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)];
