/**
 * Creates a Promise that resolves with an observed value when it fulfills the
 * conditions of a predicate.
 *
 * The generator function selects the to be observed value. If the generator
 * creates an undefined value it is ignored. If the generator creates a value,
 * it will be passed to the predicate function that indicates if that value
 * should resolve the promise.
 *
 * Any error in the generator or predicate functions will reject the promise.
 *
 * The operation can be cancelled by the cancel operation added to the returned
 * promise. This will reject the Promise.
 *
 * @example
 * const store = observed({
 *   foo:""
 * });
 *
 * //wait until store.foo contains the character "b"
 * await whenValue(()=>store.foo, (value)=>value.indexOf("b")>=0);
 *  *
 * @param generator - selects the value to be observed
 * @param predicate - optional predicate to indicate when the observed value
 *     should resolve the promise. By default accepts all values.
 * @returns a Promise that resolves with the observed value when it fulfills
 *     the conditions of the predicate
 */
export declare function whenValue<T>(generator: () => T | undefined, predicate?: (value: T) => boolean): Promise<T> & {
    cancel: () => void;
};