cocos2d中多个CCSprite循环播放CCMoveBy动画的问题

想做一个持续运动的效果。采用NSTimer来计时触发动画：

objective-c [NSTimer scheduledTimerWithTimeInterval:2.0f target:self selector:@selector(melonPush) userInfo:nil repeats:YES];


`melonPush`方法是这么写的：

• (void)melonPush
  {
  CCSprite *walnut = [walnutSprites lastObject];

  if (walnut.position.x >= 470) {

  return;

  }

  CCMoveBy *push = [CCMoveBy actionWithDuration:.1f position:ccp(10, 0)];
  [melonSprite runAction:push];
  for (int i = [walnutSprites count] - 1; i >= 0; i--) {

  CCSprite *nut = [walnutSprites objectAtIndex:i];
  [nut runAction:push];

  }
  }