ReactiveCocoa系列-隐藏显示placehold

        //创建textField获得焦点信号,当信号发生时返回@YES
        RACSignal *didBeginEditingSignal = [[self rac_signalForSelector:@selector(textFieldDidBeginEditing:) fromProtocol:@protocol(UITextFieldDelegate)] mapReplace:@YES];
        
        //创建textField失去焦点信号,当信号发生时返回@NO
        RACSignal *didEndEditingSignal = [[self rac_signalForSelector:@selector(textFieldDidEndEditing:) fromProtocol:@protocol(UITextFieldDelegate)] mapReplace:@NO];
        
        //合并两种信号到一个信号
        RACSignal *userTextSignal = [RACSignal merge:@[didBeginEditingSignal, didEndEditingSignal]];
        
        //doNext是信号发生时注入的边界触发.hiden为label的属性
        RAC(placehold, hidden) = [userTextSignal doNext:^(id x) {
            NSLog(@"toggle placehold. %@",x);
        }];