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

Question

我试图通过将一个组件的引用传递给另一个组件来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)

Answer 1

您应该使用 @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(); 
    }
}

---

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