观测按键

观测按键按下的event, 如果这个event发生, 则立即send next

[[self.signInButton
   rac_signalForControlEvents:UIControlEventTouchUpInside]
   subscribeNext:^(id x) {
     NSLog(@"button clicked");
   }];

* rac_signalForControlEvents: Create a signal from button's UIControlEventTouchUpInside