iOS Button 上图下文效果过程拆解

UIEdgeInsetsMake使用：效果上图下文，间隙可调

当对一个button同时设置了image和title时，默认情况是：image向左移动了label的宽度，标题向左移动了图片的距离，接下来通过拆解的方式来理解EdgeInsets设置实现。

1. 居中

  [button setImageEdgeInsets:UIEdgeInsetsMake(0, 0, 0, -button.titleLabel.intrinsicContentSize.width)];
  [button setTitleEdgeInsets:UIEdgeInsetsMake(0, -button.currentImage.size.width, 0, 0)];

居中

2. 上移（类似于默认情况的水平移动）

    [button setImageEdgeInsets:UIEdgeInsetsMake(-button.titleLabel.intrinsicContentSize.height, 0, 0, -button.titleLabel.intrinsicContentSize.width)];
    [button setTitleEdgeInsets:UIEdgeInsetsMake(button.currentImage.size.height, -button.currentImage.size.width, 0, 0)];

上移

3. 20高度间隙

[button setTitleEdgeInsets:UIEdgeInsetsMake(button.currentImage.size.height + 20, -button.currentImage.size.width, 0, 0)];

间隙效果