UIEdgeInsets是什么?

 typedef struct UIEdgeInsets {
    CGFloat top, left, bottom, right;  // specify amount to inset (positive) for each of the edges. values can be negative to 'outset'
} UIEdgeInsets; 

UIEdgeInsets是个结构体类型。里面有四个参数，分别是：top, left, bottom, right。这四个参数表示距离上边界、左边界、下边界、右边界的距离。

哪三个UIEdgeInsets属性

@property(nonatomic) UIEdgeInsets contentEdgeInsets UI_APPEARANCE_SELECTOR; // default is UIEdgeInsetsZero
@property(nonatomic) UIEdgeInsets titleEdgeInsets; // default is UIEdgeInsetsZero
@property(nonatomic) UIEdgeInsets imageEdgeInsets; // default is UIEdgeInsetsZero

contentEdgeInsets后面有个UI_APPEARANCE_SELECTOR是什么意思呢？

提示：UI_APPEARANCE_SELECTOR标记的属性都支持通过外观代理来定制。

举例，设置UIButton的contentEdgeInsets属性，可以直接调用：

[[UIButton appearance] setTitleEdgeInsets:UIEdgeInsetsMake(0, 0, 0, 0)];