angular – 当变量值通过服务更改时如何重新加载标头组件？

我需要在标题中显示路由组件的标题.但是当我在我的应用程序中使用ngOnInit时,它获得了默认值.即使通过服务更改变量值,它也不会改变.怎么做？

Data.service.ts

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Injectable()

export class DataService {

  public myGlobalVar : string = "Chaitanya";

  constructor() { }

  setMyGV(val : string){
    this.myGlobalVar = val;
    console.log(this.myGlobalVar);
  }

  getMyGV(){
    return this.myGlobalVar;
  }
}

header.component.ts

import { Component,OnInit } from '@angular/core';
    import { DataService } from 'src/app/data.service';

    @Component({
      selector: 'app-header',templateUrl: './header.component.html',styleUrls: ['./header.component.scss']
    })
    export class HeaderComponent implements OnInit {

      public title : string = '';

      constructor(private _emp : DataService) { }

      ngOnInit() {
        this.title = this._emp.getMyGV();    
      }
}

contact.component.ts

import { Component,OnInit } from '@angular/core';
import { DataService } from 'src/app/data.service';

@Component({
  selector: 'app-contact',templateUrl: './contact.component.html',styleUrls: ['./contact.component.scss']
})
export class ContactComponent implements OnInit {

  constructor(private _emp : DataService) { }

  ngOnInit() {
      this._emp.setMyGV('Contact');
      }
}

之前：

之后：

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { BehaviorSubject } from 'rxjs';

@Injectable()

export class DataService {

  public myGlobalVar$= new BehaviorSubject<string>("Chaitanya");

  constructor() { }

  setMyGV(val : string){
    this.myGlobalVar.next(val);
  }

  getMyGV(){
    return this.myGlobalVar$;
  }
}

import { Component,OnInit } from '@angular/core';
import { DataService } from 'src/app/data.service';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-header',styleUrls: ['./header.component.scss']
})
export class HeaderComponent implements OnInit {

  public title : Observable<string>;

  constructor(private _emp : DataService) { }

  ngOnInit() {
    this.title = this._emp.getMyGV();    
  }