Angular Typescript订阅值为空，但数据仍然提供给模板

我对Angular和Typescript很新，并且遇到了一个看似奇怪的问题 . 我正在尝试构建一个mat-table并使用我的类的值来创建一个MatTableDataSource对象 . 我将get请求中的值从后端获取，但是当我尝试在组件中使用恢复的值时，值为空，但是当加载模板时，它们突然出现了 .

这是我的组件：

import { Component, OnInit, ViewChild } from '@angular/core';
import { first } from 'rxjs/operators';
import { MatTableDataSource } from '@angular/material/table';
import { MatPaginator, MatSort } from '@angular/material';

import { Invoice } from '../_models';
import { InvoiceService } from '../_services';

@Component({
  selector: 'app-invoices',
  templateUrl: './invoices.component.html',
  styleUrls: ['./invoices.component.css']
})
export class InvoicesComponent implements OnInit {
  displayedColumns=['rating', 'amount', 'debtor', 'serial', 'dateout', 'expiration', 'daysleft', 'fid']
  invoices: Invoice[] = [];
  dataSource= new MatTableDataSource<Invoice>(this.invoices);

  @ViewChild(MatPaginator) paginator: MatPaginator;
  @ViewChild(MatSort) sort: MatSort;

  constructor(
    private invoiceService: InvoiceService
  ) { }

  ngOnInit() {
    this.loadInvoices();
    console.log(this.invoices);
    this.dataSource.paginator = this.paginator;
    this.dataSource.sort = this.sort;
  }
  private loadInvoices(){
    this.invoiceService.getUserInvoices().pipe(first()).subscribe(invoices => {
      this.invoices=invoices;
    });
    console.log(this.invoices);


  }
  applyFilter(filterValue: string) {
    filterValue = filterValue.trim(); // Remove whitespace
    filterValue = filterValue.toLowerCase(); // Datasource defaults to lowercase matches
    this.dataSource.filter = filterValue;
    if (this.dataSource.paginator) {
      this.dataSource.paginator.firstPage();
    }
  }

}

这是模板：

<h3> All Uploaded invoices:</h3>
<mat-form-field>
    <input matInput (keyup)="applyFilter($event.target.value)" placeholder="Filter">
</mat-form-field>
<div class="mat-elevation-z8">
  <mat-table #table [dataSource]="invoices" matSort class="mat-elevation-z8">

    <ng-container matColumnDef="rating">
      <mat-header-cell *matHeaderCellDef mat-sort-header> Rating </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.rating}} </mat-cell>
    </ng-container>

    <ng-container matColumnDef="amount">
      <mat-header-cell *matHeaderCellDef mat-sort-header> Amount </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.amount}} </mat-cell>
    </ng-container>

    <ng-container matColumnDef="debtor">
      <mat-header-cell *matHeaderCellDef> Debtor </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.debtor}} </mat-cell>
    </ng-container>

    <ng-container matColumnDef="serial">
      <mat-header-cell *matHeaderCellDef> Serial </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.serial}} </mat-cell>
    </ng-container>

    <ng-container matColumnDef="dateout">
      <mat-header-cell *matHeaderCellDef> Dateout </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.dateout}} </mat-cell>
    </ng-container>

    <ng-container matColumnDef="expiration">
      <mat-header-cell *matHeaderCellDef> Expiration </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.expiration}} </mat-cell>
    </ng-container>

    <ng-container matColumnDef="daysleft">
      <mat-header-cell *matHeaderCellDef mat-sort-header > Days left </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.daysleft}} </mat-cell>
    </ng-container>

    <ng-container matColumnDef="fid">
      <mat-header-cell *matHeaderCellDef> Fid </mat-header-cell>
      <mat-cell *matCellDef="let element"> {{element.fid}} </mat-cell>
    </ng-container>


    <mat-header-row *matHeaderRowDef="displayedColumns"></mat-header-row>
    <mat-row *matRowDef="let row; columns: displayedColumns;"></mat-row>
  </mat-table>
  <mat-paginator [pageSizeOptions]="[3, 100, 200]" showFirstLastButtons></mat-paginator> 
</div>
<p><a [routerLink]="['/login']">Logout</a></p>

重要的部分是 [dataSource]="invoices" ，因为它会填充从loadInvoices方法启动的get请求中收集的数据 . 如果我将其更改为 [dataSource]="dataSource" ，它将以空数组形式出现 . 同样，我的console.logs都输出一个空数组 . 有没有办法使用我的请求中的数据创建一个MatTableDataSource对象并将其传递给模板，如果是这样，我该怎么做？

@DiabolicWords解决后的新组件：

import { Component, OnInit, ViewChild } from '@angular/core';
import { first } from 'rxjs/operators';
import { MatTableDataSource } from '@angular/material/table';
import { MatPaginator, MatSort } from '@angular/material';

import { Invoice } from '../_models';
import { InvoiceService } from '../_services';

@Component({
  selector: 'app-invoices',
  templateUrl: './invoices.component.html',
  styleUrls: ['./invoices.component.css']
})
export class InvoicesComponent implements OnInit {
  displayedColumns=['rating', 'amount', 'debtor', 'serial', 'dateout', 'expiration', 'daysleft', 'fid']
  dataSource: MatTableDataSource<Invoice>;

  @ViewChild(MatPaginator) paginator: MatPaginator;
  @ViewChild(MatSort) sort: MatSort;

  constructor(
    private invoiceService: InvoiceService
  ) { }

  ngOnInit() {
    this.loadInvoices();
  }
  private loadInvoices(){
    this.invoiceService.getUserInvoices().pipe(first()).subscribe(invoices => {
      this.dataSource = new MatTableDataSource<Invoice>(invoices);
      this.dataSource.paginator = this.paginator;
      this.dataSource.sort = this.sort;
    });


  }
  applyFilter(filterValue: string) {
    filterValue = filterValue.trim(); // Remove whitespace
    filterValue = filterValue.toLowerCase(); // Datasource defaults to lowercase matches
    this.dataSource.filter = filterValue;
    if (this.dataSource.paginator) {
      this.dataSource.paginator.firstPage();
    }
  }

}