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:

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     @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

[[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

     12	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

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     @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

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

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     @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

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

• 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

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     @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

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: github.com/skytoup/Tes…

本文为博主skytoup原创文章，未经博主skytoup允许不得转载。

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