Objective-C runtime黑魔法，交换iOS系统类库方法

#pragma mark - 使用分类重写NSObject的load方法+(void)load{    //只需要运行一次    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        Class class = [self class];        //交换viewWillAppear: 和 glt_viewWillAppear: 方法        SEL oriSelector = @selector(viewWillAppear:);        SEL swiSelctor = @selector(glt_viewWillAppear:);                //注意：导入头文件 #import <objc/message.h>        Method oriMethod = class_getInstanceMethod(class, oriSelector);        Method swiMethod = class_getInstanceMethod(class, swiSelctor);                BOOL success = class_addMethod(class, oriSelector, method_getImplementation(swiMethod), method_getTypeEncoding(swiMethod));                if (success) {            class_replaceMethod(class, swiSelctor, method_getImplementation(oriMethod), method_getTypeEncoding(oriMethod));        }else{            method_exchangeImplementations(oriMethod, swiMethod);        }    });}#pragma mark - 替换后的子类viewWillAppear-(void)glt_viewWillAppear:(BOOL)animated{    [self glt_viewWillAppear:animated];}