uibutton增加漸變色的時候一定要注意不能直接addsublayer，這樣會導致新增加的layer在最上面，會遮擋title和image
應該使用insertSublayer將漸變色的layer增加在最下層
'''
[_adoptAnswerTagBtn.layer insertSublayer:self.gradientLayer atIndex:0];
'''
這時title就會顯示出來，但，image還是會被隱藏，我猜測應該是image的層級比較靠后一些吧，具體是不是沒有驗證，
我強行將imageview的優(yōu)先級提到最前面，使用bringSubviewToFront。
具體代碼如下：
'''

• (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:@"采納" 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;
  }
  '''