ReactiveCocoa系列-隐藏顯示placehold

//建立textField獲得焦點信号,當信号發生時傳回@YES
        RACSignal *didBeginEditingSignal = [[self rac_signalForSelector:@selector(textFieldDidBeginEditing:) fromProtocol:@protocol(UITextFieldDelegate)] mapReplace:@YES];
        
        //建立textField失去焦點信号,當信号發生時傳回@NO
        RACSignal *didEndEditingSignal = [[self rac_signalForSelector:@selector(textFieldDidEndEditing:) fromProtocol:@protocol(UITextFieldDelegate)] mapReplace:@NO];
        
        //合并兩種信号到一個信号
        RACSignal *userTextSignal = [RACSignal merge:@[didBeginEditingSignal, didEndEditingSignal]];
        
        //doNext是信号發生時注入的邊界觸發.hiden為label的屬性
        RAC(placehold, hidden) = [userTextSignal doNext:^(id x) {
            NSLog(@"toggle placehold. %@",x);
        }];