---

public subscribeIsLocked(callback: (isLocked: boolean) => void): Subscription {
  return this.isLockedSubject.pipe(distinctUntilChanged()).subscribe(callback);
}

public isLocked(): boolean {
  return this.isLockedSubject.value;
}

public setLocked(isLocked: boolean): void {
  if (this.isLockedSubject.value === isLocked) {
    return;
  }

  this.isLockedSubject.next(isLocked);

  if (isLocked) {
    this.showCrosshair();
  }

  this.render();
}