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什么是 NSProxy

    在苹果官方文档中是这样对 NSProxy 描述的：一个抽象的根类，通常，发送给代理的消息被转发到真实对象，或使代理加载（或转换为）真实对象。头文件代码如下：

@interface NSProxy <NSObject> {
    __ptrauth_objc_isa_pointer Class isa;
}

+ (id)alloc;
+ (id)allocWithZone:(nullable NSZone *)zone NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
+ (Class)class;

- (void)forwardInvocation:(NSInvocation *)invocation;
- (nullable NSMethodSignature *)methodSignatureForSelector:(SEL)sel NS_SWIFT_UNAVAILABLE("NSInvocation and related APIs not available");
- (void)dealloc;
- (void)finalize;
@property (readonly, copy) NSString *description;
@property (readonly, copy) NSString *debugDescription;
+ (BOOL)respondsToSelector:(SEL)aSelector;

- (BOOL)allowsWeakReference API_UNAVAILABLE(macos, ios, watchos, tvos);
- (BOOL)retainWeakReference API_UNAVAILABLE(macos, ios, watchos, tvos);

// - (id)forwardingTargetForSelector:(SEL)aSelector;

@end

NSProxy 使用

    可以看到 NSProxy 是一个实现了 <NSObject> 协议的根类，NSProxy 实现根类所需的基本方法，包括 <NSObject> 协议中定义的方法。然而，作为一个抽象类，它不提供初始化方法，并且在收到任何未响应的消息时引发异常。因此，

1. 一个具体的子类必须提供一个初始化或创建方法，
2. 并重写 forwardInvocation: 和 methodSignatureForSelector: 方法来处理它自己没有实现的消息。
   • forwardInvocation 的子类实现：应该执行处理调用所需的任何操作，例如通过网络转发调用或加载真实对象并将调用传递给它。
   • methodSignatureForSelector: 需要提供给定消息的参数类型信息；子类的实现应该能够确定它需要转发的消息的参数类型，并且应该相应地构造一个 NSMethodSignature 对象。

中介者

    使用 NSProxy 对 target 进行消息转发，并不会对 target 造成引用关系。 A ->NSProxy -> B 。例如，YYKit/YYWeakProxy，源码如下：

• YYWeakProxy.h

  
  /**
   A proxy used to hold a weak object.
   It can be used to avoid retain cycles, such as the target in NSTimer or CADisplayLink.
   
   sample code:
   
       @implementation MyView {
          NSTimer *_timer;
       }
       
       - (void)initTimer {
          YYWeakProxy *proxy = [YYWeakProxy proxyWithTarget:self];
          _timer = [NSTimer timerWithTimeInterval:0.1 target:proxy selector:@selector(tick:) userInfo:nil repeats:YES];
       }
       
       - (void)tick:(NSTimer *)timer {...}
       @end
   */
  @interface YYWeakProxy : NSProxy
  
  /**
   The proxy target.
   */
  @property (nullable, nonatomic, weak, readonly) id target;
  - (instancetype)initWithTarget:(id)target;
  + (instancetype)proxyWithTarget:(id)target;
  
  @end
  

• YYWeakProxy.m

  #import "YYWeakProxy.h"
  
  
  @implementation YYWeakProxy
  
  - (instancetype)initWithTarget:(id)target {
      _target = target;
      return self;
  }
  
  + (instancetype)proxyWithTarget:(id)target {
      return [[YYWeakProxy alloc] initWithTarget:target];
  }
  
  - (id)forwardingTargetForSelector:(SEL)selector {
      return _target;
  }
  
  - (void)forwardInvocation:(NSInvocation *)invocation {
      void *null = NULL;
      [invocation setReturnValue:&null];
  }
  
  - (NSMethodSignature *)methodSignatureForSelector:(SEL)selector {
      return [NSObject instanceMethodSignatureForSelector:@selector(init)];
  }
  
  - (BOOL)respondsToSelector:(SEL)aSelector {
      return [_target respondsToSelector:aSelector];
  }
  
  - (BOOL)isEqual:(id)object {
      return [_target isEqual:object];
  }
  
  - (NSUInteger)hash {
      return [_target hash];
  }
  
  - (Class)superclass {
      return [_target superclass];
  }
  
  - (Class)class {
      return [_target class];
  }
  
  - (BOOL)isKindOfClass:(Class)aClass {
      return [_target isKindOfClass:aClass];
  }
  
  - (BOOL)isMemberOfClass:(Class)aClass {
      return [_target isMemberOfClass:aClass];
  }
  
  - (BOOL)conformsToProtocol:(Protocol *)aProtocol {
      return [_target conformsToProtocol:aProtocol];
  }
  
  - (BOOL)isProxy {
      return YES;
  }
  
  - (NSString *)description {
      return [_target description];
  }
  
  - (NSString *)debugDescription {
      return [_target debugDescription];
  }
  
  @end
  

多继承

    NSProxy 能对多个对象方法进行转发，就使得 NSProxy 具有了多个对象的方法，可以实现“多继承”。