Angular drag and drop not working when dropping elements in n-ary tree represented using recursive template

问题

I'm new to angular material drag and drop.

My app is using an n-ary tree, and since I don't know its form apriori, I've been forced to use a recursive template in order to represent it.

The last couple of days I've been trying to use Angular material drag and drop to reorder siblings. If I remove the root node everything works just fine, but I cannot remove the root node since I need it to traverse the tree for other operations.

Bellow is a simplified form of the tree. In reality, the tree can have many more branches and leaves, but I wanted to keep it simple.

Json of the n-ary tree (root node with the 3 children nodes I want to be able to drag and drop):

[
  {
    "orderIndex": 0,
    "id": "5a4f87ce-c52d-4cc1-b587-21898ded5cc0",
    "parentId": "5a4f87ce-c52d-4cc1-b587-21898ded5cc0",
    "name": "Interface1",
    "conditions": "None",
    "text": null,
    "description": "description",
    "children": [
      {
        "orderIndex": 0,
        "id": "5a4f87ce-c527-4cc1-b587-22898ded5cc0",
        "parentId": "5a4f87ce-c52d-4cc1-b587-21898ded5cc0",
        "name": "Interface2",
        "conditions": "None",
        "text": null,
        "description": "description",
        "children": [],
        "errors": null
      },
      {
        "orderIndex": 1,
        "id": "5a4f87ce-c527-4cc8-b587-26898ded5cc0",
        "parentId": "5a4f87ce-c527-4cc8-b587-23898ded5cc0",
        "name": "Interface6",
        "conditions": "None",
        "text": null,
        "description": "description",
        "children": [],
        "errors": null
      },
      {
        "orderIndex": 2,
        "id": "5a4f87ce-c525-4cc1-b587-25898ded5cc0",
        "parentId": "5a4f87ce-c52e-4cc1-b587-24898ded5cc0",
        "name": "Interface4",
        "conditions": "None",
        "text": null,
        "description": "description",
        "children": [],
        "errors": null
      }
    ],
    "errors": null
  }
]

Component:

import { DataService } from './../data.service';
import { Node } from '../_Models/Node';
import {CdkDragDrop, moveItemInArray} from '@angular/cdk/drag-drop';

@Component({
  selector: 'app-vnf-main',
  templateUrl: './vnf-main.component.html',
  styleUrls: ['./vnf-main.component.css']
})
export class VnfMainComponent implements OnInit {
  tree: Node[];

  constructor(private dataService: DataService) {
  }

  ngOnInit() {
    this.dataService.tree$.subscribe(result => {
      this.tree = result;
    });

  drop(event: CdkDragDrop<Node[]>) {
    moveItemInArray(this.tree, event.previousIndex, event.currentIndex);
  }
}

Template code:

<table>
     <ng-template #recursiveList let-tree>
         <div cdkDropList (cdkDropListDropped)="drop($event)">
             <div *ngFor="let node of tree; let i=index " cdkDrag>
                 <tr>
                    <app-default-node [node]="node"></app-default-node>
                    <td *ngIf="node.children!==null && node.children.length > 0" cdkDrag>
                        <ng-container *ngTemplateOutlet="recursiveList; context:{ $implicit: node.children }"></ng-container>
                    </td>
                 </tr>
             </div>
         </div>
     </ng-template>
</table>
<ng-container *ngTemplateOutlet="recursiveList; context:{ $implicit: tree }"></ng-container>

Also: How can I know the Node being dragged and the node it was dropped after? I haven't been able to find an answer anywhere but if I can get these two I might be able to solve it by traversing the tree and doing everything by hand.

Thanks in advance and for getting this far and reading the whole question.

回答1:

You may use the HTML5 drag and drop. Its working fine with angular2+. Reference Link https://www.w3schools.com/html/html5_draganddrop.asp

回答2:

In case some is going through the same problem... I solved this issue passing the node dragged to the drop method by using [cdkDragData]=node since cdk drag and drop is designed for lists, not n-ary trees.

<div [cdkDragData]=node *ngFor="let node of tree; let i=index " cdkDrag>

Then, inside the method one can traverse the tree and do the changes "by hand":

const movedNode: Node = event.item.data; //Node that was dragged
const parentNode: Node = this.dataService.findNode(movedNode.parentId, this.tree[0]); //parent of the dragged node
moveItemInArray(parentNode.children, event.previousIndex, event.currentIndex); //move the node as in any other cdk drag and drop list

Thanks for the answers I received, it really encouraged me to find the best solution 😊

来源：https://stackoverflow.com/questions/59711702/angular-drag-and-drop-not-working-when-dropping-elements-in-n-ary-tree-represent