Angular/RxJS: synchronous observable

Question

I have a service that has a method foo. Inside the method I subscribe to an observable (http-client).

foo () : boolean
{
  let ret : false;

  this.http.get ("/blabla").subscribe (
  (resp) =>
  {
    ret = true;
  }

  return ret;
);

I like to return a boolean value from foo that depends on the get. That is not working because http.get is asynchrouns - return is called before http.get finished.

How can I make this synchronous?

EDIT

Returning the observable instead boolean is not an option here. That is because I handle the response of get in foo (not shown here) but I also need to act out of foo depending of its return.

EDIT2

I extended my sample with pipe and tap. Now I return the http.get-observable for outside the service and I process the http.get-result with tap.

foo () : Observable <any>
{
  return this.http.get ("/blabla").pipe (tap (x => handlehere ()));
}

As far I see there is only one uglyness with it. I have the complexity of parsing the get-result inside AND outside of foo. I would prefer a simple boolean outside of foo.

Possible duplicate of How do I return the response from an asynchronous call? — Igor
How can I make this synchronous? <= you don't, you return an observable or a promise. Returning the observable instead boolean is not an option here <= why? — Igor
Then you have no option because synchronous code could become asynchronous in one operation, but asynchronous code could never become synchronous again — smnbbrv
but I also need to act out of foo depending of its return <= then make the acting on the call also occur asynchronously. Once you understand the pattern used for asynchronous javascript (typescript) code/interaction it will be much easier for you to write your application. There is generally a fair amount of asynchronous interaction in applications because they interact with external resources (user input, databases, network streams, etc). You will be a better programmer for it. — Igor
Returning an Observable would be also possible but then it must be related to http.get. I do not know how to do that. — chris01

xrobert35 xrobert35 · Accepted Answer · 2018-09-24T19:22:59

This method can only run asynchronously so you don't have a lot of option. Returning the obsersable and subscribe or return a promise. Perhaps returning a Promise will suite more your need in term of comprehention.

In your service : the foo method :

async foo() {
   const result = await this.http.get("/blabla").toPromise();

   // do what you want with result 

   return result;
}

How to call it :

this.myService.foo().then( (result) => {
   // Do what you want with the result
});

Angular/RxJS: synchronous observable

1 Answers