/**
 * @license
 * Copyright Google Inc. All Rights Reserved.
 *
 * Use of this source code is governed by an MIT-style license that can be
 * found in the LICENSE file at https://angular.io/license
 */

import {Type} from '../interface/type';
import {getClosureSafeProperty} from './property';
import {stringify} from './stringify';



/**
 * An interface that a function passed into {@link forwardRef} has to implement.
 *
 * @usageNotes
 * ### Example
 *
 * {@example core/di/ts/forward_ref/forward_ref_spec.ts region='forward_ref_fn'}
 * @publicApi
 */
export interface ForwardRefFn { (): any; }

const __forward_ref__ = getClosureSafeProperty({__forward_ref__: getClosureSafeProperty});

/**
 * Allows to refer to references which are not yet defined.
 *
 * For instance, `forwardRef` is used when the `token` which we need to refer to for the purposes of
 * DI is declared, but not yet defined. It is also used when the `token` which we use when creating
 * a query is not yet defined.
 *
 * @usageNotes
 * ### Example
 * {@example core/di/ts/forward_ref/forward_ref_spec.ts region='forward_ref'}
 * @publicApi
 */
export function forwardRef(forwardRefFn: ForwardRefFn): Type<any> {
  (<any>forwardRefFn).__forward_ref__ = forwardRef;
  (<any>forwardRefFn).toString = function() { return stringify(this()); };
  return (<Type<any>><any>forwardRefFn);
}

/**
 * Lazily retrieves the reference value from a forwardRef.
 *
 * Acts as the identity function when given a non-forward-ref value.
 *
 * @usageNotes
 * ### Example
 *
 * {@example core/di/ts/forward_ref/forward_ref_spec.ts region='resolve_forward_ref'}
 *
 * @see `forwardRef`
 * @publicApi
 */
export function resolveForwardRef<T>(type: T): T {
  const fn: any = type;
  if (typeof fn === 'function' && fn.hasOwnProperty(__forward_ref__) &&
      fn.__forward_ref__ === forwardRef) {
    return fn();
  } else {
    return type;
  }
}
