RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
      [subscriber sendNext:@"我是发送的数据"];
      [subscriber sendError:[NSError errorWithDomain:@"www.huang"
                                                code:0
                                            userInfo:@{NSLocalizedDescriptionKey:@"错误信息"}]];
      return nil;
  }];
  [signal subscribeNext:^(id x) {
      NSLog(@"next:%@",x);
  } error:^(NSError *error) {
      NSLog(@"error:%@",error.localizedDescription);
  }];

 ---------
 打印结果:
  next:我是发送的数据
  error:错误信息

  RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
      [subscriber sendNext:@"我是发送的数据"];
      return nil;
  }];
  
  [signal subscribeNext:^(id x) {
      NSLog(@"next:%@",x);
  } error:^(NSError *error) {
      NSLog(@"error:%@",error.localizedDescription);
  }];

  RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
      [subscriber sendNext:@"我是发送的数据"];
      return nil;
  }];
  
  RACSignal *newSignal = [signal tryMap:^id(id value, NSError *__autoreleasing *errorPtr) {
      if ([value isKindOfClass:[NSNumber class]]) {
          // 符合预期 返回原本的值
          return value;
      } else {
          // 不符合预期，走error回调
          NSError *error = [NSError errorWithDomain:@"www.huang"
                                               code:0
                                           userInfo:@{NSLocalizedDescriptionKey:@"数据格式错误"}];
          *errorPtr = error;
          // 这里一定要返回nil，相当于拦截了value的值，就不会走next回调了
          return nil;
      }
  }];

  // Note: 其实可以使用 tryMap 原来的value，并返回新的value给 next回调
  
  [newSignal subscribeNext:^(id x) {
      NSLog(@"next:%@",x);
  } error:^(NSError *error) {
      NSLog(@"error:%@",error.localizedDescription);
  }];

 ---------
 打印结果:
 error:数据格式错误

   RACSignal *signal = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
      [subscriber sendNext:@"我是发送的数据"];
      return nil;
  }];
  
  /**
   返回值是一个BOOL
   当返回YES时，subscriber 订阅时走next回调
   当返回NO时，subscriber 订阅时走error回调
   */
  RACSignal *newSignal = [signal try:^BOOL(id value, NSError *__autoreleasing *errorPtr) {
      NSError *error = [NSError errorWithDomain:@"www.huang"
                                                    code:0
                                                userInfo:@{NSLocalizedDescriptionKey:@"数据格式错误"}];
      *errorPtr = error;
      return NO;
     
      /**
       这样并不能修改原来发送的value的值，因为这是形参!!
       value = @(1234);
       return YES;
       */
      
  }];
  
  [newSignal subscribeNext:^(id x) {
      NSLog(@"next:%@",x);
  } error:^(NSError *error) {
      NSLog(@"error:%@",error.localizedDescription);
  }];

 ---------
 打印结果:
 error:数据格式错误

- (RACSignal *)try:(BOOL (^)(id value, NSError **errorPtr))tryBlock {
  NSCParameterAssert(tryBlock != NULL);

  return [[self flattenMap:^(id value) {
      NSError *error = nil;
      BOOL passed = tryBlock(value, &error);
             // 关键点 passed是否为YES
      return (passed ? [RACSignal return:value] : [RACSignal error:error]);
  }] setNameWithFormat:@"[%@] -try:", self.name];
}

- (RACSignal *)tryMap:(id (^)(id value, NSError **errorPtr))mapBlock {
  NSCParameterAssert(mapBlock != NULL);

  return [[self flattenMap:^(id value) {
      NSError *error = nil;
      id mappedValue = mapBlock(value, &error);
              // 关键点: mappedValue是否为nil
      return (mappedValue == nil ? [RACSignal error:error] : [RACSignal return:mappedValue]);
  }] setNameWithFormat:@"[%@] -tryMap:", self.name];
}