另一种实现 Fruit Ninja 里刀
实现原理：

画直线

在一个 list 列表里记录所有的触摸点，在 draw 函数里开始画线，线段逐渐加粗，在末端逐渐减细。可以直接用 Cocos2d 里的 box2d 模板，添加了少量代码即可。

//使用list列表保存所有点

std::list<CGPoint> pointl;-(void)ccTouchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {       UITouch *touch = [touches anyObject];       CGPoint start = [touch locationInView: [touch view]];       start = [[CCDirector sharedDirector] convertToGL: start];       CGPoint end = [touch previousLocationInView:[touch view]];    end = [[CCDirector sharedDirector] convertToGL:end];       float distance = ccpDistance(start, end);    if (distance > 1)    {        int d = (int)distance;        for (int i = 0; i < d; i++ )        {            float difx = end.x - start.x;            float dify = end.y - start.y;            float delta = (float)i / distance;            CGPoint p;            p.x = start.x + (difx * delta);            p.y = start.y + (dify * delta);                       pointl.push_back(p);        }    }       pointcount = pointl.size();}//*************************************draw函数核心代码-(void) draw{    CGPoint pr;    glPointSize( 0.3f );    list <CGPoint>::iterator b = pointl.begin();    glColor4ub(255,255,255,32);       for(;b!=pointl.end();b++)    {        CGPoint pt = *b;        ps++;        //控制线段的粗细，使达到两头细中间粗的效果        if (ps > (pl -30 )) // initlw > 5 )        {           initlw=initlw-lwc;        }        else        {          if (initlw < 6 )          {            initlw =initlw+lwc;          }        }        glLineWidth( initlw);        if (pr.x > 1 && pr.y > 1 )        {            //画线段，也可以使用点            ccDrawLine(pr, pt );                  }        pr = *b;    }}//**********************************************//自动缩短线段-(void) tick: (ccTime) dt{    //It is recommended that a fixed time step is used with Box2D for stability    //of the simulation, however, we are using a variable time step here.    //You need to make an informed choice, the following URL is useful    //http://gafferongames.com/game-physics/fix-your-timestep/       int32 velocityIterations = 8;    int32 positionIterations = 1;       // Instruct the world to perform a single step of simulation. It is    // generally best to keep the time step and iterations fixed.    world->Step(dt, velocityIterations, positionIterations);       //*********************************************    //**    for (int i=0; i<12 ; i++)    {        if (pointl.size() >0)        {           pointl.pop_front();           pointcount--;        }        else {            break;        }    }    //为了使线段不过长    while (pointcount >200) {        pointl.pop_front();        //pointcount--;        pointcount=pointl.size();    }    //********************************************}