Angular Context.Service used to share data (aka state) across components - It's a "dynamic reactive DI" - sample usage: https://stackblitz.com/edit/angular-context-service

Context.Service.ts

import { Injectable, OnDestroy, Optional, SkipSelf } from "@angular/core";
import { Observable, Observer, BehaviorSubject, Subject } from "rxjs";
import { takeUntil, filter } from "rxjs/operators";
import { InjectionToken } from "@angular/core";

@Injectable()
export class ContextService implements OnDestroy {
	constructor(@Optional() @SkipSelf() private _parent?: ContextService) {}

	private _destroy$ = new Subject<void>();
	ngOnDestroy(): void {
		this._parent = undefined;
		this._destroy$.next();
		this._destroy$.complete();
		//cleanup internals I don't know if it's safer to execute this in nextTick Promise.resolve().then()
		for (let _key in this._) (this._[_key] as unknown) = undefined;
	}

	private _: {
		//internals use an object map string -> BehaviorSubject (ndr token -> reactive variable)
		[_key: string]: BehaviorSubject<unknown | undefined>;
	} = {};

	private _bs<T>(key: string | InjectionToken<T>): BehaviorSubject<T | undefined> | null {
		//recursive get the internal BehaviorSubject from the local _ map, or clib the _parent hierarchy
		const _key = key.toString(); //directly use string or get it from InjectionToken
		if (_key in this._) return this._[_key] as BehaviorSubject<T | undefined>;
		else if (this._parent != null) return this._parent._bs(key);
		else return null;
	}

	provide<T>(key: string | InjectionToken<T>, initVal?: T): Observer<T> {
		let _bs = this._bs<T>(key);
		if (_bs == null) _bs = this._[key.toString()] = new BehaviorSubject<T | undefined>(initVal);
		else _bs.next(initVal); //if no initVal is passed it emits undefined but it's filtered

		return {
			next: _bs.next.bind(_bs),
			error: _bs.error.bind(_bs),
			complete: _bs.complete.bind(_bs)
		};
	}

	get$<T>(key: string | InjectionToken<T>): Observable<T> {
		let _bs = this._bs<T>(key);
		if (_bs == null) _bs = this._[key.toString()] = new BehaviorSubject<T | undefined>(undefined);

		return _bs.pipe(
			filter(t => t !== undefined), //filter the initial undefined, or possible initVal not passed
			takeUntil(this._destroy$)
		);
	}

	getCurr<T>(key: string | InjectionToken<T>): T | undefined {
		const _bs = this._bs<T>(key);
		if (_bs == null) return undefined;
		else return _bs.getValue();
	}
}