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@property (nonatomic, readonly) CGFloat pointSize;
@property (nonatomic, readonly) CGFloat ascender;
@property (nonatomic, readonly) CGFloat descender;
@property (nonatomic, readonly) CGFloat capHeight;
@property (nonatomic, readonly) CGFloat xHeight;
@property (nonatomic, readonly) CGFloat lineHeight API_AVAILABLE(ios(4.0));
@property (nonatomic, readonly) CGFloat leading;  

不同字号下两个font之间 垂直居中所需要baseline offset

+ (float)baselineOffsetBetweenFont:(UIFont *)font1 anotherFont:(UIFont *)font2 {
    CGFloat base1 = font1.ascender;
    CGFloat base2 = font2.ascender;
    CGFloat lh1 = font1.lineHeight;
    CGFloat lh2 = font2.lineHeight;
    CGFloat offset1 = base2 - base1;
    CGFloat offset2 = (lh2 - lh1) / 2;
    return offset1 - offset2;
}