iOS之《Effective Objective-C 2.0》读书笔记（12）

第12条：理解消息转发机制

1.消息转发机制的作用

对象在接受到无法解读的消息时，会在编译时报警告，运行时直接崩溃，此方法可以给对象和消息更多的机会来完成成功的调用

2.消息转发流程

（1）动态方法解析：

+(BOOL)resolveInstanceMethod:(SEL)sel // 实例方法+(BOOL)resolveClassMethod:(SEL)sel    // 类方法

• 下列代码演示了如何用“resolveInstanceMethod:”来实现@dynamic属性：

id autoDictionaryGetter(id self, SEL _cmd);  void autoDictionarySetter(id self, SEL _cmd, id value);  + (BOOL)resolveInstanceMethod:(SEL)selector {      NSString *selectorString = NSStringFromSelector(selector);      if ( /* selector is from a @dynamic property */ ) {          if ([selectorString hasPrefix:@"set"]) {              class_addMethod(self,                              selector,                              (IMP)autoDictionarySetter,                              "v@:@");          } else {              class_addMethod(self,                              selector,                              (IMP)autoDictionaryGetter,                              "@@:");          }          return YES;      }  return [super resolveInstanceMethod:selector];  } 

（2）备援接受者：运行期系统把消息转给其他接收者处理

- (id)forwardingTargetForSelector:(SEL)aSelector{    class *other = [class new];    if ([other forwardingTargetForSelector:aSelector]) {        return other;    }else{        return [super forwardingTargetForSelector:aSelector];    }}

（3）完整的消息转发：创建NSInvocation对象，把与尚未处理的那条消息有关的全部细节都封于其中。此对象包含选择子、目标target及参数。在触发NSInvocation对象时，消息派发系统会把消息指派给目标对象

• TestModel

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {      if(aSelector == @selector(testMethod))      {          return [NSMethodSignature signatureWithObjCTypes:"v@:"];      }      return nil;  }  -(void)forwardInvocation:(NSInvocation *)anInvocation  {      if (anInvocation.selector == @selector(testMethod))      {          TestModelHelper1 *h1 = [[TestModelHelper1 alloc] init];          TestModelHelper2 *h2 = [[TestModelHelper2 alloc] init];          [anInvocation invokeWithTarget:h1];          [anInvocation invokeWithTarget:h2];      }  }  

• TestModelHelper1

-(void)testMethod  {      NSLog(@"i am TestModelHelper1");  }  

• TestModelHelper2

-(void)testMethod  {      NSLog(@"i am TestModelHelper2");  }  

• 主调用类 : TestModel本身没有实现testMethod方法,但最终运行后，程序没有报错，且TestModelHelper1和TestModelHelper2的testMethod方法都被执行了

TestModel *model = [[TestModel alloc] init];  [model testMethod];  

（4）消息未能处理：调用doesNotRecognizeSelector抛出异常