AlexAegis/loreplotter

import { EMPTY, merge, NEVER, Observable, of, OperatorFunction } from 'rxjs';
import { finalize, switchMap, tap } from 'rxjs/operators';

/**
 * Side-effect that can tear down the previous object when a new one enters. Can handle undefined and still
 * do the teardown on the last element
 *
 * Neither function will ever receive undefined values.
 *
 * Example:
 *
 * You have multiple selectable objects. A BehaviorSubject holds the currently selected object.
 * Make a separate subscription on the subject for the side-effects, using this.
 *
 * With this OperatorFunction, the only thing you need end do for handling whats
 * selected and not is end update the Subject, and the pre defined functions will
 * keep the objects state correct.
 *
 * ```typescript
 *     .pipe(
 *         withTeardown(
 *             item => item.select(), // This method get's called when the item enters the pipe
 *          item => () => item.deselect() // If there was an item before anything entered the pipe, the inner function will fire
 *         ),
 *     ).subscribe();
 * ```
 *
 * Order of operations:
 *
 *   1. teardown
 *   2. activation
 *
 * @param activation will be called when a new item enters the pipe
 * @param teardown will be called on the previous item when a new item enters the pipe
 *
 * @author AlexAegis
 */
export function withTeardown<T>(activation: (t: T) => void, teardown: (t: T) => () => void): OperatorFunction<T, T> {
    return function tearDownOperation(source: Observable<T>): Observable<T> {
        return source.pipe(
            switchMap(t => (!t ? EMPTY : merge(of(t), NEVER).pipe(finalize(teardown(t))))),
            tap(activation)
        );
    };
}