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Edit File: SimpleSubscriber.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\Subscriber;

use GuzzleHttp\Event\BeforeEvent;
use GuzzleHttp\Event\RequestEvents;
use GuzzleHttp\Event\SubscriberInterface;

/**
 * SimpleSubscriber is a Guzzle Subscriber that implements Google's Simple API
 * access.
 *
 * Requests are accessed using the Simple API access developer key.
 */
class SimpleSubscriber implements SubscriberInterface
{
    /**
     * @var array
     */
    private $config;

/**
     * Create a new Simple plugin.
     *
     * The configuration array expects one option
     * - key: required, otherwise InvalidArgumentException is thrown
     *
     * @param array $config Configuration array
     */
    public function __construct(array $config)
    {
        if (!isset($config['key'])) {
            throw new \InvalidArgumentException('requires a key to have been set');
        }

$this->config = array_merge([], $config);
    }

/**
     * @return array
     */
    public function getEvents()
    {
        return ['before' => ['onBefore', RequestEvents::SIGN_REQUEST]];
    }

/**
     * Updates the request query with the developer key if auth is set to simple.
     *
     * Example:
     * ```
     * use Google\Auth\Subscriber\SimpleSubscriber;
     * use GuzzleHttp\Client;
     *
     * $my_key = 'is not the same as yours';
     * $subscriber = new SimpleSubscriber(['key' => $my_key]);
     *
     * $client = new Client([
     *     'base_url' => 'https://www.googleapis.com/discovery/v1/',
     *     'defaults' => ['auth' => 'simple']
     * ]);
     * $client->getEmitter()->attach($subscriber);
     *
     * $res = $client->get('drive/v2/rest');
     * ```
     *
     * @param BeforeEvent $event
     */
    public function onBefore(BeforeEvent $event)
    {
        // Requests using "auth"="simple" with the developer key.
        $request = $event->getRequest();
        if ($request->getConfig()['auth'] != 'simple') {
            return;
        }
        $request->getQuery()->overwriteWith($this->config);
    }
}