从另一个组件调用组件函数 — 出现“无法读取属性”错误答案

【问题标题】：Call component function from another component — getting "Cannot read property" error从另一个组件调用组件函数 — 出现“无法读取属性”错误
【发布时间】：2019-07-04 05:32:45
【问题描述】：

我试图通过将一个组件的引用传递给另一个组件来从一个组件调用另一个组件函数，但出现“无法读取属性”错误。这是代码

警报组件

import { Component, OnInit, Output } from '@angular/core';    

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

export class AlertComponent implements OnInit {  
  AlertMessage:string;

  constructor() {        
   }

  showSuccessAlert(){       
      this.AlertMessage =  "Data saved successfully.";
  }

  ngOnInit() {
  }

}

警报组件 html

<p>{{AlertMessage}}</p>

我正在从另一个组件调用showSuccessAlert 函数

组件

import { Component, OnInit, Input, HostListener } from '@angular/core';
import { QueueService } from 'src/services/queue.service';
import { ActivatedRoute } from "@angular/router";
import { NgForm } from '@angular/forms';
import { Queue } from 'src/models/queue';
import { error } from 'util';
import { AlertComponent } from '../alert/alert.component';

@Component({
  selector: 'queue-edit',
  templateUrl: './queue.edit.component.html'  
})
export class QueueEditComponent implements OnInit{
    @Input() alert:AlertComponent;

    queue = new Queue();    
    constructor(private api: QueueService, private route: ActivatedRoute){
        this.route.params.subscribe(params => this.params = params);

    } 
    title = "Edit Support Queue";

    ngOnInit() { 
    }

    @HostListener('saveQueue')
    saveQueue(queue){
        this.alert.showSuccessAlert(); 
    }
}

html

<app-alert></app-alert>

错误

ERROR TypeError: Cannot read property 'showSuccessAlert' of undefined
    at QueueEditComponent.push../src/app/queue/queue.edit.component.ts.QueueEditComponent.saveQueue (queue.edit.component.ts:44)
    at Object.eval [as handleEvent] (QueueEditComponent.html:25)
    at handleEvent (core.js:19545)
    at callWithDebugContext (core.js:20639)
    at Object.debugHandleEvent [as handleEvent] (core.js:20342)
    at dispatchEvent (core.js:16994)
    at core.js:17441
    at HTMLButtonElement.<anonymous> (platform-browser.js:993)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:423)
    at Object.onInvokeTask (core.js:14051)

【问题讨论】：

标签： javascript angular typescript components

【解决方案1】：

您应该使用@ViewChild 而不是@Input。像这样的：

import { ..., ViewChild } from '@angular/core';
...
import { AlertComponent } from '../alert/alert.component';

@Component({...})
export class QueueEditComponent implements OnInit{
    @ViewChild(AlertComponent) alert:AlertComponent;

    ...

    @HostListener('saveQueue')
    saveQueue(queue){
        this.alert.showSuccessAlert(); 
    }
}

PS：在调用ngAfterViewInit 之前，您将无法访问QueueEditComponent 中的AlertComponent。因此，请确保您的 saveQueue 在 ngAfterViewInit 之后的某个时间被调用。

【讨论】：