Runtime

运行时的使用:

1. KVO , KVC 

2. 运行过程中交换两个方法的实现,改系统的方法.

　　例如:  当一个做了几年的项目要从iOS6适配到iOS7时,要把之前的图片全部换掉,可通过扩展UIImage 实现它的分类.补充一个类方法imageWithName: name.然后将系统的imageName:方法与imageWithName:name 的方法在运行时换掉,而不用修改其他代码.

#import "UIImage+Extension.h"#import 
     
      @implement UIImage(Extension)+(void) load{    Method otherMethod = class_getClassMethod([UIImage class],@selector(imageWithName:));    Method originMethod = class_getClassMethod([UIImage class],@selector(imageNamed:));    //交换两个方法的实现    method_exchangeImplementations(otherMethod, originMethod);}+ (UIImage *) imageWithName:(NSString *) name{    BOOL ios7 = [[UIDevice currentDevice].systemVersion floatValue] >= 7.0;    UIImage * image = nil;    if (ios7)    {        NSString * newName = [name stringByAppendingString:@"_os7"];        image = [UIImage imageWithName:newName];        }    if (image ==nil)    {        image = [UIImage imageWithName: name];    }    return image;}@end