使用 UIImageView 和 CGAffinetransform 绘制的重复图层

Question

我在将转换应用于 UIImageView 的图层时遇到问题。使用下面的代码，我尝试旋转这根针，但我绘制了重复的副本。我已经检查过没有其他地方添加针图像，并且注释掉 addSubview 不会让任何一个都出现，所以它肯定是绘制了两次。

左侧的指针位于图像视图应开始的位置，-20。右侧的针在旋转方面工作并显示 needleValue 的正确值。我做错了什么导致重复抽奖？

- (UIImageView *)needle {
 if (_needle == nil) {
  UIImage *image = [UIImage imageNamed:@"needle.png"];
  self.needle = [[[UIImageView alloc] initWithImage:image] autorelease];
  [_needle sizeToFit];
  [_needle setFrame:CGRectMake(floor((self.width - _needle.width)/2),
          self.height - _needle.height + 68, // this - offset comes from the lowering of the numbers on the meter
          _needle.width, _needle.height)];
  _needle.contentMode = UIViewContentModeCenter;
  _needle.autoresizingMask = UIViewAutoresizingFlexibleTopMargin|UIViewAutoresizingFlexibleLeftMargin|UIViewAutoresizingFlexibleRightMargin;
  _needle.autoresizesSubviews = YES;

  // CALayer's transform property is a CATransform3D.
  // rotate around a vector (x, y, z) = (0, 0, 1) where positive Z points 
  // out of the device's screen.
  _needle.layer.anchorPoint = CGPointMake(0.05,1);

  [self addSubview:_needle];
 }
 return _needle;
}



- (void)updateNeedle {
 [UIView beginAnimations:nil context:NULL]; // arguments are optional
 [UIView setAnimationDuration:VU_METER_FREQUENCY];
 CGFloat radAngle = [self radianAngleForValue:needleValue];
 self.needle.transform = CGAffineTransformMakeRotation(radAngle);
 [UIView commitAnimations];
}

Answer 1

尝试在 [self addSubview:...] 行下断点，看看它是否被调用了两次。 _needle 变量可能在其他地方设置为 nil ？