Tag: OOP

Categories
PHP Programming

Design patterns: singletons

Singleton classes basically mean there is only one object class of a certain type running at all times. That could be used to store Global variables, for example.

The way to implement it on PHP:

class Preferences {
    private $props = array();
    private static $instance;

    private function __construct() { }  // By setting the class constructor to private, it can’t be instantiated from outside

    public static function getInstance() { // This static method will serve as instantiation, or to serve the only instance available
         if ( empty( self::$instance ) ) {
              self::$instance = new Preferences();
         }
         return self::$instance;
      }

      public function setProperty( $key, $val ) {
          $this->props[$key] = $val;
      }

      public function getProperty( $key ) {
          return $this->props[$key];
      }
}

Example of the driver class that is calling this:

$pref = Preferences::getInstance();
$pref->setProperty( “name”, “matt” );

unset( $pref ); // remove the reference

$pref2 = Preferences::getInstance();
print $pref2->getProperty( “name” ) .”n”; // demonstrate value is not lost

Categories
Programming

Design patterns: Composition and Inheritance (Strategy pattern)

Instead of creating child classes, you create a new tree of classes and move some of the functionality away from the main tree.

You connect the trees via object creation, meaning some of the classes in one tree contrain objects with the other classes type.

The advantage is that the trees are more specialized in what they do, so functionality in one tree is independent from the other tree, and it just complement it.

The disadvantages is that now you are creating more classes, so it may be difficult to maintain and keep track of it.

Categories
PHP

PHP __toString() method

Useful to convert objects that, when printed (say in debugging), will give you output like “Resource ID blah blah”, or error messages telling you you can’t print the object.

Example:

class Person {
    function getName()  { return "Bob"; }
    function getAge() { return 44; }
    function __toString() { // Controls how this object will represent itself when printed
        $desc  = $this->getName();
        $desc .= " (age ".$this->getAge().")";
        return $desc;
    }
}
$person = new Person();
print $person; // We get the printout defined in __toString() here...
Categories
PHP

PHP __clone() method

In PHP5, objects are passed by reference, and when we do assignments with the equal sign in objects, we are in fact using the same object for both variables, all pointing to the same reference:

$a = new TestObject();

$b = $a; // We end up with one object, two variables pointing to it.

In order to create two different objects, we can use the clone keyword instead:

$b = clone $a;

We can also clone parts of the object at the method or variable level, for instance, if we put the following method inside the class TestObject used above, everytime we clone it, we end up with two objects with some differences regarding the variables or property objects that compose them:

function __clone() {
    $this->id  = 0; // No matter what the value of id is on the original object, we will set it to 0 on the clone
    $this->account = clone $this->account; // If we have an “account” property object in the original class, we

                                                                                        // ensure its “uniqueness” this way in the clone object, making it a different

                                                                                        // object as well
}

Categories
PHP Programming

PHP interfaces

Interfaces are pure class templates that define functionality. It can contain properties and methods, but no method implementation. See the example below:

interface Chargeable {
    public function getPrice();
}

So any class that implements this interface, will look something like this:

class ShopProduct implements Chargeable {
    // …
    public function getPrice() {
    return ( $this->price – $this->discount );
    }
    // …

When a class implements an interface, it takes on the Type of the interface as well, effectively having more than two types if that same class is also extending other classes, or implementing other interfaces. This way you can join types that are otherwise unrelated when creating a new class.

In PHP you can only inherit from one parent, but you can have multiple interfaces:

class Consultancy extends TimedService implements Bookable, Chargeable {

    // ...
}
Categories
PHP Programming

Public, private and protected methods and variables.

  • Public: access from everywhere.
  • Private: access is only permitted from inside the instantiated object.
  • Protected: accessed is permitted from inside the instantiated object, or from objects that are subclasses of the current object.
Categories
PHP Programming

OOP Best Practices and architecture

  1. When creating classes, make a really general abstaction of the object you have in mind (usually an abstact class or interface), and then start working your way down to the specifics using inheritance, encapsulation and polymorphism. For example: to create a book object for an ecommerce site, create something like Product / Item / Books / Book.
    • Advantages: This way your architecture can accomodate other objects later (cars / clothes / other categories)
    • Inside your classes, you avoid using hard to maintain “If” flags and clauses to perform different tasks per type of class
    • At the same time, you maintain a hierarquical structure of your objects that help you control variables and methods per level.
    • Make sure your classes are not overloaded with responsibilities. Spread responsibilities around the hierarquical structure of classes, if you can’t define a class responsibility with 25 words, and without using “and” and “or” words, you may be overloading your class with tasks. This will be hard to maintain.
    • But always remember: OOP best practices are not set on stone. If you have a good reason to bend the rules (performance, etc), do it, but think hard before you go ahead.
  2. As a general safety rule, you can set all methods and variables to private, and start resetting them to protected or public as you go along.
  3. Add error handling to all places things can go wrong (unexpected input / output usually). See PHP Error Handling section for more details, but basically extend the provided Exception class with a new class for each kind of error, and at the catch part of your try catch statement, have several catches that will deal with the particular error the proper way based on Exception type detection.
  4. Here are four possible indications of design problems:
    1. Code duplication: if you get a deja vu feeling at code writing a class, think: if I change something fundamental in this routine, will the other simmilar routine need ammendments? If so, then you may have a dup class you can compress in one.
    2. If a class depends too much on Globals variables and its context, this class will be hard to maintain. Try to make each class its own environment.
    3. If a class is doing too much, think of ways to reduce responsibility and extend down to other classes.
    4. Testing for the same condition with “if” statements throughout diffent classes may call to review your polymorphism. Where you can do the “if” test once, and spawn different objects with no “if” conditions down that fork
  5. Inside your classes, seek for those lines of code that express variation and fork the behavior of the current class given different values or conditions, and try to encapsulate them in their own type class.
  6. The golden rules: “Do the simplest thing that work”, and “are you sure you are going to need this extra complexity?”
  7. Avoid global variables like the plague, they make your classes highly coupled with the system, can’t be reused easily, and it’s hard to encapsulate stuff when using those. If you are to use Globals (say, to avoid passing too many variables or objects around) use singletons instead.
  8. Use phpDocumentor to add documentation to your code, even the default, do nothing and just run it option is better than nothing: http://www.phpdoc.org/
  9. Make sure you include Unit testing with PHPUnit
Categories
PHP Programming

PHP instanceof to determine what kind of object we are dealing with

Example:

class ShopProductWriter {
    public function write( $shopProduct ) {
        if ( ! ( $shopProduct instanceof CdProduct )  &&
             ! ( $shopProduct instanceof BookProduct ) ) {
            die( "wrong type supplied" );
        }
         $str  = "{$shopProduct->title}: " .
                $shopProduct->getProducer() .
                " ({$shopProduct->price})n";
        print $str;
    }
}
Categories
PHP Programming

PHP Object type hinting

Introduced with PHP 5.1, it basically looks like this:

function setWriter( ObjectWriter $objwriter=null ) {
    $this->writer = $objwriter;
}

Where the $objwriter has to be either null or of the type ObjectWriter, or it will produce a runtime error. The =null part is optional.

Advantages:

Forces to pass the right kind of object to a method, saving us the trouble of testing before passing. It also makes the code clearer regarding method signature.

Disadvantage:

Since it only runs at runtime, if it is buried within conditional statements that only run on xmas, you may get a call on your holiday to get your budd to the office and fix it.

Not that you can’t use type hinting with primitives.

Categories
PHP

PHP abstract classes

Basically, abstract classes define methods but don’t have any functionality inside of them, and can’t be instantiated directly. They are just blueprints for classes that extend them, inheriting their methods (and being forced to define them). Abstract classes can’t be instantiated, just extended.