HTML5之2D物理引擎 Box2D for javascript Games 系列 翻外篇--如何结合createJS应用box2d.js

太久没有更新了，新年回来工作，突然有收到网友的邮件提问，居然还有人在关注，惭愧，找了下电脑上还有一点儿存着，顺便先发这一个番外篇吧，好歹可以看到真实的效果，等我考完英语，一定会更新下一章，“愤怒的小鸟篇”

如：createjs, pixi.js 等，Egret或者其它游戏引擎有自己的物理引擎扩展库，所以就不说了。

现在通过之前的学习，基本掌握了刚体等基础概念。那如何如何应用于现实画面中呢？

box2d只是模拟了物体，是虚拟的，如果不是通过debug是看不到任何画面的，要让用户看到画面，必须得结合canvas绘图能力，自己操作canvas绘图的原始API太麻烦，所以就有了createjs等其它流行的canvas库.

以下都以createJS代替canvas，当然你用其它库或者直接操作canvas也都可以

完成代码位于

https://github.com/willian12345/Box2D-for-Javascript-Games/blob/master/5-c.html

box2d呈现于createJS，贴上图的基本原理，就是将物理引擎世界中刚体的所有状态复制到createJS舞台对象！

function init() {
         var   b2Vec2 = Box2D.Common.Math.b2Vec2
            ,  b2AABB = Box2D.Collision.b2AABB
            ,  b2BodyDef = Box2D.Dynamics.b2BodyDef
            ,  b2Body = Box2D.Dynamics.b2Body
            ,  b2FixtureDef = Box2D.Dynamics.b2FixtureDef
            ,  b2Fixture = Box2D.Dynamics.b2Fixture
            ,  b2World = Box2D.Dynamics.b2World
            ,  b2MassData = Box2D.Collision.Shapes.b2MassData
            ,  b2PolygonShape = Box2D.Collision.Shapes.b2PolygonShape
            ,  b2CircleShape = Box2D.Collision.Shapes.b2CircleShape
            ,  b2DebugDraw = Box2D.Dynamics.b2DebugDraw
            ,  b2MouseJointDef =  Box2D.Dynamics.Joints.b2MouseJointDef
            ;
         var worldScale = 30; // box2d中以米为单位，1米=30像素
         var gravity = new b2Vec2(0, 5);
         var sleep = true;
         var world;
         var stage,debug;

         function main(){
            stage = new createjs.Stage("canvas");
            debug = new createjs.Stage("debug");

            setupPhysics();

            debugDraw();

            debug.on("stagemousedown", stagemousedown);

            createjs.Ticker.timingMode = createjs.Ticker.RAF;
            createjs.Ticker.on("tick", function(){
               stage.update();
               world.DrawDebugData(); // 为了显示出createjs对象，这里不再绘制box2d对象至canvas
               world.Step(1/30, 10, 10);// 更新世界模拟
               world.ClearForces(); // 清除作用力
            });
         }
         main();

         function Ball(){
            this.view = new createjs.Bitmap(‘soccer.png‘);
            this.view.regX = this.view.regY = 50;

            // 创建box2d球形体
            var bodyDef = new b2BodyDef();
            bodyDef.position.Set(Math.random()*640 / worldScale, 0/worldScale);
            bodyDef.type = b2Body.b2_dynamicBody
            bodyDef.userData = 0;
            var circleShape = new b2CircleShape(50 / worldScale);
            var fixtureDef = new b2FixtureDef();
            fixtureDef.shape = circleShape;
            fixtureDef.density = 1;
            fixtureDef.restitution = .4
            fixtureDef.friction = .5;
            this.view.body = world.CreateBody(bodyDef);
            this.view.body.CreateFixture(fixtureDef);

            this.view.on("tick", function(){
               // 让createjs的bitmap对象实时复制box2d对象的位置与旋转角度
               this.x = this.body.GetPosition().x * worldScale;
               this.y = this.body.GetPosition().y * worldScale;
               this.rotation = this.body.GetAngle() * (180 / Math.PI);
            });
         }

         function setupPhysics(){
            world = new b2World(new b2Vec2(0, 50), true);
            floor();
         }

         function stagemousedown(){
            var b = new Ball();
            stage.addChild(b.view); // 将产生的createjs对象添加至舞台上
         }

         function floor(){
            var bodyDef = new b2BodyDef();
            bodyDef.position.Set(320/worldScale, 465/worldScale);
            var polygonShape = new b2PolygonShape();
            polygonShape.SetAsBox(320/worldScale, 15/worldScale);
            var fixtureDef = new b2FixtureDef();
            fixtureDef.shape = polygonShape;
            fixtureDef.restitution = .4;
            fixtureDef.friction = .5;
            var theFloor = world.CreateBody(bodyDef);
            theFloor.CreateFixture(fixtureDef);
         }

         //setup debug draw
         function debugDraw(){
            var debugDraw = new b2DebugDraw();
            debugDraw.SetSprite(debug.canvas.getContext(‘2d‘));
            debugDraw.SetDrawScale(worldScale);
            debugDraw.SetFillAlpha(0.5);
            debugDraw.SetFlags(b2DebugDraw.e_shapeBit | b2DebugDraw.e_jointBit);
            world.SetDebugDraw(debugDraw);
         }
      };

这一句

this.view = new createjs.Bitmap(‘soccer.png‘);

通过createjs的Bitmap对象读取图片，创建一个足球

this.view这个显示对象即createjs的Bitmap对象，用于显示在舞台即canvas上

this.view.on("tick", function(){
               // 让createjs的bitmap对象实时复制box2d对象的位置与旋转角度
               this.x = this.body.GetPosition().x * worldScale;
               this.y = this.body.GetPosition().y * worldScale;
               this.rotation = this.body.GetAngle() * (180 / Math.PI);
            });

在Bitmap对象上侦听tick事件，tick事件可以理解为FLASH中的ENTER_FRAME事件，即每一帧调用一次，在每帧中将刚体的x,y位置属性与角度属性复制到createJS的显示对象上，就完成了结合

注释掉这一句，就可以隐藏掉box2Djs的调试状态变成一个正常的带物理效果的足球了

// debugDraw();

相关系列：

HTML5之2D物理引擎 Box2D for javascript Games 系列 第一部分

注：转载请注明出处博客园：sheldon(willian12345@126.com)

https://github.com/willian12345

原文地址：https://www.cnblogs.com/willian/p/10391319.html