Programming would be simple, but boring. Programmers would continue to build applications on top of the same technologies that they did years ago. They would never need to introduce different databases, implement new best practices, or consume different APIs. But these things do change. Luckily, programmers have the Adapter Design Pattern to help update legacy systems with new code and functionality.
The solution is to build another object, using the Adapter Design Pattern. This Adapter object works as an intermediary between the original application and the new functionality. The Adapter Design Pattern defines a new interface for an existing object to match what the new object requires.
Code Example
In the original code base of the project, an object exists that handles all of the error messages and codes called errorObject. The original programmers didn’t think their code would ever generate any errors, so they designed the system to output the errorObject’s error information directly to the console.
In this example, a 404:Not Found error is being generated. You are going to assume that the error message content and code may change, but the text will always stay in the same format.
class ErrorObject
{
private $error;
public function __construct($error){
$this->error = $error;
}
public function getError(){
return $this->error;
}
}
class LogToConsole
{
private $errorObject;
public function __construct($errorObject){
$this->errorObject = $errorObject;
}
public function write() {
fwrite(STDERR, $this->errorObject->getError() . "\n");
}
}
Here is the testing...
include "ErrorObject.php";
include "LogToConsole.php";
/** create the new 404 error object **/
$error = new ErrorObject("404:Not Found");
/** write the error to the console **/
$log = new LogToConsole($error);
$log->write();
How about new requirement comes that error need to output to csv format, with the code and description.
Lets look at how we going to do that with Adapter pattern.
class LogToCsvAdapter extends ErrorObject
{
private $errorNumber, $errorText;
public function __construct($error)
{
parent::__construct($error);
$parts = explode(':', $this->getError());
$this->errorNumber = $parts[0];
$this->errorText = $parts[1];
}
public function getErrorNumber()
{
return $this->errorNumber;
}
public function getErrorText()
{
return $this->errorText;
}
}
include "ErrorObject.php";
include "LogToCsv.php";
include "LogToCsvAdapter.php";
/** create the new 404 error object adapted for csv **/
$error = new LogToCsvAdapter("404:Not Found");
/** write the error to the csv file **/
$log = new logToCSV($error);
$log->write();
