Determining whether an Observable has updated its value or if the final value has been established

Question

Determining whether an Observable has updated its value or if the final value has been established

I am working with a variable called

returnDate: Observable<string>

, which is assigned a value from another function of type Observable. Here is an example of how the function looks:

getDates(): Observable<string> {
   return new Observable((observer) => { 
   observer.next(moment(this.data.Date).format('DD MMM'));}
   });
 }

My question is, how can I determine if the value of returnDate has changed or if the observable has received a different value? Essentially, I am updating dates using these observables and need to track if a specific object (this.data.Date) has had its value changed (i.e., when observer.next is completed), or if there is a method like finalize for observables.

angular typescript observable angular7 angular-observable

Answer 1

Answer №1

It may be challenging to decipher from the code you provided how you are implementing this exact process. However, regarding your query -

I am looking to determine if a specific object (this.data.Date) has undergone any changes.

When working with rxjs observables, there exists an operator called distinctUntilChanged which can fulfill this requirement effectively. More details can be found here.

Only emit when the current value is different than the last.

Here is a sample solution:

getDates.pipe(distinctUntilChanged())
  .subscribe(data => console.log("This will only trigger if data changed"));

Feel free to give it a try and see if it meets your needs.

Answer 2

It may be challenging to decipher from the code you provided how you are implementing this exact process. However, regarding your query -

I am looking to determine if a specific object (this.data.Date) has undergone any changes.

When working with rxjs observables, there exists an operator called distinctUntilChanged which can fulfill this requirement effectively. More details can be found here.

Only emit when the current value is different than the last.

Here is a sample solution:

getDates.pipe(distinctUntilChanged())
  .subscribe(data => console.log("This will only trigger if data changed"));

Feel free to give it a try and see if it meets your needs.

Answer 3

Answer №2

When you consistently return a new Observable() and subscribe to it, you will receive different values each time. For instance, if you return

return new Observable(Math.random()*10)

and subscribe in 2 components, you will get varying values. If your intention is to receive the same value, then consider using Subject().

To achieve this, first declare the subject, set the value, and then return this.subject.asObservable(). This approach allows you to subscribe to the subject consistently.

Here's a simple example:

// Set initial value for the subject, typically set data above constructor
dataSubject = new BehaviorSubject(null);

// Retrieve data from the server
// The response can be data from the server or an eventEmitter, depending on your needs

this.dataSubject.next(response);

// Return this subject

return this.dataSubject.asObservable()

In this scenario, utilize BehaviorSubject because it can retain a value and also enable setting an initial value for the subject.

If you have any further questions, please feel free to ask.

Answer 4

When you consistently return a new Observable() and subscribe to it, you will receive different values each time. For instance, if you return

return new Observable(Math.random()*10)

and subscribe in 2 components, you will get varying values. If your intention is to receive the same value, then consider using Subject().

To achieve this, first declare the subject, set the value, and then return this.subject.asObservable(). This approach allows you to subscribe to the subject consistently.

Here's a simple example:

// Set initial value for the subject, typically set data above constructor
dataSubject = new BehaviorSubject(null);

// Retrieve data from the server
// The response can be data from the server or an eventEmitter, depending on your needs

this.dataSubject.next(response);

// Return this subject

return this.dataSubject.asObservable()

In this scenario, utilize BehaviorSubject because it can retain a value and also enable setting an initial value for the subject.

If you have any further questions, please feel free to ask.

Determining whether an Observable has updated its value or if the final value has been established

Answer №1

Answer №2

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