javascript - 立即创建 Observable 并排队发出直到订阅

Question

我有一个消息 Observable，用于在页面上显示通知。我遇到的问题是，在消息组件初始化之前页面加载时发生错误，这意味着 Observable 尚未订阅，因此当我尝试发出错误时未定义。

export class MessageService {
    public messagesSource: Observable<IMessage>;
    private messagesObserver: any;

    constructor() {
        console.log('MessageService constructor called');
        this.messagesSource = new Observable(observer => {
            console.log('MessageService observer called');
            this.messagesObserver = observer;
        }).share();
    }

    public show = (message: IMessage) => {
        if (this.messagesObserver) {
            this.messagesObserver.next(message);
        } else {
            console.error('this.messagesObserver is not initialised', this.messagesObserver);
        }
    }
}


export class MessagingComponent {
    public messages: IMessage[] = [];

    constructor(private messageService: MessageService) {
        console.log('messaging component constructor');
        this.messageService.messagesSource.subscribe(message => this.setMessage(message));
    }
}

// and within my HttpService:
private errorHandler = (error: Response): Observable<any> => {
    const err: string = error.text() || JSON.stringify(error);
    console.error('App error occurred:', err);
    this.messageService.show({ content: err, level: 'danger'});
    return Observable.from([null]);
}

正常操作日志:

Navigated to http://localhost:8090/

MessageService constructor called

messaging component constructor

MessageService observer called

Angular is running in the development mode.

当解析器出现错误时:

Navigated to http://localhost:8090/jobs

MessageService constructor called

GET http://localhost:7777/api/jobs 401 (Unauthorized)

App error occurred: Authorization failure, invalid bearer token.

this.messagesObserver is not initialised undefined

messaging component constructor

MessageService observer called

Angular is running in the development mode.

所以我需要一种方法让 new Observable(observer) 立即创建观察者，这样它就不是未定义的，并且当组件准备订阅时消息不会丢失和发出.

Answer 1

您需要创建一个BehaviorSubject ,

Observers can subscribe to the subject to receive the last (or initial) value and all subsequent notifications.

您还可以使用ReplaySubject ，如果您想缓冲所有响应。

希望这有帮助!!