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();
    }
    //********************************************
}