<script>

   import Sortable from 'sortablejs'

   export default {
      data() {
        const generateData = _ => {
          const data = []; for (let i = 1; i <= 15; i++) {
            data.push({
              key: i,
              label: `备选项 ${i}`,
              disabled: i % 4 === 0 });
          } return data;
        }; return {
          data: generateData(),
          value: [1, 4],
          draggingKey : null }
      },
      mounted() {
        const transfer = this.$refs.transfer.$el
        const leftPanel = transfer.getElementsByClassName("el-transfer-panel")[0].getElementsByClassName("el-transfer-panel__body")[0];
        const rightPanel = transfer.getElementsByClassName("el-transfer-panel")[1].getElementsByClassName("el-transfer-panel__body")[0];
        const rightEl = rightPanel.getElementsByClassName("el-transfer-panel__list")[0]
        Sortable.create(rightEl,{
          onEnd: (evt) => {
            const {oldIndex,newIndex} = evt;
            const temp = this.value[oldIndex]
            if (!temp || temp === 'undefined') {
              return
            }// 解决右边最后一项从右边拖左边，有undefined的问题
            this.$set(this.value,oldIndex,this.value[newIndex]) 
            this.$set(this.value,newIndex,temp)
          }
        })
        const leftEl = leftPanel.getElementsByClassName("el-transfer-panel__list")[0]
        Sortable.create(leftEl,{
          onEnd: (evt) => {
            const {oldIndex,newIndex} = evt;
            const temp = this.data[oldIndex]
            if (!temp || temp === 'undefined') {
              return
            } // 解决右边最后一项从右边拖左边，有undefined的问题
            this.$set(this.data,oldIndex,this.data[newIndex])
            this.$set(this.data,newIndex,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>