// (C) Copyright 2015 Moodle Pty Ltd.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import { FormControl } from '@angular/forms';
import { Observable, OperatorFunction } from 'rxjs';
import { filter } from 'rxjs/operators';

/**
 * Create an observable that emits the current form control value.
 *
 * @param control Form control.
 * @returns Form control value observable.
 */
export function formControlValue<T = unknown>(control: FormControl): Observable<T> {
    return control.valueChanges.pipe(
        startWithOnSubscribed(() => control.value),
        filter(value => value !== null),
    );
}

/**
 * Observable operator that waits for a promise to resolve before emitting the result.
 *
 * @returns Operator.
 */
export function resolved<T>(): OperatorFunction<Promise<T>, T> {
    return source => new Observable(subscriber => {
        const subscription = source.subscribe(async promise => {
            const value = await promise;

            return subscriber.next(value);
        });

        return subscription;
    });
}

/**
 * Same as the built-in startWith operator, but evaluates the starting value for each subscriber
 * on subscription.
 *
 * @param onSubscribed Callback to calculate the starting value.
 * @returns Operator.
 */
export function startWithOnSubscribed<T>(onSubscribed: () => T): OperatorFunction<T, T> {
    return source => new Observable(subscriber => {
        subscriber.next(onSubscribed());

        return source.subscribe(value => subscriber.next(value));
    });
}