Vue Transition not Triggering on button click

Question

I am new to Vue JS and I am creating a thumbnail viewer wherein I'll be getting a list of images and videos as an array of objects. At first, I will be showing just 5 items and when the user clicks Top / Bottom button, I want to slide the thumbnails vertically.

I have created a codepen by referring some links on StackOverflow.

I am using Vue Transitions and my data seems to be reactive but somehow I can't see the smooth transition (sliding to the top / bottom by 100px) when I click on the Top and Bottom buttons.

HTML Code:

<div id="app" class="container-fluid">
    <div class="row row-eq-height">
        <div class="thumbnail-container">
            <button class="up" @click="moveTop" :disabled="currentTopIndex === 0">Top</button>
            <div :class="'slider' + (isSlidingToPrevious ? ' sliding-to-previous' : '')">
                <transition-group name='list' tag="ul">
                    <li v-for="(item,index) in currentCarouselData" v-bind:key="index" class="list-item"><img :src="item.itemImage" :alt="item.itemImageAlt" /></li>
                </transition-group>
            </div>
            <button @click="moveBottom" class="down" :disabled="currentBottomIndex === totalCount">Down</button>
        </div>
    </div>
    <pre>
    totalCount {{totalCount}}
    currentTopIndex {{currentTopIndex}}
    currentBottomIndex {{currentBottomIndex}}
    itemsToDisplay {{itemsToDisplay}}
    currentCarouselData {{currentCarouselData}}
</pre>
</div>

CSS / LESS Code:

.row-eq-height {
  display: flex;
  ul {
    list-style-type: none;
    display: flex;
    flex-direction: column;
    overflow: hidden;
    height: auto;
    border: 1px solid black;
  }
  li {
    flex: 1;
    width: 64px;
    height: 64px;
    position: relative;
    margin: 8px 0;
    border: 1px solid red;
    img {
      max-width: 100%;
      max-height: 100%;
    }
  }
}

.list-leave-active,
.list-enter-active {
  transition: 0.5s;
}
.list-enter {
  transform: translate(0, 100px);
}
.list-leave-to {
  transform: translate(0, -100px);
}
.sliding-to-previous {
  .list-enter {
    transform: translate(0, -100px);
  }
  .list-leave-to {
    transform: translate(0, 100px);
  }
}

Javascript / VUE Code:

new Vue({
    el: "#app",
    data() {
        return {
            totalCarouselData: [{
                    itemImage: "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
                    itemImageAlt: "Test1"
                },
                {
                    itemImage: "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
                    itemImageAlt: "Test2"
                },
                {
                    itemImage: "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
                    itemImageAlt: "Test3"
                },
                {
                    itemImage: "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
                    itemImageAlt: "Test4"
                },
                {
                    itemImage: "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
                    itemImageAlt: "Test5"
                },
                {
                    itemImage: "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
                    itemImageAlt: "Test6"
                },
                {
                    itemImage: "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
                    itemImageAlt: "Test7"
                }
            ],
            currentCarouselData: [],
            isSlidingToPrevious: false,
            totalCount: 0,
            currentTopIndex: 0,
            currentBottomIndex: 0,
            itemsToDisplay: 5
        };
    },
    computed: {},
    mounted() {
        //At first show only 5 items
        this.currentCarouselData = this.totalCarouselData.slice(
            this.currentTopIndex,
            this.itemsToDisplay
        );
        //Get Total Count
        this.totalCount = this.totalCarouselData.length;
        //Update current bottom index
        this.currentBottomIndex = this.itemsToDisplay;
    },
    methods: {
        moveTop() {
            this.isSlidingToPrevious = true;
            this.currentTopIndex += 1;
            this.currentBottomIndex -= 1;
            this.addToTopComputedArr(this.currentBottomIndex);
        },
        moveBottom() {
            this.isSlidingToPrevious = false;
            this.currentTopIndex -= 1;
            this.currentBottomIndex += 1;
            this.addToBottomComputedArr(this.currentBottomIndex);
        },
        addToBottomComputedArr(index) {
            //Splice the first item
            this.currentCarouselData.splice(0, 1);
            //Add the next item to the array
            this.currentCarouselData.push(this.totalCarouselData[index - 1]);
        },
        addToTopComputedArr(index) {
            //Splice the last item
            this.currentCarouselData.splice(index - 1, 1);
            //Add item to the beginning of the array
            this.currentCarouselData.unshift(
                this.totalCarouselData[index - this.itemsToDisplay]
            );
        }
    }
});

Answer 1

No transition issue you met is caused by :key="index".

Check Vue Guide: key of v-for,

When Vue is updating a list of elements rendered with v-for, by default it uses an “in-place patch” strategy.

This default mode is efficient, but only suitable when your list render output does not rely on child component state or temporary DOM state (e.g. form input values).

To give Vue a hint so that it can track each node’s identity, and thus reuse and reorder existing elements, you need to provide a unique key attribute for each item.

In your codes, your five images always have the same key=[1, 2, 3, 4, 5], so Vue will in-place patch them, it causes the transition is not triggered.

So simply modify the :key="index" to :key="item.itemImageAlt", then it works.

Finally, adjust the css by yourself to make the transition effetcs meet your requirements.

Below is one working demo:

Vue.config.productionTip = false
new Vue({
  el: "#app",
  data() {
    return {
      totalCarouselData: [
        {
          itemImage:
            "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
          itemImageAlt: "Test1"
        },
        {
          itemImage:
            "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
          itemImageAlt: "Test2"
        },
        {
          itemImage:
            "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
          itemImageAlt: "Test3"
        },
        {
          itemImage:
            "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
          itemImageAlt: "Test4"
        },
        {
          itemImage:
            "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
          itemImageAlt: "Test5"
        },
        {
          itemImage:
            "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
          itemImageAlt: "Test6"
        },
        {
          itemImage:
            "https://www.publicdomainpictures.net/pictures/150000/velka/banner-header-tapete-145002399028x.jpg",
          itemImageAlt: "Test7"
        }
      ],
      currentCarouselData: [],
      isSlidingToPrevious: false,
      totalCount: 0,
      currentTopIndex: 0,
      currentBottomIndex: 0,
      itemsToDisplay: 5
    };
  },
  computed: {
    computedCarouseData: function () { // added computed property
        return this.totalCarouselData.slice(
        -this.currentTopIndex,
        this.itemsToDisplay-this.currentTopIndex
      )
    }
  },
  mounted() {
    //At first show only 5 items
    this.currentCarouselData = this.totalCarouselData.slice(
      this.currentTopIndex,
      this.itemsToDisplay
    );
    //Get Total Count
    this.totalCount = this.totalCarouselData.length;
    //Update current bottom index
    this.currentBottomIndex = this.itemsToDisplay;
  },
  methods: {
    moveTop() {
      this.isSlidingToPrevious = true;
      this.currentTopIndex += 1;
      this.currentBottomIndex -= 1;
      this.addToTopComputedArr(this.currentBottomIndex);
    },
    moveBottom() {
      this.isSlidingToPrevious = false;
      this.currentTopIndex -= 1;
      this.currentBottomIndex += 1;
      this.addToBottomComputedArr(this.currentBottomIndex);
    },
    addToBottomComputedArr(index) {
      //Splice the first item
      this.currentCarouselData.splice(0, 1);
      //Add the next item to the array
      this.currentCarouselData.push(this.totalCarouselData[index - 1]);
    },
    addToTopComputedArr(index) {
      //Splice the last item
      this.currentCarouselData.splice(index - 1, 1);
      //Add item to the beginning of the array
      this.currentCarouselData.unshift(
        this.totalCarouselData[index - this.itemsToDisplay]
      );
    }
  }
});

.row-eq-height {
  display: flex;
}
.row-eq-height ul {
  list-style-type: none;
  display: flex;
  flex-direction: column;
  overflow: hidden;
  height: auto;
  border: 1px solid black;
}
.row-eq-height li {
  flex: 1;
  width: 64px;
  height: 64px;
  position: relative;
  margin: 8px 0;
  border: 1px solid red;
}
.row-eq-height li img {
  max-width: 100%;
  max-height: 100%;
}
.list-leave-active,
.list-enter-active {
  transition: all 2s ease;
}
.list-enter {
  opacity: 0;
  transform: translateX(-300px);
}
.list-leave-to {
  opacity: 0;
  transform: translateX(-300px);
}
.sliding-to-previous .list-enter {
  transform: translateY(-100px);
}
.sliding-to-previous .list-leave-to {
  transform: translateY(100px);
}
.list-move {
  transition: transform 1s;
}

<script src="https://unpkg.com/vue@2.5.16/dist/vue.js"></script>
<div id="app" class="container-fluid">
  <div class="row row-eq-height">
    <div class="thumbnail-container">
      <button class="up" @click="moveTop" :disabled="currentTopIndex === 0">Top</button>
      <button @click="moveBottom" class="down" :disabled="currentBottomIndex === totalCount">Down</button>
      <div :class="'slider' + (isSlidingToPrevious ? ' sliding-to-previous' : '')">
        <transition-group name='list' tag="ul">
          <li v-for="(item,index) in computedCarouseData" v-bind:key="item.itemImageAlt" class="list-item"><img :src="item.itemImage" :alt="item.itemImageAlt" style=""/>{{item.itemImageAlt}}</li>
        </transition-group>
      </div>
    </div>
  </div>
  <pre>
    totalCount {{totalCount}}
    currentTopIndex {{currentTopIndex}}
    currentBottomIndex {{currentBottomIndex}}
    itemsToDisplay {{itemsToDisplay}}
    currentCarouselData {{computedCarouseData}}
</pre>
</div>

Answer 2

Vue ( all Reactive frameworks ) use a Virtual DOM, as you change the one of your data elements or anything visually the page gets rerendered.

Coming to your question, it is not Smooth because once your value is changed it gets re-rendered.

Absolute / relative position your thumbnail, once you change currentCarouselData set top as 100px. Use setInterval to decrement top till you get 0 this will bring the sliding effect

---

EDIT transition using transition-group

new Vue({
  el: '#flip-list-demo',
  data: {
    items: [1,2,3,4,5]
  },
  methods: {
    shuffle: function () {
      var x = this.items.splice(0,1)[0]
      console.log(x)
      this.items.push(x)
    }
  }
})

.a-move {
  transition: transform 1s;
}

.a-enter {
  transform: translateY(10px) 1s;
}

.a-leave {
  transform: translateY(10px) 1s;
  opacity : 0
}

<script src="https://cdn.jsdelivr.net/npm/vue@2.5.16/dist/vue.js"></script>

<div id="flip-list-demo" class="demo">
  <button v-on:click="shuffle">Shuffle</button>
  <transition-group name="a" tag="ul">
    <li v-for="(item, index) in items" v-bind:key="item" v-show="index<5">
      {{ item }}
    </li>
  </transition-group>
</div>