特别注意：定义的command中的封装信号一定要sendCompleted；否则在生命周期内无法被重新激活；

1. 普通的command

普通的RACCommand的使用很简单了，只需要在封装的信号中sendCompleted、sendError就可以。例如：

- (RACCommand *)commandForDataSelecting {
  if (!_commandForDataSelecting) {
    @weakify(self);
    _commandForDataSelecting = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
        return [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
            @strongify(self);
            [[DBManager sharedDB] selectDataFromTabel:kCardInfoTableName forConditions:nil sucBlock:^(NSArray *datasSelected) {
                [subscriber sendCompleted];
            } errBlock:^(NSError *error) {
                [subscriber sendError:error];
            }];
            return nil;
        }] replayLast];
    }];
  }
  return _commandForDataSelecting;
}

2. 获取响应状态

要处理的状态有3种：开始,完成,失败

• 开始

  [[command executing] subscribeNext:^id(id x) {
      NSLog(@"数据加载中...");
  } // 或者在 'subscribeNext' 前面加上 'deliverOn:' 在主线程中处理;
  

• 完成

  [[[command executionSignals] flattenMap:^ RACStream(RACSignal* innerSignal) {
      return [innerSignal materialize];
  }] subscribeNext:^(RACEvent* event) {
      if (event.eventType == RACEventTypeCompleted) {
          NSLog(@"数据加载完毕!"); // 1
      }
      else if (event.eventType == RACEventTypeError) {
          NSLog(@"数据加载失败!"); //2
      }
      else if (event.eventType == RACEventTypeNext) {
          NSLog(@"数据加载中..."); //3
      }
  }];
  

实际测试发现，subscribeNext只能获取到 completed的状态，所以可以在 subscribeNext 前对结果进行过滤:filter:,过滤出completed的状态。

• 失败

  [[command errors] subscribeNext:^(NSError* error) {
      NSLog(@"%@",[error localizedDescription]); //4
  }
  

3. 疑惑和解惑

• 当command能正常结束时，一切都没问题。可是当command执行异常时，不仅异常errors被抛出://4，completed也被抛出了//1。
• 如果上面 // 1-4 都用蒙版 MBProgressHUD 显示的话，线程调度会有问题，蒙版显示会一闪而过；

所以，我们要解决的目标：当 command 抛出 error 时，不要同时抛出 completed .

• 解决方法
  在command的定义中，将返回的RACSignal再次封装materialize，封装后产生的信号的值是RACEvent类型；
  然后executionSignals通过解封装dematerialize出一个新的信号,对它进行'异常'、'完成'捕获:subscribeError: completed:分别处理异常和完成状态.

新的command定义和executionSignals处理如下:

// command定义
- (RACCommand *)commandForDataSelecting {
  if (!_commandForDataSelecting) {
    @weakify(self);
    _commandForDataSelecting = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
        return [[[[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
            @strongify(self);
            [[DBManager sharedDB] selectDataFromTabel:kCardInfoTableName forConditions:nil sucBlock:^(NSArray *datasSelected) {
                self.cardInfosSelected = [datasSelected copy];
                [subscriber sendCompleted];
            } errBlock:^(NSError *error) {
                [subscriber sendError:error];
            }];
            return nil;
        }] replayLast] materialize] delay:1];
    }];
  }
  return _commandForDataSelecting;
}

 // 结果处理
 [command.executionSignals subscribeNext:^(RACSignal* signal) {
    @strongify(self);
    [[[signal dematerialize] deliverOn:[RACScheduler mainThreadScheduler]] subscribeError:^(NSError *error) {
        [self.progressHUD showFailWithText:[error localizedDescription] andDetailText:nil onCompletion:nil];
    } completed:^{
        [self.progressHUD showSuccessWithText:@"加载成功" andDetailText:nil onCompletion:^{}];
    }];
}];