概(fei)述(hua)

...(省略1024亿个byte)

探究

为了研究Objective-C的消息转发，做了一系列测试

Test 0

• 内容: 调用对象存的方法

• 步骤:

  1. 定义一个TestObject类，继承NSObject, 重写的方法如下:

     @implementation TestObject
     
     + (BOOL)resolveClassMethod:(SEL)sel {
         BOOL ret = [super resolveClassMethod:sel];
         NSLog(@"-->> %@ %p resolveClassMethod: %@, return: %d", [self class], self, NSStringFromSelector(sel), ret);
         return ret;
     }
     
     - (BOOL)respondsToSelector:(SEL)aSelector {
         BOOL ret = [super respondsToSelector:aSelector];
         NSLog(@"-->> %@ %p respondsToSelector: %@, return: %d", [self class], self, NSStringFromSelector(aSelector), ret);
         return ret;
     }
     
     - (id)forwardingTargetForSelector:(SEL)aSelector {
         id ret = [super forwardingTargetForSelector:aSelector];
         NSLog(@"-->> %@ %p forwardingTargetForSelector: %@, return: %@", [self class], self, NSStringFromSelector(aSelector), ret);
         return ret;
     }
     
     - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
         NSMethodSignature* ret = [super methodSignatureForSelector:aSelector];
         NSLog(@"-->> %@ %p methodSignatureForSelector: %@, return: %@", [self class], self, NSStringFromSelector(aSelector), ret);
         return ret;
     }
     
     - (void)forwardInvocation:(NSInvocation *)anInvocation {
         NSLog(@"-->> %@ %p forwardInvocation: %@", [self class], self, anInvocation);
         [super forwardInvocation:anInvocation];
     }
     
     @end

  2. 代码

     [[TestObject alloc] init];

• 结果: 什么都没有log
• 结论: 调用存在的方法, 没有发生任何消息转发

Test 1

• 内容: 调用对象不存在的方法

• 步骤:

  1. 代码

     TestObject* obj = [[TestObject alloc] init];
        [obj performSelector:NSSelectorFromString(@"not exists sel")];

• 结果: TestObject调用了forwardingTargetForSelector:(返回nil), 然后调用methodSignatureForSelector:(返回nil), 最后log unrecognized selector sent to instance
• 结论: 调用不存在的方法, 发生了消息转发

Test 2

• 内容: TestObject_1重写forwardingTargetForSelector:(返回TestObject对象), 调用它们都不存在的方法

• 步骤:

  1. 定义TestObject_1, 继承TestObject

  @implementation TestObject_1
  
  - (id)forwardingTargetForSelector:(SEL)aSelector {
      id ret = [[TestObject alloc] init];
      NSLog(@"-->> %@ %p forwardingTargetForSelector: %@, return: %@", [self class], self, NSStringFromSelector(aSelector), ret);
      return ret;
  }
  
  @end

  1. 代码:

  TestObject_2* obj = [[TestObject_2 alloc] init];
     [obj performSelector:NSSelectorFromString(@"not exists sel")];

• 结果: TestObject_1对象先调用了forwardingTargetForSelector:(返回TestObject对象), 然后TestObject对象调用forwardingTargetForSelector:(返回nil), 跟着调用methodSignatureForSelector:(返回nil), 最后log unrecognized selector sent to instance
• 结论: 不存在的方法转发到了其它对象处理失败

Test 3

• 内容: TestObject_2重写forwardingTargetForSelector:(返回TestObject_1), 调用TestObject_2不存在, 但TestObject_1存在的方法

• 步骤:

  1. 定义TestObject_2, 继承TestObject

  @implementation TestObject_2
  
  - (id)forwardingTargetForSelector:(SEL)aSelector {
      id ret = [[TestObject_1 alloc] init];
      NSLog(@"-->> %@ %p forwardingTargetForSelector: %@, return: %@", [self class], self, NSStringFromSelector(aSelector), ret);
      return ret;
  }
  
  @end

  1. TestObject_1, 添加方法

  - (void)testObject_1ExistsMethod {
         NSLog(@"-->> %@ %p I am exists", [self class], self);
  }

  1. 代码

  TestObject_2* obj = [[TestObject_2 alloc] init];
     [obj performSelector:NSSelectorFromString(@"testObject_1ExistsMethod")];

• 结果: TestObject_2对象先调用了forwardingTargetForSelector:(返回TestObject_1对象), 然后TestObject_1对象调用了testObject_1ExistsMethod
• 结论: 不存在的方法转发到了其它对象处理成功

Test 4

• 内容: TestObject_3重写methodSignatureForSelector:(不返回nil)和forwardInvocation:, 调用不存在的方法

• 步骤:

  1. 定义TestObject_3, 继承TestObject

  @implementation TestObject_3
  
  - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
      NSLog(@"-->> %@ %p methodSignatureForSelector: %@", [self class], self, NSStringFromSelector(aSelector));
      return [NSObject methodSignatureForSelector:@selector(init)];
  }
  
  - (void)forwardInvocation:(NSInvocation *)anInvocation {
      NSLog(@"-->> %@ %p forwardInvocation: %@", [self class], self, anInvocation);
  //    [super forwardInvocation:anInvocation]; // will crash, because the method signature is not right
  }
  
  @end

  1. 代码

  TestObject_3* obj = [[TestObject_3 alloc] init];
     [obj performSelector:NSSelectorFromString(@"not exists sel")];

• 结果: 先调用了forwardingTargetForSelector:, 然后methodSignatureForSelector:, 最后forwardInvocation:, 没发生崩溃
• 结论: TestObject_3自己处理了不存在方法

总结

如下图

测试代码

TestMessageForwarding: https://github.com/skytoup/TestMessageForwarding