// (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 { Injector, Type } from '@angular/core';

/**
 * Stub class used to type anonymous classes created in CoreSingletonsFactory#makeSingleton method.
 */
class CoreSingleton {}

/**
 * Token that can be used to resolve instances from the injector.
 */
export type CoreInjectionToken<Service> = Type<Service> | Type<unknown> | string;

/**
 * Singleton class created using the factory.
 */
export type CoreSingletonClass<Service> = typeof CoreSingleton & {
    instance: Service;
    setInstance(instance: Service): void;
};

/**
 * Factory used to create CoreSingleton classes that get instances from an injector.
 */
export class CoreSingletonsFactory {

    /**
     * Angular injector used to resolve singleton instances.
     */
    private injector?: Injector;

    /**
     * Set the injector that will be used to resolve instances in the singletons created with this factory.
     *
     * @param injector Injector.
     */
    setInjector(injector: Injector): void {
        this.injector = injector;
    }

    /**
     * Make a singleton that will hold an instance resolved from the factory injector.
     *
     * @param injectionToken Injection token used to resolve the singleton instance. This is usually the service class if the
     * provider was defined using a class or the string used in the `provide` key if it was defined using an object.
     */
    makeSingleton<Service>(injectionToken: CoreInjectionToken<Service>): CoreSingletonClass<Service> {
        // eslint-disable-next-line @typescript-eslint/no-this-alias
        const factory = this;

        return class {

            private static serviceInstance: Service;

            static get instance(): Service {
                // Initialize instances lazily.
                if (!this.serviceInstance) {
                    if (!factory.injector) {
                        throw new Error('Can\'t resolve a singleton instance without an injector');
                    }

                    this.serviceInstance = factory.injector.get(injectionToken);
                }

                return this.serviceInstance;
            }

            static setInstance(instance: Service): void {
                this.serviceInstance = instance;
            }

        };
    }

}
