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app的注册界面，一般都要求注册信息都要填写完全注册按钮才能点击，使用RAC一段代码实现这样的效果

RAC(self, registerBtn.enabled) = [RACSignal combineLatest:@[RACObserve(self, myRegister.userName),
                                                          RACObserve(self, myRegister.phoneCode),                                                        
                                                          RACObserve(self, myRegister.password),
                                                          RACObserve(self, myRegister.captcha),
                                                 reduce:^id(NSString *userName,
                                                            NSString *phoneCode,                                                           
                                                            NSString *password,                                                                
                                                            NSString *captcha, {
                                                     BOOL enabled = (userName> 0 &&
                                                                     password.length > 0 &&                                                                     
                                                                     captcha.length > 0)                                                                         
                                                     return @(enabled);
                                                 }];