export type SubscriptionHandle = () => undefined;
export declare abstract class BaseObservable<T> {
    protected _handlers: Set<T>;
    onSubscribeFirst(): void;
    onUnsubscribeLast(): void;
    subscribe(handler: T): SubscriptionHandle;
    unsubscribe(handler?: T): undefined;
    unsubscribeAll(): void;
    get hasSubscriptions(): boolean;
}
export declare function tests(): {
    test_unsubscribe(assert: any): void;
};