芯資管專案要求el-transfer增加拖拽排序，左右上下互相拖拽功能；
原來的元件不支援拖拽，這裡要用個第三方脫宅元件sortablejs

首先安裝

 sudo npm i sortablejs --save-dev

html程式碼

  <template>
  <el-transfer ref="transfer" id="transfer" v-model="value" :data="data">
   <span slot-scope="{ option }" :draggable="!option.disabled" @dragstart="drag($event,option)">{{ option.key }} - {{ option.label }}</span>
  </el-transfer>
</template>```

create

  <script>

   import Sortable from 'sortablejs'

   export default {
      data() {
        const generateData = _ => {
          const data = []; for (let i = 1; i <= 15; i++) {
            data.push({
              key: i,label: `備選項 ${i}`,disabled: i % 4 === 0 });
          } return data;
        }; return {
          data: generateData(),value: [1,4],draggingKey : null }
      },mounted() {
        const transfer = this.$refs.transfer.$el
        const leftPanel = transfer.getElementsByClassName("el-transfer-panel")[0].getElementsByClassName("el-transfer-panel__body")[0];
        const rightPanel = transfer.getElementsByClassName("el-transfer-panel")[1].getElementsByClassName("el-transfer-panel__body")[0];
        const rightEl = rightPanel.getElementsByClassName("el-transfer-panel__list")[0]
        Sortable.create(rightEl,{
          onEnd: (evt) => {
            const {oldIndex,newIndex} = evt;
            const temp = this.value[oldIndex] 
            if (!temp || temp === 'undefined') {
              return
            }// 解決右邊最後一項從右邊拖左邊，有undefined的問題
            this.$set(this.value,oldIndex,this.value[newIndex])  
            this.$set(this.value,newIndex,temp)
          }
        })
        const leftEl = leftPanel.getElementsByClassName("el-transfer-panel__list")[0]
        Sortable.create(leftEl,newIndex} = evt;
            const temp = this.data[oldIndex] 
            if (!temp || temp === 'undefined') {
              return
            } // 解決右邊最後一項從右邊拖左邊，有undefined的問題
            this.$set(this.data,this.data[newIndex]) 
            this.$set(this.data,temp)
          }
        })
        leftPanel.ondragover = (ev) => {
          ev.preventDefault()
        }
        leftPanel.ondrop = (ev) => {
          ev.preventDefault();
          const index = this.value.indexOf(this.draggingKey) if(index !== -1){ this.value.splice(index,1)
          }
        }
        rightPanel.ondragover = (ev) => {
          ev.preventDefault()
        }
        rightPanel.ondrop = (ev) => {
          ev.preventDefault();  if(this.value.indexOf(this.draggingKey) === -1){ this.value.push(this.draggingKey)
          }
        }
      },methods: {
        drag(ev,option) { this.draggingKey = option.key
        }
      }

    } 
</script>

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