rxjs - buffer stream until function returns true

Question

I have a stream of numbers which increase by a constant amount which I want to sub-sample. Given a constant sample interval , I want to buffer the stream until the difference between the first and last buffered value are greater or equal to interval . It would then emit this array, so similar to the buffer operator.

I've searched through the different rxjs operators but can't figure out how to make this work. A bufferUntil operator would be perfect but doesn't seem to exist. How can I implement this?

For example:

const interval = 15;
//example stream would be: 5, 10 , 15, 20, 25, 30..

Observable.pipe(
   bufferUntil(bufferedArray => {
       let last = bufferedArray.length - 1;
       return (bufferedArray[last] - bufferedArray[0] >= interval);
   })
).subscribe(x => console.log(x));

//With an expected output of [5, 10, 15, 20], [ 25, 30, 35, 40],..

Answer 1

You can implement an operator that maintains a custom buffer. Push all incoming values into the buffer, emit the buffer when it meets a given condition and reset it. Use defer to supply every subscriber with its own buffer.

function bufferUntil<T>(emitWhen: (currentBuffer: T[]) => boolean): OperatorFunction<T, T[]> {
  return (source: Observable<T>) => defer(() => {
    let buffer: T[] = []; // custom buffer
    return source.pipe(
      tap(v => buffer.push(v)), // add values to buffer
      switchMap(() => emitWhen(buffer) ? of(buffer) : EMPTY), // emit the buffer when the condition is met
      tap(() => buffer = []) // clear the buffer
    )
  });
}

https://stackblitz.com/edit/rxjs-7awqmv