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                <?php namespace Arcanedev\Currencies\Services;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use Arcanedev\Currencies\Exceptions\ApiException;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                use Illuminate\Support\Arr;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * Class     CurrencyLayerService  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * @package  Arcanedev\Currencies\Services  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 * @author   ARCANEDEV <[email protected]>  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                 */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                class CurrencyLayerService extends AbstractApiService  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /* ------------------------------------------------------------------------------------------------  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     |  Properties  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     | ------------------------------------------------------------------------------------------------  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * The base URL.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @var string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    protected $baseUrl = 'http://apilayer.net/api';  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * The Access Key.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @var string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    protected $accessKey;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /* ------------------------------------------------------------------------------------------------  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     |  Getters & Setters  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     | ------------------------------------------------------------------------------------------------  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * Set the configs.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @param  array  $configs  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    protected function setConfigs(array $configs)  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $this->accessKey = Arr::get($configs, 'access-key');  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * Get the Access Key.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @return string  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @throws \Arcanedev\Currencies\Exceptions\ApiException  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    private function getAccessKey()  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        if ( ! $this->accessKey) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                            throw new ApiException('Currencylayer.com requires an access key.'); | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        return $this->accessKey;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * Get cache key.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @return string  | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
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                     */  | 
            
            
                                                                        
                            
            
                                    
            
            
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                    protected function getCacheKey()  | 
            
            
                                                                        
                            
            
                                    
            
            
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                        return parent::getCacheKey() . '.currencylayer';  | 
            
            
                                                                        
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /* ------------------------------------------------------------------------------------------------  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     |  Main Functions  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     | ------------------------------------------------------------------------------------------------  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * Make an API request.  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @param  string  $from  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @param  array   $to  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     *  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     * @return \Arcanedev\Currencies\Entities\RateCollection  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                     */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    protected function request($from, array $to = [])  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $response = $this->client->get('/live', [ | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                            'access_key' => $this->getAccessKey(),  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                            'source'     => $from,  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                            'currencies' => empty($to) ? null : implode(',', $to), | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        ]);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $rates = [];  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $data  = json_decode($response, true);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        foreach ($data['quotes'] as $key => $ratio) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                            $rates[substr($key, 3)] = $ratio;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        return $rates;  | 
            
                            
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                    }  | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
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                }  | 
            
            
                                                        
            
                                    
            
            
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If you return a value from a function or method, it should be a sub-type of the type that is given by the parent type f.e. an interface, or abstract method. This is more formally defined by the Lizkov substitution principle, and guarantees that classes that depend on the parent type can use any instance of a child type interchangably. This principle also belongs to the SOLID principles for object oriented design.
Let’s take a look at an example:
Our function
my_functionexpects aPostobject, and outputs the author of the post. The base classPostreturns a simple string and outputting a simple string will work just fine. However, the child classBlogPostwhich is a sub-type ofPostinstead decided to return anobject, and is therefore violating the SOLID principles. If aBlogPostwere passed tomy_function, PHP would not complain, but ultimately fail when executing thestrtouppercall in its body.