Angular 7中，可以使用单例服务在两个组件之间进行通信。下面是一个示例解决方法：

1. 创建一个单例服务，例如CommunicationService，并在app.module.ts文件中将其提供为全局的单例服务。

import { Injectable } from '@angular/core';
import { Subject } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class CommunicationService {
  private messageSource = new Subject();
  message$ = this.messageSource.asObservable();

  sendMessage(message: string) {
    this.messageSource.next(message);
  }
}

2. 在发送消息的组件中，将CommunicationService注入并调用sendMessage方法发送消息。

import { Component } from '@angular/core';
import { CommunicationService } from './communication.service';

@Component({
  selector: 'app-sender',
  template: `
    发送消息
  `
})
export class SenderComponent {
  constructor(private communicationService: CommunicationService) {}

  sendMessage() {
    this.communicationService.sendMessage('Hello from SenderComponent!');
  }
}

3. 在接收消息的组件中，将CommunicationService注入并订阅message$监听消息。

import { Component, OnDestroy } from '@angular/core';
import { CommunicationService } from './communication.service';
import { Subscription } from 'rxjs';

@Component({
  selector: 'app-receiver',
  template: `
    
{{ message }}
  `
})
export class ReceiverComponent implements OnDestroy {
  message: string;
  subscription: Subscription;

  constructor(private communicationService: CommunicationService) {
    this.subscription = this.communicationService.message$.subscribe(message => {
      this.message = message;
    });
  }

  ngOnDestroy() {
    this.subscription.unsubscribe();
  }
}

4. 在父组件中，将SenderComponent和ReceiverComponent添加到模板中。

这样，当点击“发送消息”按钮时，SenderComponent会调用CommunicationService的sendMessage方法发送消息，然后ReceiverComponent会监听到该消息并将其展示在页面上。