@Viewchild在ngOnInit期间没有初始化

Question

I'm running two MatTables in different components with data sources from different observables. One of my tables sort functionality is working fine and but on my second table it seems as if the @ViewChild for MatSort doesn't initialize during ngOnInit.

Data renders and the material table has sort buttons but the functionality is nothing. Checked my imports and the module and everything is fine.
On logging the MatSort one component logs a MatSort object and the other is undefined

Sorting not working.

Feed.component:

   import { PostService } from './../../services/post.service';
   import { Post } from './../../models/post';
   import { Component, OnInit, ViewChild, ChangeDetectorRef} from 
     '@angular/core';
   import { MatSort, MatTableDataSource, MatCheckbox, MatPaginator, 
     MatTabChangeEvent, MatDialog, MatDialogActions, MatTable}  from 
   "@angular/material"



export class FeedComponent implements OnInit {
  @ViewChild(MatSort) sort: MatSort;
  @ViewChild(MatPaginator) paginator: MatPaginator;
  postData: Post[] =[];
  dataSource : MatTableDataSource<any> 
  currentUser = JSON.parse(localStorage.getItem('user'))
  displayedColumns:string[] = ['User','Title', "Description", 
  "Contact" ]
  posts = this.ps.getPosts();

  constructor(private ps: PostService, public dialog:MatDialog, 
    public change:ChangeDetectorRef, public ms:MessageService) { 

  }



refreshPosts(){
   console.log(this.sort) < -------comes back undefined
  this.posts.subscribe(posts=>{
    this.dataSource.sort = this.sort
     this.postData = posts.filter(post => post.uid != 
       `${this.currentUser.uid}` && post.claimedBy 
        !=`${this.currentUser.uid}`);
     this.dataSource= new MatTableDataSource(this.postData)
     this.dataSource.paginator = this.paginator;
    });

  }
ngOnInit() {
   this.refreshPosts()
   console.log(this.sort)
   }


Post.service
  getPosts(){
    return  this.afs.collection('posts').snapshotChanges()
     .pipe(map(actions => 
     actions.map(this.documentToDomainObject)))
  }
 documentToDomainObject = _ => {
  const object = _.payload.doc.data();
  object.id = _.payload.doc.id;
  return object;
}

Now my next component initializes in the same way but the @ViewChild shows up as a MatSort Object

Message.component:

export class MessageComponent implements OnInit {


 @ViewChild(MatSort) sort: MatSort;
  userReceived: MatTableDataSource<any>;
  userSent: MatTableDataSource<any>;
  displayedColumns:string[] = ["createdAt",'author',"title", "Delete"]
  sentColumns:string[] = ["createdAt","recipient", "title", "Delete"]


  currentUserId= this.currentUser['uid']
  currentUsername = this.currentUser['displayName']
  recipient:any;
  selectedMessage: MatTableDataSource<Message>;
  messageColumns= ['From','Title',"Body"];




  constructor(public ms:MessageService, public change:ChangeDetectorRef, public dialog: MatDialog  ) { }

  ngOnInit() {
    console.log(this.sort)
    this.updateMessages()
    this.currentUserId = this.currentUserId;
    this.currentUsername = this.currentUsername;

 }


updateMessages(){
    this.ms.getUserSent().subscribe(messages => {
      console.log(this.sort) <------logs MatSort object
      this.userSent = new MatTableDataSource(messages)
      this.userSent.sort = this.sort
      console.log(this.userSent.sort)
      console.log(this.userSent.data)

    })

message.service

 getUserSent() {
    let messages:any[] = [];
    this.userSent = this.afs
      .collection('messages', ref => ref.where('uid', '==', `${this.currentUser.uid}`)).snapshotChanges() 
return this.userSent
  }

feed.component.html

<div class = "mat-elevation-z8">
    <mat-form-field>
        <input matInput (keyup)="applyFilter($event.target.value)" placeholder="Search Posts">
      </mat-form-field>
  <table matSort mat-table [dataSource]="dataSource" style="text-align:left">
      <ng-container matColumnDef="User">
          <th mat-header-cell *matHeaderCellDef mat-sort-header>User</th>
          <td mat-cell *matCellDef="let post">{{post.displayName}}</td>
       </ng-container>

  <ng-container matColumnDef="Title">
    <th mat-header-cell *matHeaderCellDef>Title</th>
    <td mat-cell *matCellDef="let post">{{post.title | truncate:15:false }}</td>
 </ng-container>

  <ng-container matColumnDef="Description">
    <th mat-header-cell *matHeaderCellDef >Description</th>
    <td mat-cell *matCellDef="let post">{{post.description | truncate: 20 : false}}</td>
  </ng-container>





  <ng-container matColumnDef="Contact">
    <th mat-header-cell *matHeaderCellDef> Contact </th>
    <td mat-cell *matCellDef="let post">
      <button  id="{{post.id}}" color="primary" (click)="openDialog($event.target.id)" style = "outline:none" value={{post.id}}>Claim</button>
    </td>

  </ng-container>

  <tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
  <tr mat-row *matRowDef='let row; columns: displayedColumns'></tr>
</table>
</div>
  <mat-paginator [length]="this.postData.length" [pageSize]="5" [pageSizeOptions]="[5,10,25]"></mat-paginator>

I really cant find why in my first component the sort returns undefined when in my second, working component it returns and object. Am I missing something about the order of @ViewChild?

Answer 1

From official docs: https://angular.io/api/core/ViewChild#description

View queries are set before the ngAfterViewInit callback is called.

In order to get @ViewChild property inited, you need to call it in ngAfterViewInit lifecycle hook.

export class MessageComponent implements OnInit, AfterViewInit {

   @ViewChild(MatSort) sort: MatSort;

   ngAfterViewInit(){
      console.log(this.sort)
   }
}

---

If you are using Angular 8, you need to refactor the implementation of @ViewChild properties since static flag is required

Answer 2

the problem in FeedComponent is that you are making this.dataSource.sort = this.sort assignment before initializing this.dataSource

refreshPosts(){
  console.log(this.sort) < -------comes back undefined
  this.posts.subscribe(posts=>{
     this.postData = posts.filter(post => post.uid != `${this.currentUser.uid}` && post.claimedBy !=`${this.currentUser.uid}`);
     this.dataSource= new MatTableDataSource(this.postData)
     this.dataSource.sort = this.sort // make assignment after initializing this.dataSource
     this.dataSource.paginator = this.paginator;
    });
  }

please note that console.log(this.sort) will still print undefined because of lifecycle sequences. on ngOnInit view queries are not set.

so the question arises; then how would this.dataSource.sort = this.sort assignment work in ngOnInit in MessageComponent ?

the answer is long but to put it in simple terms; because that code gets executed in subscribe callback. since the code in subscribe callback gets executed when observable emits, an asynchronous operation happens. and that async operation happens in a subsequent change detection cycle after a cycle where ngAfterViewInit hook gets executed.

you are not getting undefined output in your second component because that console.log statement is also executed in a subscribe callback. moving that log statement out of subscribe callback will also print undefined.

if you place console.log statements in ngAfterViewInit hook, they both will print actual values whether they are placed in a subscribe callback or not.

as a summary;

make assignment after initializing this.datasource

this.dataSource= new MatTableDataSource(this.postData)
this.dataSource.sort = this.sort // make assignment after initializing 

and do it in ngAfterViewInit hook, even though it works in ngOnInit due to the async operation.