状态，休眠物体不参与物理仿真计算
        box2d.world = new b2World(gravity, allowSleep);
        //设置调试绘图
        var debugContext = document.getElementById('debugcanvas').getContext('2d');
        var debugDraw = new b2DebugDraw();
        debugDraw.SetSprite(debugContext);
        debugDraw.SetDrawScale(box2d.scale);
        debugDraw.SetFillAlpha(0.3);
        debugDraw.SetLineThickness(1.0);
        debugDraw.SetFlags(b2DebugDraw.e_shapeBit | b2DebugDraw.e_jointBit);    
        box2d.world.SetDebugDraw(debugDraw);
        //监听事件
        var listener = new Box2D.Dynamics.b2ContactListener;
        //参数为接触和冲击力(法向和切向冲击力)
        listener.PostSolve = function(contact, impulse){
            var body1 = contact.GetFixtureA().GetBody();
            var body2 = contact.GetFixtureB().GetBody();
            var entity1 = body1.GetUserData();
            var entity2 = body2.GetUserData();
            var impulseAlongNormal = Math.abs(impulse.normalImpulses[0]);
            // 监听器被调用得有些太频繁了，滤去非常小的冲击
            // 尝试不同的值后，5似乎比较好
            if (impulseAlongNormal > 5) {
                //如果对象有生命值，用冲击值消弱生命值
                if (entity1.health) {
                    entity1.health -= impulseAlongNormal;
                }
                if (entity2.health) {
                    entity2.health -= impulseAlongNormal;
                }
                //如果物体具有弹跳音，则播放它
                if (entity1.bounceSound){
                    entity1.bounceSound.play();
                }
                if (entity2.bounceSound){
                    entity2.bounceSound.play();
                }
            }
        };
        box2d.world.SetContactListener(listener);
    },
    step: function(timeStep){
        if (timeStep > 2/60){
            timeStep = 2/60;
        }
        box2d.world.Step(timeStep, 8, 3);
    },
    createRectangle:function(entity, definition){
        var bodyDef = new b2BodyDef;
        if (entity.isStatic) {
            bodyDef.type = b2Body.b2_staticBody;
        } else {
            bodyDef.type = b2Body.b2_dynamicBody;
        }
        bodyDef.position.x = entity.x/box2d.scale;
        bodyDef.position.y = entity.y/box2d.scale;
        if (entity.angle){
            bodyDef.angle = Math.PI*entity.angle/180;
        }
        var fixtureDef = new b2FixtureDef;
        fixtureDef.density = definition.density;
        fixtureDef.friction = definition.friction;
        fixtureDef.restitution = definition.restitution;
        fixtureDef.shape = new b2PolygonShape;  //多边形
        fixtureDef.shape.SetAsBox(entity.width/2/box2d.scale, 
           entity.height/2/box2d.scale); //60宽，100高

        var body = box2d.world.CreateBody(bodyDef);
        body.SetUserData(entity);
        var fixture = body.CreateFixture(fixtureDef);
        return body;
    },
    createCircle:function(entity, definition){
        var bodyDef = new b2BodyDef;
        if (entity.isStatic) {
            bodyDef.type = b2Body.b2_staticBody;
        } else {
            bodyDef.type = b2Body.b2_dynamicBody;
        }
        bodyDef.position.x = entity.x/box2d.scale;
        bodyDef.position.y = entity.y/box2d.scale;
        if (entity.angle){
            bodyDef.angle = Math.PI*entity.angle/180;
        }
        var fixtureDef = new b2FixtureDef;
        fixtureDef.density = definition.density;
        fixtureDef.friction = definition.friction;
        fixtureDef.restitution = definition.restitution;
        fixtureDef.shape = new b2CircleShape(entity.radius/box2d.scale);

        var body = box2d.world.CreateBody(bodyDef);
        body.SetUserData(entity);
        var fixture = body.CreateFixture(fixtureDef);
        return body;
    },
}

//图像/声音资源加载器loader
var loader = {
    loaded: true,
    loadedCount: 0, //已加载的资源数
    totalCount: 0,  //需要被加载的资源总数

    init: function(){
        //检查浏览器支持的声音格式；
        var mp3Support, oggSupport;
        var audio = document.createElement('audio');
        if (audio.canPlayType){
            // 
            mp3Support = "" != audio.canPlayType('audio/mpeg');
            oggSupport = "" != audio.canPlayType('audio/ogg; codecs="vorbis"');
        } else {
            // audio标签不被支持
            mp3Support = false;
            oggSupport = false;
        }
        // 都不支持，就将soundFileExtn设置为undefined
        loader.soundFileExtn = oggSupport?".ogg":mp3Support?".mp3":undefined;
        console.log(loader.soundFileExtn);
    },
    loadImage: function(url){
        this.totalCount++;
        this.loaded = false;
        $('#loadingscreen').show();
        var image = new Image();
        image.src = url;
        image.onload = loader.itemLoaded;
        return image;
    },
    soundFileExtn: ".ogg",
    loadSound: function(url){
        this.totalCount++;
        this.loaded = false;
        $('#loadingscreen').show();
        var audio = new Audio();
        audio.src = url + loader.soundFileExtn;
        audio.addEventListener("canplaythrough", loader.itemLoaded, false);
        return audio;
    },
    itemLoaded: function() {
        loader.loadedCount++;
        $('#loadingmessage').html('Loaded ' + loader.loadedCount + ' of ' + loader.totalCount);
        if (loader.loadedCount === loader.totalCount) {
            // loader完成了资源加载
            loader.loaded = true;
            $('#loadingscreen').hide();
            if (loader.onload){
                loader.onload();
                loader.onload = undefined;
            }
        }
    }
}
//处理鼠标事件
var mouse = {
    x: 0,
    y: 0,
    down: false,
    init: function() {
        $('#gamecanvas').mousemove(mouse.mousemovehandler);
        $('#gamecanvas').mousedown(mouse.mousedownhandler);
        $('#gamecanvas').mouseup(mouse.mouseuphandler);
        $('#gamecanvas').mouseout(mouse.mouseuphandler);
    },
    mousemovehandler: function(ev) {
        var offset = $('#gamecanvas').offset();
        mouse.x = ev.pageX - offset.left;
        mouse.y = ev.pageY - offset.top;
        if (mouse.down) {
            mouse.dragging = true;
        }
    },
    mousedownhandler: function(ev) {
        mouse.down = true;
        mouse.downX = mouse.x;
        mouse.downY = mouse.y;
        ev.originalEvent.preventDefault();
    },
    mouseuphandler: function(ev) {
        mouse.down = false;
        mouse.dragging = false;
    }
}

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