uibutton增加渐变色后title和image不显示问题

uibutton增加渐变色的时候一定要注意不能直接addsublayer，这样会导致新增加的layer在最上面，会遮挡title和image
应该使用insertSublayer将渐变色的layer增加在最下层
'''
[_adoptAnswerTagBtn.layer insertSublayer:self.gradientLayer atIndex:0];
'''
这时title就会显示出来，但，image还是会被隐藏，我猜测应该是image的层级比较靠后一些吧，具体是不是没有验证，
我强行将imageview的优先级提到最前面，使用bringSubviewToFront。
具体代码如下：
'''

• (UIButton *)adoptAnswerTagBtn
  {
  if(!_adoptAnswerTagBtn) {
  _adoptAnswerTagBtn = [UIButton buttonWithType:UIButtonTypeCustom];
  _adoptAnswerTagBtn.titleLabel.font = [UIFont systemFontOfSize:14];
  [_adoptAnswerTagBtn setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
  [_adoptAnswerTagBtn addTarget:self action:@selector(adoptAnswerBtnClick) forControlEvents:UIControlEventTouchUpInside];
  _adoptAnswerTagBtn.layer.cornerRadius = 11.5;
  _adoptAnswerTagBtn.layer.masksToBounds = YES;
  _adoptAnswerTagBtn.size = CGSizeMake(64, 23);
  self.gradientLayer.frame = _adoptAnswerTagBtn.bounds;
  [_adoptAnswerTagBtn.layer insertSublayer:self.gradientLayer atIndex:0];
  [_adoptAnswerTagBtn setImage:[UIImage imageNamed:@"ef_fundbar_answeradopt_white"] forState:UIControlStateNormal];
  [_adoptAnswerTagBtn setTitle:@"采纳" forState:UIControlStateNormal];
  [_adoptAnswerTagBtn bringSubviewToFront:_adoptAnswerTagBtn.imageView];
  _adoptAnswerTagBtn.hidden = YES;
  }
  return _adoptAnswerTagBtn;
  }
• (CAGradientLayer *)gradientLayer {
  if (!_gradientLayer) {
  _gradientLayer = [CAGradientLayer layer];
  _gradientLayer.colors = @[(id)[RGB_HEX(0xE8CC94) CGColor],
  (id)[RGB_HEX(0xD6AC6C) CGColor]
  ];
  _gradientLayer.startPoint = CGPointMake(0, 0);
  _gradientLayer.endPoint = CGPointMake(1.0, 1.0);
  }
  return _gradientLayer;
  }
  '''