作者：xx明
链接：https://www.jianshu.com/p/a90c47b8d26c
來源：简书
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。
首先要了解UILabel的用来设置和显示内部文本的方法，只需根据textAlignment的值，来动态的修改textRect的位置来达到目的（下面是UILabel需要重写的两个主要方法）
/** 返回UILabel中内容所占的rect*/
-(CGRect)textRectForBounds:(CGRect)bounds limitedToNumberOfLines:(NSInteger)numberOfLines;
/** 在一定区域内显示出来UILabel的内容*/
-(void)drawTextInRect:(CGRect)rect;
使用起来最简单的方式就是为UILabel写个分类，这就需要runtime的一些知识（通过类别实现的关键就是要通过自定的方法与UILabel中的方法进行交换，来达到方法的override效果）
/** 获取某个类中的某个方法*/
OBJC_EXPORT Method class_getInstanceMethod(Class cls, SEL name);
/** 交换两个方法*/
OBJC_EXPORT void method_exchangeImplementations(Method m1, Method m2)
下面是对UILbel分类具体的实现
#import <UIKit/UIKit.h>
typedef NS_ENUM(NSInteger, NSTextAlignmentExpand) {
NSTextAlignmentTop = 5, // Visually top aligned
NSTextAlignmentBottom = 6, // Visually bottom aligned
NSTextAlignmentLeftTop = 7, // Visually left and top aligned
NSTextAlignmentRightTop = 8, // Visually right and top aligned
NSTextAlignmentLeftBottom = 9, // Visually left and bottom aligned
NSTextAlignmentRightBottom = 10, // Visually right and bottom aligned
} NS_ENUM_AVAILABLE_IOS(6_0);
@interface UILabel (TextAlignmentAdditions)
#import "UILabel+TextAlignmentAdditions.h"
#import <objc/runtime.h>
@implementation UILabel (TextAlignmentAdditions)
+(void)load {
/** 获取原始的 textRectForBounds:limitedToNumberOfLines:方法*/
Method originalTextRectForBounds = class_getInstanceMethod([self class], @selector(textRectForBounds:limitedToNumberOfLines:));
Method exchageTextRectForBounds = class_getInstanceMethod([self class], @selector(ll_textRectForBounds:limitedToNumberOfLines:));
method_exchangeImplementations(originalTextRectForBounds, exchageTextRectForBounds);
/** 获取原始的 drawTextInRect:方法*/
Method originalDrawTextInRect = class_getInstanceMethod([self class], @selector(drawTextInRect:));
Method exchageDrawTextInRect = class_getInstanceMethod([self class], @selector(ll_drawTextInRect:));
method_exchangeImplementations(originalDrawTextInRect, exchageDrawTextInRect);
}
- (CGRect)ll_textRectForBounds:(CGRect)bounds limitedToNumberOfLines:(NSInteger)numberOfLines {
/** 先调用系统的textRect...方法*/
CGRect textRect = [self ll_textRectForBounds:bounds limitedToNumberOfLines:numberOfLines];
switch ((NSTextAlignmentExpand)self.textAlignment) {
case NSTextAlignmentTop:
textRect.origin.y = bounds.origin.y;
textRect.origin.x = 0.5 *(bounds.size.width - textRect.size.width);
break;
case NSTextAlignmentLeftTop:
textRect.origin.y = bounds.origin.y;
break;
case NSTextAlignmentRightTop:
textRect.origin.y = bounds.origin.y;
textRect.origin.x = bounds.size.width - textRect.size.width;
break;
case NSTextAlignmentBottom:
textRect.origin.y = bounds.origin.y + bounds.size.height - textRect.size.height;
textRect.origin.x = 0.5 *(bounds.size.width - textRect.size.width);
break;
case NSTextAlignmentLeftBottom:
textRect.origin.y = bounds.origin.y + bounds.size.height - textRect.size.height;
break;
case NSTextAlignmentRightBottom:
textRect.origin.y = bounds.origin.y + bounds.size.height - textRect.size.height;
textRect.origin.x = bounds.size.width - textRect.size.width;
break;
default:
textRect.origin.y = bounds.origin.y + (bounds.size.height - textRect.size.height) / 2.0;
}
return textRect;
}
-(void)ll_drawTextInRect:(CGRect)requestedRect {
/** 相当于调用ll_textRect...方法*/
CGRect actualRect = [self textRectForBounds:requestedRect limitedToNumberOfLines:self.numberOfLines];
/** 后调用系统的drawTextInRect:方法*/
[self ll_drawTextInRect:actualRect];
}
@end
举个例子，有个titleLabel，产品要求title最多显示两行，但是不管显示几行都要内容的上端起点都一样。这样你设置一个大Label，把textAline改为我扩展的leftTopAline就OK了。
