ReactiveCocoa - study

    RAC(self.loginViewModel, userName) = self.userNameTF.rac_textSignal;
    RAC(self.loginViewModel, password) = self.passwordTF.rac_textSignal;

    @weakify(self)

    [[self.loginButton rac_signalForControlEvents: UIControlEventTouchUpInside] subscribeNext:^(id x) {

        @strongify(self)

        if (self.loginViewModel.userName.length == 0) {

            self.loginViewModel.alertTip = @"用户名不能为空";

            return ;

        }

        if (self.loginViewModel.password.length == 0) {

            self.loginViewModel.alertTip = @"密码不能为空";

            return;

        }

        [self.loginViewModel clickLogin];

     }];

 [[RACObserve(self, alertTip) filter:^BOOL(NSString * value) {
        return value.length != 0 && value;
    }] subscribeNext:^(NSString * tip) {

        Alert(tip);
    }];