从另一个组件调用组件函数 — 出现"Cannot read property"错误

Question

我正在尝试通过将一个组件的引用传递给另一个组件但收到"无法读取属性"错误来调用组件中的另一个组件函数。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

元件

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)

Answer 1

你应该使用 @ViewChild 而不是 @Input 。像这样：

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后的某个时间被调用。