使用 Observable 检测变量的变化

Question

我想我误解了应该如何使用 Observables。我想输入一个值，当值发生变化时，它应该发出新值。我认为这就是它们的用途，但所有教程和文档似乎都没有这样做，但与此同时，我总是看到它们以这种方式应用。例如，当您订阅“FirebaseListObservable”时，当您订阅“FirebaseListObservable”时，当 firebase 中的值发生更改时，它会触发订阅中的快照。我想为我自己的变量做这个。假设我只有一个字符串变量，当它发生变化时，它会触发所有订阅。

Answer 1

通常我会将我的 observables 放在组件中订阅的服务中，但为了方便回答，我将它们全部捆绑在一个类中。我列出了 cmets 来解释每个步骤。我希望这有帮助。 :)

import { Subject } from 'rxjs/Subject';

export class ClassName {
    // ------ Creating the observable ----------
   // Create a subject - The thing that will be watched by the observable
   public stringVar = new Subject<string>();

   // Create an observable to watch the subject and send out a stream of updates (You will subscribe to this to get the update stream)
   public stringVar$ = this.stringVar.asObservable() //Has a $ 

   // ------ Getting Your updates ----------
   // Subscribe to the observable you created.. data will be updated each time there is a change to Subject
   public subscription = this.stringVar$.subscribe(data => {
         // do stuff with data
         // e.g. this.property = data
   });

  // ------ How to update the subject ---------
   // Create a method that allows you to update the subject being watched by observable
   public updateStringSubject(newStringVar: string) {
     this.stringVar.next(newStringVar);
   }
   // Update it by calling the method..
   // updateStringSubject('some new string value')

   // ------- Be responsible and unsubscribe before you destory your component to save memory ------
   ngOnDestroy() {
     this.subscription.unsubscribe()
   }
}

Answer 2

使用ReplySubject 试试这个。我在下面的代码示例中使用了 typescript、angularfire 来解释

export class MessageService {
  private filter$: ReplaySubject<any> = new ReplaySubject(1);

  getMessagesOfType():FirebaseListObservable<any>{
   return this.af.database.list(this.messagesPath, {
    query: {
      orderByChild: 'type',
      equalTo: this.filter$
    }
  });
  }
  getClosedMessage(): void {
    this.filter$.next('closed');
  }
  getOpenMessage(): void {
    this.filter$.next('open');
  }
}

// in some other class
// MessagesSubject is assigned to messageService 
this.messageService.getMessagesOfType().subscribe((listOfMessages)=>{
 // this list will be updated when the this.filter$ updated see below functions
 console.log(listOfMessages); 
});
// update this.filter$ like this to get 
this.messageService.getClosedMessage();
// to get open messges in 
this.messageService.getOpenMessage();