学习iOS的同学都知道ojbc一种runtime的语言，runtime表明函数的真正执行的时候来确定函数执行的。这样的好处就是我们能很灵活的设计我们的代码，也能在看似合法的情况下做一些非常有意思的事情，要了解iOS的runtime，我们需要了解iOS的类结构，iOS所有的类的基类都是NSObject这个类，从这个类来分析iOS的runtime机制。
下面我们在xcode 中打开 NSObject 的声明,为了简单明了，我省略了很多，类型和宏的声明。
NS_ROOT_CLASS
@interface NSObject <NSObject> {
Class isa;
}
+ (void)load;
+ (void)initialize;
- (id)init;
+ (id)new;
+ (id)allocWithZone:(NSZone *)zone;
+ (id)alloc;
- (void)dealloc;
- (void)finalize;
- (id)copy;
- (id)mutableCopy;
+ (id)copyWithZone:(NSZone *)zone NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
+ (id)mutableCopyWithZone:(NSZone *)zone NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
+ (Class)superclass;
+ (Class)class;
+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
+ (BOOL)conformsToProtocol:(Protocol *)protocol;
- (IMP)methodForSelector:(SEL)aSelector;
+ (IMP)instanceMethodForSelector:(SEL)aSelector;
- (void)doesNotRecognizeSelector:(SEL)aSelector;
- (id)forwardingTargetForSelector:(SEL)aSelector NS_AVAILABLE(10_5, 2_0);
- (void)forwardInvocation:(NSInvocation *)anInvocation;
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector;
+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector;
- (BOOL)allowsWeakReference NS_UNAVAILABLE;
- (BOOL)retainWeakReference NS_UNAVAILABLE;
+ (NSString *)description;
+ (BOOL)isSubclassOfClass:(Class)aClass;
+ (BOOL)resolveClassMethod:(SEL)sel NS_AVAILABLE(10_5, 2_0);
+ (BOOL)resolveInstanceMethod:(SEL)sel NS_AVAILABLE(10_5, 2_0);