2. 기본 UI 만들기(2) - 부드럽게 움직이는 Transition, with [React-Transition-Group]

 .entering {
        opacity: 0;
    }
    .entered {
        animation: popup 400ms ease-out;
    }
    .exiting {
        animation: unpopup 400ms ease-out;
    }
    .exited {
        opacity: 0;
    }
    
    @keyframes popup {
        0% {
        opacity: 0;
        }
        100% {
        opacity: 1;
        }
    }
    
    @keyframes unpopup {
        0% {
        opacity: 1;
        }
        100% {
        opacity: 0;
        }
    }