javascript - Angular - 在事件监听器函数内调用服务函数返回未定义的错误

Question

免责声明 - 如果标题不明确/问题本身很愚蠢，我们深表歉意。我对 Angular 和 JavaScript 还很陌生

我在服务中有一个函数，我试图在 JavaScript eventListener 函数中调用它，如下所示:

import { WebSocketService } from '../web-socket.service';

@Component({
 selector: 'app-board',
 templateUrl: './board.component.html',
 styleUrls: ['./board.component.css']
})

export class BoardComponent implements OnInit, OnDestroy {

 constructor(private webSocketService: WebSocketService) { }

 ngOnInit() {
   let cells = document.querySelectorAll('.cell');
   for (let i = 0; i < cells.length; i++) {
       cells[i].addEventListener('click', this.cellClicked);
   }
 }

 cellClicked(clickedCellEvent) {
   console.log(this.webSocketService.printSomething());
 }

}  

当我从前端触发事件(从而调用“cellClicked”函数)时，出现以下错误:

core.js:6014 ERROR TypeError: Cannot read property 'printSomething' of undefined

编辑 1:添加下面的服务定义:

import { Injectable } from '@angular/core';
import * as io from 'socket.io-client';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';
import { HttpClient } from '@angular/common/http';

@Injectable({
  providedIn: 'root'
})
export class WebSocketService {
      printSomething() {
         return "Called a function inside webSocketService!";
      }
}

Answer 1

您需要将cellClicked方法绑定(bind)到this当前上下文

import { WebSocketService } from '../web-socket.service';

@Component({
 selector: 'app-board',
 templateUrl: './board.component.html',
 styleUrls: ['./board.component.css']
})

export class BoardComponent implements OnInit, OnDestroy {

 constructor(private webSocketService: WebSocketService) { }

 ngOnInit() {
   let cells = document.querySelectorAll('.cell');
   for (let i = 0; i < cells.length; i++) {
       cells[i].addEventListener('click', this.cellClicked.bind(this);
   }
 }

 cellClicked(clickedCellEvent) {
   console.log(this.webSocketService.printSomething());
 }

}