Use Vue+Sortable

This time I will bring you the use of Vue Sortable and what are the precautions for using Vue Sortable. The following is a practical case, let's take a look.

I previously developed a backend management system, which used the component library Vue and Element-UI. I encountered a very interesting problem and would like to share it with you.

The scene is like this. On a list display page, I used the table component of Element-UI. The new requirement is to support drag-and-drop sorting based on the original table. However, the original component itself does not support drag-and-drop sorting, and since it is directly introduced from Element-UI, it is inconvenient to modify its source code, so the only feasible method is to directly operate the DOM.

The specific method is to perform real DOM operations on this.$el in the mounted life cycle function, listen to a series of drag events, move the DOM in the event callback, and update the data.

There are quite a lot of HTML5 Drag events, which are similar to Touch events. You can also implement them manually. However, I am lazy here and directly use an open source Sortable library and directly pass in this.$el. Listen to the encapsulated callback, and according to Vue's development model, update the actual Data data in the callback of the mobile DOM to maintain consistency between the data and the DOM.

If you think it’s over here, you are totally wrong. Sooner or later, you will have to pay back the laziness you have stolen. . . I thought this solution was very good, but as soon as I wanted to debug it, a strange phenomenon occurred: after A and B were dragged and swapped, B and A were magically swapped back again! How is this going? It seems that there is no problem with our operation. After the real DOM is moved, we also move the corresponding data. The order of the data array and the order of the rendered DOM should be consistent.

where is the problem? Let's recall the implementation principle of Vue. Before Vue2.0, two-way binding was achieved through defineProperty dependency injection and tracking. For the v-for array instruction, if a unique Key is specified, the difference of elements in the array will be calculated through an efficient Diff algorithm, and minimal movement or deletion operations will be performed. After the introduction of Virtual Dom after Vue2.0, the Dom Diff algorithm of the Children element is actually similar to the former. The only difference is that before 2.0, Diff directly targeted the array object of the v-for instruction, while after 2.0, it targeted Virtual Dom. The DOM Diff algorithm will not be described in detail here. The virtual-dom diff algorithm is explained more clearly here

Assume that our list element array is

['A','B', 'C','D']

The rendered DOM node is

[$A,$B,$C,$D]

Then the corresponding structure of Virtual Dom is

[{elm:$A,data:'A'},
{elm:$B,data:'B'} ,
{elm:$C,data:'C'},
{elm:$D,data:'D'}]

Assuming that after drag and drop sorting, the real The DOM becomes

[$B,$A,$C,$D]

At this time we only operate the real DOM and modify its position, and The structure of Virtual Dom has not changed, it is still

[{elm:$A,data:'A'},
{elm:$B,data:'B'},
{elm:$C,data:'C'},
{elm:$D,data:'D'}]

At this time, we also change the list elements according to the real DOM After sorting, it becomes

['B','A','C','D']

At this time, according to the Diff algorithm, the calculated Patch is , the first two items of VNode are nodes of the same type, so they are updated directly, that is, the $A node is updated to $B, the $B node is updated to $A, and the real DOM changes back to

[ $A,$B,$C,$D]

So there is a problem that it is updated by the Patch algorithm after dragging. The operation path can be simply understood as

Drag and move the real DOM -> Manipulate the data array -> Patch algorithm and then update the real DOM

The root cause

The root cause is between the Virtual DOM and the real DOM There is an inconsistency.

So before Vue2.0, because Virtual DOM was not introduced, this problem did not exist.

When using the Vue framework, try to avoid directly manipulating the DOM

Solution

1. Uniquely mark each VNode by setting the key. This is also the way Vue recommends using the v-for instruction. Because the sameVnode method will be called when judging whether two VNodes are of the same type, the priority is to judge whether the key is the same

function sameVnode (a, b) {
 return (
  a.key === b.key &&
  a.tag === b.tag &&
  a.isComment === b.isComment &&
  isDef(a.data) === isDef(b.data) &&
  sameInputType(a, b)
 )
}

2. Because the fundamental reason is that the real DOM and VNode are inconsistent, you can move the real DOM by dragging Operation restoration, that is, in the callback function, restore [$B,$A,$C,$D] to [$A,$B,$C,$D], and return the DOM operation to Vue

Drag and move the real DOM ->Restore the move operation-> Manipulate the data array-> Patch algorithm and then update the real DOM

The code is as follows

var app = new Vue({
    el: '#app', 
    mounted:function(){
      var $ul = this.$el.querySelector('#ul')
      var that = this
      new Sortable($ul, {
        onUpdate:function(event){
          var newIndex = event.newIndex,
            oldIndex = event.oldIndex
            $li = $ul.children[newIndex],
            $oldLi = $ul.children[oldIndex]
          // 先删除移动的节点
          $ul.removeChild($li)  
          // 再插入移动的节点到原有节点，还原了移动的操作
          if(newIndex > oldIndex) {
            $ul.insertBefore($li,$oldLi)
          } else {
            $ul.insertBefore($li,$oldLi.nextSibling)
          }
          // 更新items数组
          var item = that.items.splice(oldIndex,1)
          that.items.splice(newIndex,0,item[0])
          // 下一个tick就会走patch更新
        }
      })
    },
    data:function() {
      return {
        message: 'Hello Vue!',
        items:[{
          key:'1',
          name:'1'
        },{
          key:'2',
          name:'2'
        },{
          key:'3',
          name:'3'
        },{
          key:'4',
          name:'4'
        }]
      }
    },
    watch:{
      items:function(){
        console.log(this.items.map(item => item.name))
      }
    }
  })

3. Violent solution! Without patch update, directly re-render through v-if settings. Of course it is not recommended to do this, I just provide this idea~

    mounted:function(){
      var $ul = this.$el.querySelector('#ul')
      var that = this
      var updateFunc = function(event){
        var newIndex = event.newIndex,
          oldIndex = event.oldIndex
        var item = that.items.splice(oldIndex,1)
        that.items.splice(newIndex,0,item[0])
        // 暴力重新渲染！
        that.reRender = false
        // 借助nextTick和v-if重新渲染
        that.$nextTick(function(){
          that.reRender = true
          that.$nextTick(function(){
            // 重新渲染之后，重新进行Sortable绑定
            new Sortable(that.$el.querySelector('#ul'), {
              onUpdate:updateFunc
            })
          })
        })
      }
      new Sortable($ul, {
        onUpdate:updateFunc
      })
    },

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the PHP Chinese website!

Recommended reading:

Vue Nuxt.js makes server-side rendering

Angular uses HMR code parsing

Use vue to make a mobile drop-down list

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