import { isNotifyPropertyChanged, notificationsFor, NotifyPropertyChangedType } from '../../notify/notifyPropertyChangedSymbol';
import { filter, map } from 'rxjs/operators';
import { concat, defer, Observable, of } from 'rxjs';

export type PropertyValue<TObject, TProperty extends keyof TObject> = { sender: TObject; value: TObject[TProperty] };

export function whenChanged<TObject, TProperty extends keyof TObject>(value: TObject, key: TProperty, notifyInitial = true, fallbackValue?: () => TObject[TProperty]) {
    if (!isNotifyPropertyChanged(value)) {
        throw new Error(
            'Object must implement the notifyPropertyChangedSymbol or inherit from the NotifyPropertyChangedBase class or be wrapped by the proxy method observePropertyChanges',
        );
    }
    return whenChangedValues(value, key, notifyInitial, fallbackValue).pipe(
        filter(x => !!x.value),
        map(z => z.value),
    );
}

function whenChangedValues<TObject, TProperty extends keyof TObject>(
    value: TObject,
    key: TProperty,
    notifyInitial = true,
    fallbackValue?: () => TObject[TProperty],
): Observable<PropertyValue<NotifyPropertyChangedType<TObject>, TProperty>> {
    if (!isNotifyPropertyChanged(value)) {
        throw new Error(
            'Object must implement the notifyPropertyChangedSymbol or inherit from the NotifyPropertyChangedBase class or be wrapped by the proxy method observePropertyChanges',
        );
    }
    const propertyChanged = notificationsFor(value).pipe(
        filter(x => x === key),
        map(t => ({ sender: value, value: value[key] } as PropertyValue<NotifyPropertyChangedType<TObject>, TProperty>)),
    );
    return notifyInitial
        ? concat(
              defer(() =>
                  of({
                      sender: value,
                      value: value[key] || fallbackValue?.(),
                  } as PropertyValue<NotifyPropertyChangedType<TObject>, TProperty>),
              ),
              propertyChanged,
          )
        : propertyChanged;
}