startWithDefer.md

The startWithDefer Operator

Depricated

Use startWithStream instead.

---

This is a version of startWith that generates the first value at the time of subscription rather than the time of creation. This is useful if you want each new subscriber to get a value based on the (possibly changing) state of the program.

function startWithDefer<T, R>(fn: () => R): OperatorFunction<T, T | R> {
	return input$ => defer(() => input$.pipe(startWith(fn())));
}

startWithDefer in use:

A stream of the current username and future updates of that name. Since we don't know when the caller will subscribe (or if they'll subscribe multiple times), we defer grabbing the current username until the observable is subscribed.

function getUserName(): Observable<string> {
  return this.userNameUpdates$.pipe(
    startWithDefer(() => this.currentUserName)
  );
}