Sunday, June 01, 2008

PHP or JavaScript implicit Factory method design pattern

Update
Above technique could be used to create an implicit Singleton as well.

class Demo {
// your unbelievable stuff
}

function Demo($some, $arg){
static $instance;
return isset($instance) ? $instance : ($instance = new Demo($some, $arg));
}

Demo(1,2)->doStuff();
Demo(1,2) === Demo(1,2); // true


from Wikipedia
The factory method pattern is an object-oriented design pattern ... More generally, the term factory method is often used to refer to any method whose main purpose is creation of objects.

I have talked about JavaScript possibility different times in my prototypal inheritance documentation, and in other posts of this blog. The summary is that thanks to perfect this behaviour, that for some unknown reason someone would like to modify in JS2, making them ambiguous when you are using a constructor as a function and not, for example, as private method, we can create intelligent constructors that does not require the usage of new keyword - in a Pythonic way:

// JavaScript
function Person(name, age){
if(this instanceof Person){
this.name = name;
this.age = age;
} else
return new Person(name, age);
};
Person.prototype.getInfo = function(){
return this.name + " is " + this.age + " years old";
};

// Example
alert(
Person("Andrea", 30).getInfo() // Andrea is 30 years old
);

var a = Person("Luca", 25),
b = Person("Fabio", 31);

The main advantage, using this technique, is that we write less code and we are able to concatenate every public method, allowing us to do different stuff in few lines.

alert(
Person(name, age).transFormToEmployee(company).getInfo()
);


The new keyword


We could use simply constructor when we would like to create an instance and use then directly a public method.

var time = (new Date).getTime();

// same as
var time = new Date().getTime();

// different of
var time = new Date.getTime(); // Date.getTime is not a constructor !!

Above examples show JavaScript new keyword nature, and its priority when we use brackets around, or at the of the constructor - passing, or not, one or more arguments. With implicit factory method purpose, we could simply do stuff like this one:

// just a basic (and wrong) example of implicit factory implementation
Date = function(Date){
return function(){
return this instanceof Date ? this : new Date;
};
}(Date);

var time = Date().getTime();

To be honest, with native or defined constructors, we should put more effort to implement my proposed technique, but the aim of this post is to show how could be possibile to do the same, with PHP.

PHP implicit Factory method


I suppose that if you are an Object Oriented PHP Programmer, you have tried, at least once, to do something like this and without success:

echo new Person("Me", 30)->getInfo();
echo (new Person("Me", 30))->getInfo();

Since in PHP both usage of brackets and syntax are different, the "only" way we have to concatenate public methods is to create a manual factory one:

class Person {

// ... our stuff ...

static public function create($name, $age){
return new Person($name, $age);
}
}

echo Person::create($name, $age)->getInfo();

Above example is a very common one in many OO frameworks or libraries.

Ambiguity


You probably don't know that thanks to some intrinsic PHP language ambiguity, it is possible to create a function with the same name of a class, as is, for example, for constants, but this time without future features problems.

// PHP
class Person {
function Person($name, $age){
$this->name = $name;
$this->age = $age;
}
function getInfo(){
return "{$this->name} is {$this->age} years old";
}
}

// factory method!!!
function Person($name, $age){
return new Person($name, $age);
}

// Example
echo Person("Andrea", 30)->getInfo(); // Andrea is 30 years old
?>

The PHP interpreter is able to recognize if we are calling the class or the function, simply looking at new keyword.
If we are using them, it will be obviously an instance creation, while if we are not using them, it will be obviously a function call.
The last case is if we do not use brackets, and then it will be the defined constant, if any, or, in a better future, the static public __toString method, if any again.

The __autofactory function


Since this way to code and use classes and functions could be truly interesting, and since we could have one to thousands of classes, it is natural to think how to automatically implement this kind of feature in our project.
The answer is a function, called __autofactory, that will be able to create a file to include, or require, that will contain every defined class as function, using best practices to speed up implicit factory method usage.
The only thing to write before logic code execution, is this piece of code:

// ... application classes inclusions

// before code logic
require '__autofactory.php';
!file_exists($fileName = 'factory.function.php') && __autofactory($fileName);

The first parameter will be the name of the file that will contain informations, while the second one will specify, if present and with a false evaluable data, if function should require them or not.

At this point, you have basis and / or code to use this particular feature for every kind of purpose.

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