// Send an error message 
_loginButton.rac_command = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
    return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        [subscriber sendError:error]; // Pretend this is a real error
        return nil;
    }];
}];
 
// Subscribe to loginButton's returned signal
[_loginButton.rac_command.executionSignals subscribeNext:^(RACSignal *loginSignal) {
    [loginSignal subscribeError:^(NSError *error) {
         NSLog(@"A");
    } completed:^{
         NSLog(@"B");
    }];
}];

这打印“B”.知道为什么吗？如果在订阅者上调用-sendError：,为什么完成块会收到它？