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Edit File: FetchAuthTokenInterface.php

<?php
/*
 * Copyright 2015 Google Inc.
 *
 * 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.
 */

namespace Google\Auth;

/**
 * An interface implemented by objects that can fetch auth tokens.
 */
interface FetchAuthTokenInterface
{
    /**
     * Fetches the auth tokens based on the current state.
     *
     * @param callable $httpHandler callback which delivers psr7 request
     * @return array a hash of auth tokens
     */
    public function fetchAuthToken(callable $httpHandler = null);

/**
     * Obtains a key that can used to cache the results of #fetchAuthToken.
     *
     * If the value is empty, the auth token is not cached.
     *
     * @return string a key that may be used to cache the auth token.
     */
    public function getCacheKey();

/**
     * Returns an associative array with the token and
     * expiration time.
     *
     * @return null|array {
     *      The last received access token.
     *
     * @var string $access_token The access token string.
     * @var int $expires_at The time the token expires as a UNIX timestamp.
     * }
     */
    public function getLastReceivedToken();
}