相对于iOS来说，UILabel系统只提供了五种样式，左右中等等，但是，内容相对于整个布局都是垂直居中的，想垂直居上和居下就不行，所以，自己搞了一个轮子。
实现的思路
首先要获取文字的真正宽高，其次，绘制在屏幕之前需要改变文字的坐标，所以需要用到下面的函数：
//获取真正的文字宽高
- (CGRect)textRectForBounds:(CGRect)bounds limitedToNumberOfLines:(NSInteger)numberOfLines;
//将文字绘制在屏幕上
- (void)drawTextInRect:(CGRect)rect;
第一种：子类实现
\\子类布局类型
typedef NS_ENUM(NSInteger, SDTextAlignment) {
SDTextAlignmentLeft = 0,
SDTextAlignmentCenter = 1,
SDTextAlignmentRight = 2,
SDTextAlignmentJustified = 3,
SDTextAlignmentNatural = 4,
SDTextAlignmentLeftTop = 5,
SDTextAlignmentLeftBottom = 6,
SDTextAlignmentCenterTop = 7,
SDTextAlignmentCenterBottom = 8,
SDTextAlignmentRightTop = 9,
SDTextAlignmentRightBottom = 10,
SDTextAlignmentJustifiedTop = 11,
SDTextAlignmentJustifiedBottom = 12,
SDTextAlignmentNaturalTop = 13,
SDTextAlignmentNaturalBottom = 14
};
新建一个子类，并添加一个SDTextAlignment类型枚举属性：
@interface SDTextAlignmentLabel : UILabel
@property (nonatomic, assign) SDTextAlignment sdTextAlignment;
@end
在sdTextAlignment 的set方法设值：
- (void)setSdTextAlignment:(SDTextAlignment)sdTextAlignment {
if (sdTextAlignment <= 4) {
self.textAlignment = (int)sdTextAlignment;
} else if (sdTextAlignment > 4 && sdTextAlignment <= 6) {
self.textAlignment = NSTextAlignmentLeft;
} else if (sdTextAlignment > 6 && sdTextAlignment <= 8) {
self.textAlignment = NSTextAlignmentCenter;
} else if (sdTextAlignment > 8 && sdTextAlignment <= 10) {
self.textAlignment = NSTextAlignmentRight;
} else if (sdTextAlignment > 10 && sdTextAlignment <= 12) {
self.textAlignment = NSTextAlignmentJustified;
} else if (sdTextAlignment > 12 && sdTextAlignment <= 14) {
self.textAlignment = NSTextAlignmentNatural;
}
_sdTextAlignment = sdTextAlignment;
}
重写drawTextInRect：方法，重写布局：
- (void)drawTextInRect:(CGRect)rect {
CGRect textRect = [self textRectForBounds: rect limitedToNumberOfLines: self.numberOfLines];
switch (self.sdTextAlignment) {
case SDTextAlignmentLeftTop:
case SDTextAlignmentCenterTop:
case SDTextAlignmentRightTop:
case SDTextAlignmentJustifiedTop:
case SDTextAlignmentNaturalTop:
{
textRect.origin.y = 0;
break;
}
case SDTextAlignmentLeftBottom:
case SDTextAlignmentCenterBottom:
case SDTextAlignmentRightBottom:
case SDTextAlignmentJustifiedBottom:
case SDTextAlignmentNaturalBottom:
{
textRect.origin.y = rect.size.height - textRect.size.height;
break;
}
default:
{
textRect = rect;
break;
}
}
[super drawTextInRect: textRect];
}
这样和系统默认的布局不冲突，需要新的布局，只需要设置相应的sdTextAlignment类型就行。但是，毕竟是新的子类，如果旧的代码想用这个布局，代码修改量就有点大了，所以，想到了第二种实现方法，使用类别。
第二种：类别实现
新建一个类别，并关联sdTextAlignmentEX属性：
@interface UILabel (SDTextAlignmentExtension)
@property (nonatomic, assign) SDTextAlignment sdTextAlignmentEX;
@end
一样，在set方法里需要设置类型：
- (void)setSdTextAlignmentEX:(SDTextAlignment)sdTextAlignmentEX {
if (sdTextAlignmentEX <= 4) {
self.textAlignment = (int)sdTextAlignmentEX;
} else if (sdTextAlignmentEX > 4 && sdTextAlignmentEX <= 6) {
self.textAlignment = NSTextAlignmentLeft;
} else if (sdTextAlignmentEX > 6 && sdTextAlignmentEX <= 8) {
self.textAlignment = NSTextAlignmentCenter;
} else if (sdTextAlignmentEX > 8 && sdTextAlignmentEX <= 10) {
self.textAlignment = NSTextAlignmentRight;
} else if (sdTextAlignmentEX > 10 && sdTextAlignmentEX <= 12) {
self.textAlignment = NSTextAlignmentJustified;
} else if (sdTextAlignmentEX > 12 && sdTextAlignmentEX <= 14) {
self.textAlignment = NSTextAlignmentNatural;
}
objc_setAssociatedObject(self, @selector(sdTextAlignmentEX), @(sdTextAlignmentEX), OBJC_ASSOCIATION_ASSIGN);
}
重点来了，需要重写load方法，然后hook系统的drawTextInRect:方法：
+ (void)load {
Method originalMethod = class_getInstanceMethod([UILabel class], @selector(drawTextInRect:));
Method swizzleMethod = class_getInstanceMethod([UILabel class], @selector(sd_drawTextInRect:));
BOOL didAddMethod = class_addMethod([UILabel class], @selector(sd_drawTextInRect:), method_getImplementation(originalMethod), method_getTypeEncoding(originalMethod));
if (didAddMethod) {
class_replaceMethod([UILabel class], @selector(drawTextInRect:), method_getImplementation(swizzleMethod), method_getTypeEncoding(swizzleMethod));
} else {
method_exchangeImplementations(originalMethod, swizzleMethod);
}
}
相应的，在hook的方法里重新布局：
- (void)sd_drawTextInRect:(CGRect)rect {
CGRect textRect = [self textRectForBounds: rect limitedToNumberOfLines: self.numberOfLines];
switch (self.sdTextAlignmentEX) {
case SDTextAlignmentLeftTop:
case SDTextAlignmentCenterTop:
case SDTextAlignmentRightTop:
case SDTextAlignmentJustifiedTop:
case SDTextAlignmentNaturalTop:
{
textRect.origin.y = 0;
break;
}
case SDTextAlignmentLeftBottom:
case SDTextAlignmentCenterBottom:
case SDTextAlignmentRightBottom:
case SDTextAlignmentJustifiedBottom:
case SDTextAlignmentNaturalBottom:
{
textRect.origin.y = rect.size.height - textRect.size.height;
break;
}
default:
{
textRect = rect;
break;
}
}
[self sd_drawTextInRect: textRect];
}
效果显示：
Demo.gif
总结
总的来说，使用类别实现修改的代码比较少，无侵入，使用子类，如果想修改旧代码就比较麻烦。但是，类别也不能滥用，一是类别的代码载入顺序不确定，和添加文件的顺序有关，二是，如果一个类的类别很多会大大加大加载时间，最好同一个类的类别写到一起。最后，给出demo。
