什么是 NSProxy

????在蘋果官方文檔中是這樣對 NSProxy 描述的：一個抽象的根類，通常今膊，發(fā)送給代理的消息被轉(zhuǎn)發(fā)到真實對象咙边，或使代理加載（或轉(zhuǎn)換為）真實對象。頭文件代碼如下：

@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 是一個實現(xiàn)了 <NSObject> 協(xié)議的根類拗胜，NSProxy 實現(xiàn)根類所需的基本方法，包括 <NSObject> 協(xié)議中定義的方法怒允。然而埂软，作為一個抽象類，它不提供初始化方法纫事，并且在收到任何未響應(yīng)的消息時引發(fā)異常勘畔。因此所灸，

1. 一個具體的子類必須提供一個初始化或創(chuàng)建方法，
2. 并重寫 forwardInvocation: 和 methodSignatureForSelector: 方法來處理它自己沒有實現(xiàn)的消息炫七。
   • forwardInvocation 的子類實現(xiàn)：應(yīng)該執(zhí)行處理調(diào)用所需的任何操作爬立，例如通過網(wǎng)絡(luò)轉(zhuǎn)發(fā)調(diào)用或加載真實對象并將調(diào)用傳遞給它。
   • methodSignatureForSelector: 需要提供給定消息的參數(shù)類型信息诉字；子類的實現(xiàn)應(yīng)該能夠確定它需要轉(zhuǎn)發(fā)的消息的參數(shù)類型懦尝，并且應(yīng)該相應(yīng)地構(gòu)造一個 NSMethodSignature 對象知纷。

中介者

????使用 NSProxy 對 target 進行消息轉(zhuǎn)發(fā)壤圃，并不會對 target 造成引用關(guān)系。 A ->NSProxy -> B 琅轧。例如伍绳，YYKit/YYWeakProxy，源碼如下：

• 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
  

多繼承

????NSProxy 能對多個對象方法進行轉(zhuǎn)發(fā)乍桂，就使得 NSProxy 具有了多個對象的方法冲杀，可以實現(xiàn)“多繼承”。