Add a spinner when Mat-table is loading?

table.component.html

<table mat-table [dataSource]="dataSource" class="mat-elevation-z8">

  <!-- table here ...-->

  <tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
  <tr mat-row *matRowDef="let row; columns: displayedColumns;"></tr>
</table>

<div *ngIf="isLoading" 
   style="display: flex; justify-content: center; align-items: center; background: white;">
  <mat-progress-spinner 
    color="primary" 
    mode="indeterminate">
  </mat-progress-spinner>
</div>

table.component.ts

isLoading = true;
dataSource = null;

ngOnInit() {
    this.annuairesService.getMedecins()
       subscribe(
        data => {
          this.isLoading = false;
          this.dataSource = data
        }, 
        error => this.isLoading = false
    );
}

Live demo

Set showSpinner to true when you start requesting your data, and set it to false when you receive it (aka, in the subscribe of your service method)

ngOnInit() {
  this.showSpinner = true;
  this.annuairesService.getMedecins()
    .subscribe(res => {
      this.showSpinner = false;
      this.dataSource.data = res;
    });
}