Introduction to PHP Design Patterns: Value Object Pattern Page 1/5_PHP Tutorial

For example: an object is usually used to describe a date, a number or currency. Class definitions for dates, integers, or dollars are easy to use, fast, easy to encapsulate, and easy to copy, compare with each other, and even create.

On the surface, these simple description objects are easy to execute: they have very few statements and make no difference whether they apply to Customer or SKU when constructing the class. This idea seems correct, but the so-called "seems correct" can easily produce some bugs.

Please look at the following code, which is a definition and execution operation of an object that pays employees wages in US dollars. In most cases, it operates without problems. (This class was named BadDollar because it was buggy). Think about it and see if you can find a bug in it.

// PHP5
class BadDollar {
protected $amount;
public function __construct($amount=0) {
$this->amount = (float)$amount ;
}
public function getAmount() {
return $this->amount;
}
public function add($dollar) {
$this->amount + = $dollar->getAmount();
}
}
class Work {
protected $salary;public function __construct() {
$this->salary = new BadDollar( 200);}
public function payDay() {
return $this->salary;
}
}
class Person {
public $wallet;
}
function testBadDollarWorking() {
$job = new Work;
$p1 = new Person;
$p2 = new Person;
$p1->wallet = $job->payDay ();
$this->assertEqual(200, $p1->wallet->getAmount());
$p2->wallet = $job->payDay();
$this->assertEqual(200, $p2->wallet->getAmount());
$p1->wallet->add($job->payDay());
$this->assertEqual(400, $p1->wallet->getAmount());
//this is bad — actually 400
$this->assertEqual(200, $p2- >wallet->getAmount());
//this is really bad — actually 400
$this->assertEqual(200, $job->payDay()->getAmount()) ;
}
So, what is a bug? If you can't spot the problem intuitively in the above code example, here's a tip: Employee object $p1 and object $p2 use the same BadDollar object instance.

First, instances of class Work and class Person have been created. Then, assuming that each employee initially has an empty electronic wallet, the employee's electronic wallet Person:wallet is assigned through the object resource variable returned by the Work::payDay() function, so it is set to an object instance of the BadDollar class.

Do you remember PHP5’s object assignment method? Because of the way object assignment is handled in PHP5, although the three seemingly different object instances $job::salary, $p1::wallet and $p2::wallet use different "identifiers", in fact , they are all assigned to the same object instance.

Therefore, for the next salary payment operation (PayDay represents the day when wages are paid, here represents the action of paying wages), using $job->payDay() originally only wanted to increase the salary of $P1. But unexpectedly, $P2 was also distributed for the first time. Moreover, this action also changes the amount of the basic salary of the job. Therefore, the detection of the last two values ​​reports an error.

Value Object PHP5 Unit Test
1) Equal expectation fails because [Integer: 200] differs from [Float: 400] by 200
in testBadDollarWorking
in ValueObjTestCase
2) Equal expectation fails because [Integer: 200] differs from [Float: 400] by 200
in testBadDollarWorking
in ValueObjTestCase
FAILURES!!!