Cocos2d Box2D之简介

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　　Box2D是一个用于模拟2D刚体物体的C++引擎。Box2D集成了大量的物理力学和运动学的计算，并将物理模拟过程封装到类对象中，将对物体的操作，以简单友好的接口提供给开发者。我们只需要调用引擎中相应的对象或函数，就可以模拟现实生活中的加速、减速、抛物线运动、万有引力、碰撞反弹等等各种真实的物理运动。

　　Box2D中的名词：

　　>>世界(World)

　　传说世界本是一片混沌，自从盘古开天辟地之后，盘古神斧砍下出了天和地，形成了真正的世界，而后形成才有了山川、河流、花草、树木。Box2D也一样，现在Box2D在我们的脑中也是混沌的一片，我们要创建物体或进行物理模拟之前，首先也是先创建物理世界。

　　在Box2D中用b2World类来表示世界。它是Box2D的一个核心类之一，集成了Box2D对所有对象的创建、删除、碰撞模拟的相关接口。

　　>>刚体(b2Body)

　　生活中我们看到的任何物体都可以用东西来描述，飞的小鸟，马路上行驶的汽车，等等。“东西”这个词在Box2D的字典中叫做“刚体”（b2Body），其实刚体也就是物理世界中的物体。b2Body是Box2D的核心类，是学习Box2D的基础，也是重中之重。b2Body用来模拟现实物理世界中的所有物体。Box2D中的任何碰撞、反弹、运动轨迹等各种物理现象模拟和数据计算都是基于刚体实现的，所以刚体b2Body所包含的信息有很多，如物体的坐标、角度、受力大小、速度、质量等大量的信息。

Box2D引擎中b2Body定义：

 /// The body type.
 /// static: zero mass, zero velocity, may be manually moved
 /// kinematic: zero mass, non-zero velocity set by user, moved by solver
 /// dynamic: positive mass, non-zero velocity determined by forces, moved by solver
 enum b2BodyType
 {
         b2_staticBody = ,    //静止Body
         b2_kinematicBody,     //浮动Body
         b2_dynamicBody        //动态Body
         // TODO_ERIN
         //b2_bulletBody,
 };

 /// A body definition holds all the data needed to construct a rigid body.
 /// You can safely re-use body definitions. Shapes are added to a body after construction.
 struct b2BodyDef
 {
         /// This constructor sets the body definition default values.
         b2BodyDef()
         {
                userData = NULL;
                position.Set(0.0f, 0.0f);          //位置
                angle = 0.0f;                      //弧度
                linearVelocity.Set(0.0f, 0.0f);    //直线速度设置
                angularVelocity = 0.0f;
                linearDamping = 0.0f;              //直线阻尼
                angularDamping = 0.0f;
                allowSleep = true;
                awake = true;
                fixedRotation = false;             //角度
                bullet = false;
                type = b2_staticBody;
                active = true;
                gravityScale = 1.0f;
         }

         /// The body type: static, kinematic, or dynamic.
         /// Note: if a dynamic body would have zero mass, the mass is set to one.

     ///如果一个动态的身体将有零质量，质量被设置为一。
         b2BodyType type;

         /// The world position of the body. Avoid creating bodies at the origin
         /// since this can lead to many overlapping shapes.

     /// 刚体在物理世界中的位置
         b2Vec2 position;

         /// The world angle of the body in radians.
         float32 angle;

         /// The linear velocity of the body's origin in world co-ordinates.
         b2Vec2 linearVelocity;

         /// The angular velocity of the body.
         float32 angularVelocity;

         /// Linear damping is use to reduce the linear velocity. The damping parameter
         /// can be larger than 1.0f but the damping effect becomes sensitive to the
         /// time step when the damping parameter is large.
         float32 linearDamping;

         /// Angular damping is use to reduce the angular velocity. The damping parameter
         /// can be larger than 1.0f but the damping effect becomes sensitive to the
         /// time step when the damping parameter is large.
         float32 angularDamping;

         /// Set this flag to false if this body should never fall asleep. Note that
         /// this increases CPU usage.
         bool allowSleep;

         /// Is this body initially awake or sleeping?
         bool awake;

         /// Should this body be prevented from rotating? Useful for characters.
         bool fixedRotation;

         /// Is this a fast moving body that should be prevented from tunneling through
         /// other moving bodies? Note that all bodies are prevented from tunneling through
         /// kinematic and static bodies. This setting is only considered on dynamic bodies.
         /// @warning You should use this flag sparingly since it increases processing time.
         bool bullet;

         /// Does this body start out active?
         bool active;

         /// Use this to store application specific body data.
         void* userData;

         /// Scale the gravity applied to this body.
         float32 gravityScale;
 };

　　>>夹具(Fixture)

　　Fixture在Box2D中是一种夹具，主要作用是用来定义刚体所固有的一些属性，并保存在b2Fixture对象中。现实中通常是物体材料特性相关的一些属性，如刚体的密度、摩擦系数等属性都是由b2FixtureDef保存的。

Box2D中b2FixtureDef结构体定义：

 /// A fixture definition is used to create a fixture. This class defines an
 /// abstract fixture definition. You can reuse fixture definitions safely.
 struct b2FixtureDef
 {
         /// The constructor sets the default fixture definition values.
         b2FixtureDef()
         {
                shape = NULL;
                userData = NULL;
                friction = 0.2f;
                restitution = 0.0f;
                density = 0.0f;
                isSensor = false;
         }

         /// The shape, this must be set. The shape will be cloned, so you
         /// can create the shape on the stack.
         const b2Shape* shape;

         /// Use this to store application specific fixture data.
         void* userData;

         /// The friction coefficient, usually in the range [0,1].
         float32 friction;

         /// The restitution (elasticity) usually in the range [0,1].
         float32 restitution;

         /// The density, usually in kg/m^2.
         float32 density;

         /// A sensor shape collects contact information but never generates a collision
         /// response.
         bool isSensor;

         /// Contact filtering data.
         b2Filter filter;
 };

　　>>形状(Shape)

　　形状是一个b2Shape类型的对象，实现了刚体的具体形状，Box2D将基于这个形状进行精确的物理碰撞模拟。实际上，b2Shape只是一个抽象的父类，没有实际创建形状的过程。在实际开发过程中，b2FixtureDef.shape的属性值都是b2CircleShape、b2PolygonShape等b2Shape的子类对象。

Box2D中Shape的定义：

 /// A shape is used for collision detection. You can create a shape however you like.
 /// Shapes used for simulation in b2World are created automatically when a b2Fixture
 /// is created. Shapes may encapsulate a one or more child shapes.

 class b2Shape
 {
 public:

         enum Type
         {
                e_circle = ,   //圆形
                e_edge = ,     //边界
                e_polygon = ,  //自定义
                e_chain = ,
                e_typeCount =
         };
         //余下部分省略
 };