Target-Action示例代码如下：

    UIButton *btn = [[UIButton alloc]initWithFrame:CGRectMake(20,20,50,50)];
    btn.backgroundColor = [UIColor redColor];
    btn.rac_command = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
        NSLog(@"按钮被点击！！");
        return [RACSignal empty];
    }];

KVO

[RACObserve(self,username) subscribeNext:^(NSString *newName) {
  NSLog(@"%@",newName);
}];

[[[NSNotificationCenter defaultCenter] rac_addObserverForName:UITextFieldTextDidBeginEditingNotification object:self.emailTextField] subscribeNext:^(id x) {
    [self.emailImageView animate];
    self.emailImageView.image = [UIImage imageNamed:@"icon-mail-active"];
    self.emailTextField.background = [UIImage imageNamed:@"input-outline-active"];
}];

[[[NSNotificationCenter defaultCenter] rac_addObserverForName:UITextFieldTextDidEndEditingNotification object:self.emailTextField] subscribeNext:^(id x) {
    self.emailTextField.background = [UIImage imageNamed:@"input-outline"];
    self.emailImageView.image = [UIImage imageNamed:@"icon-mail"];
}];