一、@ViewChild

父组件中使用@ViewChild拿到子组件的变量和方法（父组件可调用子组件的方法和变量）

parent.component.ts:

import { Component,OnInit,ViewChild } from '@angular/core';
import { ChildComponent } from './child.component';

@Component({
  selector: 'my-parent',templateUrl: './parent.component.html',styleUrls: [ './parent.component.css' ],})
export class ParentComponent implements OnInit {
  //通过@ViewChild注册子组件
  @ViewChild(ChildComponent) public child:ChildComponent;
  public countNum: number;
  public firstName:string = "Jeck";
  public fullName:string = "";

  constructor() {}

  ngOnInit(): void {

  }
  displayFull(){
    this.fullName = this.firstName + this.child.lastName;
　　 console.log(this.fullName)  //"Jeck wang"
  }
}

child.component.ts:

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

@Component({
  selector: 'my-child',templateUrl: './child.component.html',styleUrls: [ './child.component.css' ],})
export class ChildComponent implements OnInit {
  public lastName:string = "wang";

  constructor() {}

  ngOnInit(): void {

  }

}

二、@Inject

子组件中使用@Inject调用父组件中的变量和方法

parent.component.ts:

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

@Component({
  selector: 'my-parent',})
export class ParentComponent implements OnInit {

  constructor() {}

  ngOnInit(): void {

  }
  sayHello(){
    console.log("Hello!")
  }
}

child.component.ts:

import { Component,Inject,forwardRef} from '@angular/core';
import { ParentComponent } from './parent.component';

@Component({
  selector: 'my-child',})
export class ChildComponent implements OnInit {

  constructor(
    @Inject(forwardRef(()=>ParentComponent)) public parent:ParentComponent
  ) {}

  ngOnInit(): void {
    this.parent.sayHello();   //"Hello!"
  }
}

注意：如果父子模块通过以上方式相互引用，请在父模块中使用 @ViewChild(forwardRef(()=>ChildComponent)) public child:ChildComponent 方式避免父子组件循环引用报错