假如現(xiàn)在有一個(gè)業(yè)務(wù)需求，在項(xiàng)目中每個(gè)控制器界面將要顯示的時(shí)候執(zhí)行一段代碼，可能你第一時(shí)間想到的就是新建一個(gè)基類控制器BaseViewController,然后在BaseViewController中重寫viewWillAppear方法，后面的控制器都繼承自這個(gè)BaseViewController,這樣不是不行，只是后面新建的每一個(gè)控制器都對(duì)父類進(jìn)行了修改，這樣是很消耗內(nèi)存和資源的一個(gè)操作，現(xiàn)在用runtime，交換UIViewController中的viewWillAppear方法的實(shí)現(xiàn)：

先附上BaseViewController中的代碼：

+(void)load
{
    NSString*className=NSStringFromClass(self.class);
    NSLog(@"classname%@",className);
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken,^{
        Class class =[self class];
        SEL originalSelector=@selector(viewWillAppear:);
        SEL swizzledSelector=@selector(xxx_viewWillAppear:);
        Method originalMethod=class_getInstanceMethod(class,originalSelector);
        Method swizzledMethod=class_getInstanceMethod(class,swizzledSelector);
        BOOL didAddMethod=
        class_addMethod(class,
                        originalSelector,
                        method_getImplementation(swizzledMethod),
                        method_getTypeEncoding(swizzledMethod));
        if(didAddMethod){
            class_replaceMethod(class,
                                swizzledSelector,
                                method_getImplementation(originalMethod),
                                method_getTypeEncoding(originalMethod));
        }else{
            method_exchangeImplementations(originalMethod,swizzledMethod);
        }
    });
}

-(void)xxx_viewWillAppear:(BOOL)animated
{
    NSLog(@"viewWillAppear:%@",self);
    [self xxx_viewWillAppear:animated];
    self.navigationController.navigationBar.hidden = YES;
}

這段代碼的作用是替換UIViewController的viewWillAppear方法，所以以后每個(gè)頁面出現(xiàn)的時(shí)候就會(huì)執(zhí)行代碼中-(void)xxx_viewWillAppear:(BOOL)animated這個(gè)方法，因?yàn)樘鎿Q系統(tǒng)方法的業(yè)務(wù)需求只需要執(zhí)行一次，所以代碼中用到的GCD的dispatch_once函數(shù)