NSObject is the root class of most Objective-C class hierarchies. Through NSObject, objects inherit a basic interface to the runtime system and the ability to behave as Objective-C objects.
NSObject 是 Objective-C 的大多数类的根类，自身实现了 <NSObject> 协议。
头文件申明为：
@interface NSObject <NSObject> {
Class isa OBJC_ISA_AVAILABILITY;
}
+ (void)load;
+ (void)initialize;
- (instancetype)init
#if NS_ENFORCE_NSOBJECT_DESIGNATED_INITIALIZER
NS_DESIGNATED_INITIALIZER
#endif
;
+ (instancetype)new OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
+ (instancetype)allocWithZone:(struct _NSZone *)zone OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
+ (instancetype)alloc OBJC_SWIFT_UNAVAILABLE("use object initializers instead");
- (void)dealloc OBJC_SWIFT_UNAVAILABLE("use 'deinit' to define a de-initializer");
- (void)finalize;
- (id)copy;
- (id)mutableCopy;
+ (id)copyWithZone:(struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;
+ (id)mutableCopyWithZone:(struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;
+ (BOOL)instancesRespondToSelector:(SEL)aSelector;
+ (BOOL)conformsToProtocol:(Protocol *)protocol;
- (IMP)methodForSelector:(SEL)aSelector;
+ (IMP)instanceMethodForSelector:(SEL)aSelector;
- (void)doesNotRecognizeSelector:(SEL)aSelector;
- (id)forwardingTargetForSelector:(SEL)aSelector __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);
- (void)forwardInvocation:(NSInvocation *)anInvocation OBJC_SWIFT_UNAVAILABLE("");
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");
+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");
- (BOOL)allowsWeakReference UNAVAILABLE_ATTRIBUTE;
- (BOOL)retainWeakReference UNAVAILABLE_ATTRIBUTE;
+ (BOOL)isSubclassOfClass:(Class)aClass;
+ (BOOL)resolveClassMethod:(SEL)sel __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);
+ (BOOL)resolveInstanceMethod:(SEL)sel __OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);
+ (NSUInteger)hash;
+ (Class)superclass;
+ (Class)class OBJC_SWIFT_UNAVAILABLE("use 'aClass.self' instead");
+ (NSString *)description;
+ (NSString *)debugDescription;
@end
new / alloc / copy / mutableCopy 之类的是创建/拷贝方法。
instancesRespondToSelector / conformsToProtocol 之类的方法也比较简单，判断该类/该类的实例对象是否响应某 Selector / Protocol 等。
forwardingTargetForSelector / forwardingInvocation / resolveClassMethod / resolveInstanceMethod 之类的方法是 NSObject 的消息转发机制，如果某对象没有实现某方法，就会进行消息转发。内容比较多，这里先不说。
<NSObject>协议申明为：
@protocol NSObject
- (BOOL)isEqual:(id)object;
@property (readonly) NSUInteger hash;
@property (readonly) Class superclass;
- (Class)class OBJC_SWIFT_UNAVAILABLE("use 'anObject.dynamicType' instead");
- (instancetype)self;
- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;
- (BOOL)isProxy;
- (BOOL)isKindOfClass:(Class)aClass;
- (BOOL)isMemberOfClass:(Class)aClass;
- (BOOL)conformsToProtocol:(Protocol *)aProtocol;
- (BOOL)respondsToSelector:(SEL)aSelector;
- (instancetype)retain OBJC_ARC_UNAVAILABLE;
- (oneway void)release OBJC_ARC_UNAVAILABLE;
- (instancetype)autorelease OBJC_ARC_UNAVAILABLE;
- (NSUInteger)retainCount OBJC_ARC_UNAVAILABLE;
- (struct _NSZone *)zone OBJC_ARC_UNAVAILABLE;
@property (readonly, copy) NSString *description;
@optional
@property (readonly, copy) NSString *debugDescription;
@end
performSelector 由 另外一篇文章 从多线程的角度总结了点知识点，其他略过不表。
和 UIKit 相关的继承自 NSObject的组织架构图：
常用到需要注意的关键点在于 UIResponder 和 UIControl 的特性。
- UIResponder
- (nullable UIResponder*)nextResponder;
- (BOOL)canBecomeFirstResponder; // default is NO
- (BOOL)becomeFirstResponder;
- (BOOL)canResignFirstResponder; // default is YES
- (BOOL)resignFirstResponder;
- (BOOL)isFirstResponder;
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesEnded:(NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesCancelled:(nullable NSSet<UITouch *> *)touches withEvent:(nullable UIEvent *)event;
- (void)touchesEstimatedPropertiesUpdated:(NSSet * _Nonnull)touches NS_AVAILABLE_IOS(9_1);
这些是比较常用到的 Api，还有一些 pressesBegan:withEvent: / motionBegan:withEvent: 之类的同 touchesBegan:withEvent: 类似，只是手势不同。
可以看出 UIResponder 的特点，它是为了处理交互事件的。所以能够处理交互的类大都继承自 UIResponder，包括 UIView，UIViewController，UIApplication。这里提一句，项目自动生成的 AppDelegate 类也是继承自 UIResponder。
- UIControl
@property(nonatomic,getter=isEnabled) BOOL enabled;
@property(nonatomic,getter=isSelected) BOOL selected;
@property(nonatomic,getter=isHighlighted) BOOL highlighted;
@property(nonatomic,readonly) UIControlState state;
// add target/action for particular event. you can call this multiple times and you can specify multiple target/actions for a particular event.
// passing in nil as the target goes up the responder chain. The action may optionally include the sender and the event in that order
// the action cannot be NULL. Note that the target is not retained.
- (void)addTarget:(nullable id)target action:(SEL)action forControlEvents:(UIControlEvents)controlEvents;
- (void)removeTarget:(nullable id)target action:(nullable SEL)action forControlEvents:(UIControlEvents)controlEvents;
- (NSSet *)allTargets;
- (UIControlEvents)allControlEvents;
- (nullable NSArray<NSString *> *)actionsForTarget:(nullable id)target forControlEvent:(UIControlEvents)controlEvent;
UIControl 的一个重要的特性就是拥有 enabled / selected / highlighted 之类的属性，而且它还有个很重要的方法 addTarget:action:forControlEvents: 。很明显的 目标 - 动作 模式。当该目标触发 **ControlEvents 时调用对应的 action。
主要总结了下 UIResponder 和 UIControl 的比较明显的特点。所有 UIView 的事件处理都是在 UIResponder 类中实现的，所有带有交互处理的比如 UIButton，UISlider 都是继承自 UIControl，实现了比较重要的 addTarget:action:forControlEvents: 方法。
