I'm wondering if it is possible in PHP to force a class to have its constructor be protected as part of a design pattern.
So far I've tried to implement it with interfaces and abstract classes but it doesn't seem to work. I want all of my Service classes to be Singletons and I achieve this (to some degree) by making the counstructor protected. How can I enforce this?
It's possible to make a constructor protected.
Here an example for singleton pattern:
<?php
class Test {
private static $instance = null;
protected function __construct()
{
}
public static function getSingleton()
{
if (self::$instance === null) {
self::$instance = new self();
}
return self::$instance;
}
}
// Does work
$test = Test::getSingleton();
// doesn't work
$test = new Test();
For "services" use a dependency injection container.
As example I use a simple container implementation, but there are a lot more.
http://container.thephpleague.com/2.x/getting-started/
<?php
interface ExampleServiceInterface {
}
class ImplementationA implements ExampleServiceInterface {
}
class ImplementationB implements ExampleServiceInterface {
}
$container = new League\Container\Container;
// add a service to the container
$container->share(ExampleServiceInterface::class, function() {
$yourChoice = new ImplementationA();
// configure some stuff? etc
return $yourChoice;
});
// retrieve the service from the container
$service = $container->get(ExampleServiceInterface::class);
// somewhere else, you will get the same instance
$service = $container->get(ExampleServiceInterface::class);
you can force it by throwing an exception?
final class Foo {
private static $meMyself = null;
protected function __construct() {
if(!is_null(Foo::$meMyself)) {
throw new \Exception("ouch. I'm seeing double");
}
// singleton init code
}
}
but there against speaks: People who use it will probably have access to your methods/code and can just change it.
Related
I am still trying to wrap my head around some of the aspects of the Dependency Injection design pattern using Pimple. I completely get the concept of using a constructor or setter function belonging to class Foo to establish it's dependency on class Bar.
The part I don't quite get is how to properly instantiate multiple new instances of class Bar from inside a method belonging to Foo while using a Pimple factory.
Basically I want to accomplish the equivalent of this:
Block.php
class Block {
private $filepath;
public function setFilePath($filepath) {
$this->filepath = $filepath;
}
}
Page.php
class Page {
private function newBlock() {
//do something here to get $filepath
$block = new Block();
$block->setFilePath($filepath);
return $block;
}
}
I am using Pimple for my container like so:
bootstrap.php
$container = new Container();
$container['page'] = $container->factory(function ($c) {
return new Page();
});
$container['block'] = $container->factory(function ($c) {
return new Block();
});
The idea is that there can be multiple pages defined, and each page is potentially comprised of multiple blocks. The properties for each block are defined by methods within Page. And it needs to happen using entirely decoupled code.
It is my understanding that injecting the entire container into Page as a dependency is actually the Service Locator anti-pattern. So the following is BAD code:
class Page {
private $container;
public function __construct(Container $container) {
$this->container = $container;
}
private function newBlock() {
//do something here to get $filepath
$block = $this->container['block'];
$block->setFilePath($filepath);
return $block;
}
}
How do I go about giving Page the ability to use the Block factory that is defined in the DIC?
So, in your DI container setup, you can pass around container references. So if you have the need to use the Block class a lot in your page class, you could do this:
$container = new Container();
$container['page'] = $container->factory(function ($c) {
return new Page( $c['block'] );
});
$container['block'] = $container->factory(function ($c) {
return new Block();
});
But this means you must add a parameter to your Page class's constructor:
class Page {
private $block = NULL;
public function __construct( $block )
{
$this->block = $block;
}
private function newBlock() {
//do something here to get $filepath
$this->block->setFilePath($filepath);
return $this->block;
}
}
So, we're not actually passing the entire container into each class, but rather allowing the DI container to pass objects as needed.
I have a Factory Method to instance a class. Is there a way to prevent this class from direct instancing?
The only option I see is to use an argument passed into the __construct(), but that's not something I'm looking for.
On the other hand, making the __construct() private would be ideal, but I don't want MyClass to extend the Factory without actual need.
What do you guys think?
Factory Method:
class Factory
{
public static function instance()
{
return new MyClass(true);
}
}
MyClass:
class MyClass
{
public function __construct($isFactory = false)
{
if (!$isFactory) {
throw new Exception('Use Factory::instance() to create an object');
}
}
}
There are hacks to do that:
abusing inheritance to use a protected constructor
putting the factory method inside the class so that it can call the private constructor, which is actually not a hack. But then why not using the constructor in the first place?
using reflection to access the private constructor
I'm not promoting anything of that. What I personally do is documenting the API with things like #internal and leave it to the client following that contract.
In essence, your code should have read something like this:
THE FACTORY
<?php
class Factory {
public static function instance(){
return new MyClass(true); //HERE YOU ARE INSTANTIATING
}
}
THE CLASS TO BE INSTANTIATED VIA THE FACTORY
<?php
//NOT MyClass() <--- YOU ARE DEFINING.... NOT INSTANTIATING...
class MyClass {
public function __construct($isFactory = false) {
if (!$isFactory) {
throw new Exception('Use Factory::instance() to create an object');
}
}
//...MORE METHODS
}
Could you try this instead?
<?php
class Factory
{
private static $FACTORY_GUARANTOR; //ONLY SET DURING INSTANTIATION
public static function instance($type) {
if (class_exists($type)) {
self::$FACTORY_GUARANTOR = 1;
$instance = new $type();
self::$FACTORY_GUARANTOR = null;
return $instance;
}
else {
throw new Exception("Class not found...");
}
}
//YOU CAN GET $FACTORYGUARANTOR EXTERNALLY BUT NEVER SET IT;
public static function getGuarantor(){
return self::$FACTORY_GUARANTOR;
}
}
class MyClass {
protected $property1;
protected $property3;
protected $property2;
public function __construct() {
// IF SOMEONE TRIES TO INSTANTIATE THE CLASS OUTSIDE OF THE FACTORY... BLOW A WHISTLE
if(!Factory::getGuarantor()){
throw new Exception('Use Factory::instance() to create an object');
}
// IF THE PROGRAM MADE IT TO THIS POINT;
// JUST INSTANTIATE THE CLASS BECAUSE MOST LIKELY IT IS COMING FROM THE FACTORY
var_dump($this); // A LITTLE CONFIRMATION....
}
//...MORE METHODS
}
// TRY IT OUT:
/*INSTANCE A: RIGHT*/ $theClass = Factory::instance("MyClass"); //INSTANTIATES THE CLASS
/*INSTANCE B: WRONG*/ $theClass = new MyClass(); //THROWS AN EXCEPTION
The easiest way is to define your base class as abstract. The abstract classes cannot be directly instanced, so you will have to redefine their abstract members in the inherited classes:
abstract class Factory
{
abstract public function foo();
}
class InheritedClass extends Factory
{
public function foo()
{
// Do something
}
}
// $obj1 = new Factory(); // Will produce an error
$obj1 = new InheritedClass(); // Will be executed successfully
You can read more for the abstract classes here: PHP: Class Abstraction - Manual.
For me, the best way is to use ReflectionClass:
class MyClass
{
public const FRIEND_CLASSES = [Factory::class];
protected function __construct() {}
}
trait Constructor
{
protected function createObject(string $className, array $args = [])
{
if (!in_array(static::class, $className::FRIEND_CLASSES)) {
throw new \Exception("Call to private or protected {$className}::__construct() from invalid context");
}
$reflection = new ReflectionClass($className);
$constructor = $reflection->getConstructor();
$constructor->setAccessible(true);
$object = $reflection->newInstanceWithoutConstructor();
$constructor->invokeArgs($object, $args);
return $object;
}
}
class Factory
{
use Constructor;
public function MyClass(): MyClass
{
return $this->createObject(MyClass::class);
}
}
In constant FRIEND_CLASSES you can define in which classes the class can be instanced.
trait is used because this functionality can be used in different factories that are not related.
If you need to put parameters into constructor of the class, put them as second parameter of createObject.
Details I described in the article "Forbidding of creating objects outside factory in PHP"
I know how to create a singleton class in php. But i am confused that if we can able to clone that then it is waste of that class.
May i know how to restrict the singleton class from cloning? please check the code below:
class DatabaseConnection {
private static $singleton_instance = null;
private function construct__() {
// Declaring the constructor as private ensures
// that this object is not created as a new intance
// outside of this class. Therefore you must use the
// global_instance function to create this object.
}
public static function global_instance() {
static $singleton_instance = null;
if($singleton_instance === null) {
$singleton_instance = new DatabaseConnection();
}
return($singleton_instance);
}
}
Just make your function __clone() private.
private function __clone() { }
It will throw a Fatal Error if you try to access it.
There is a __clone() method that will help you to prevent cloning.
public function __clone()
{
trigger_error('Cloning forbidden.', E_USER_ERROR);
}
I'd like to have a library class that maintains state across the same request. My use case is that I want to pass 'messages' to the class, and then call them at any time from a view. Messages can be added from any part of the application.
I had originally done this via static methods, which worked fine. However, as part of the lib, I also need to call __construct and __destruct(), which can't be done on a static class.
Here's a very simple example of what I am trying to do:
class Messages
{
private static $messages = array();
public function __construct()
{
// do something
}
public function __destruct()
{
// do something else
}
public static function add($message)
{
self::$messages[] = $message;
}
public static function get()
{
return self::$messages;
}
}
I can then add messages anywhere in my code by doing
Messages::add('a new message');
I'd like to avoid using static if at all possible (testability). I have looked at DI, but it doesn't seem appropriate, unless I'm missing something.
I could create a class (non-static) instead, but how do I then ensure that all messages are written to the same object - so that I can retrieve them all later?
What's the best way to tackle this?
I looks like you could benefit from using the Singleton pattern - it is designed for an object that must have only one instance throughout a request. Basically, you create a private constructor and a static method to retrieve the sole instance. Here is an example of a singleton that will do what you describe.
<?php
class Messages
{
private static $_instance;
private $_messages = array();
private function __construct() {
// Initialize
}
static public function instance() {
if (! self::$_instance) {
self::$_instance = new self();
}
return self::$_instance;
}
public function add_message( $msg ) {
$this->_messages[] = $message;
}
public function get_messages() {
return $this->_messages;
}
private function __destruct() {
// Tear-down
}
}
$my_messages = Messages::instance();
$my_messages->add_message( 'How now, brown cow?' );
// ...
$your_messages = Messages::instance();
$msgs = $your_messages->get_messages();
echo $your_messages[0]; // Prints, "How now, brown cow?"
Since the constructor is private, you can only create a Messages object from within a method of the object itself. Since you have a static method, instance(), you can create a new Messages instance from there. However, if an instance already exists, you want to return that instance.
Basically, a singleton is the gatekeeper to its own instance, and it stubbornly refuses to ever let more than one instance of itself exist.
Sounds like you are wanting to do a Singleton class. This will create an instance in one class and allow you to access that same instance in another class. Check out http://www.developertutorials.com/tutorials/php/php-singleton-design-pattern-050729-1050/ for more information.
How about making it a singleton class?
class Messages
{
// singleton instance of Messages
private static $instance;
public function __construct() { ... }
public static function getInstance()
{
if (!self::$instance)
{
self::$instance = new Messages();
}
return self::$instance;
}
}
This would ensure that all your messages get written to the same object, and also allow you to call __construct and __destruct
What you need is the Singleton pattern:
final class Singleton {
// static variable to store the instance
private static $instance = NULL;
// disable normal class constructing
private function __construct() {}
// instead of using the normal way to construct the class you'll use this method
public static function getInstance() {
if (NULL === self::$instance) {
self::$instance = new self;
}
return self::$instance;
}
// disable external cloning of the object
private function __clone() {}
}
// get the instance across some of your scripts
$singleton = Singleton::getInstance();
Sounds a bit like you want a singleton, although as an anti-pattern I'd avoid it.
You could do a full static class where every static member calls a self::_isBuilt(); method to do your construct elements. Destruct is a little trickier.
The best case for your needs might be a normal (non-static) class that you build right away and then access from a global... not super neat, but allows construct/destruct and members, and your statics to use $this which could be helpful. If you don't like the global variable, you could also wrap it in a method (a trick used in JS a fair bit) but it's not really any neater.
As a normal global class:
$myClass=new myClass();
//Access anywhere as:
globals['myClass']->myFunction(..);
Wrapped in a function
function my_class() {
static $var=null;
if ($var===null) $var=new myClass();
return $var;
}
//Access anywhere as:
my_class()->myFunction(..);
Let's imagine that we have Registry pattern...
<?php
class Registry
{
private static $objects = array();
private static $instance = null;
public static function getInstance() {
if (self::$instance == null) {
self::$instance = new Registry();
}
return self::$instance;
}
protected function _get($key) {
return ($this->objects[$key]) ? $this->objects[$key] : null;
}
protected function _set($key, $val) {
$this->objects[$key] = $val;
}
public static function get($key) {
return self::getInstance()->_get($key);
}
public static function set($key, $object) {
return self::getInstance()->_set($key, $object);
}
}
?>
Using this realization is really easy...
<?
Registry::set('db', $db_client);
Registry::set('redis', $redis_client);
//Using registered objects is really easy
Registry::get('db')->query("...");
Registry::get('redis')->get("...");
?>
But as you can see, we're adding instances into registry even if we don't need them (yes, it's all about performance).
So, the question is... How to modify Registry pattern to be able to do lazy instantiation?
Here is what I'm looking for...
<?
class Registry
{
private static $objects = array();
private static $instance = null;
public static function getInstance() {
if (self::$instance == null) {
self::$instance = new Registry();
}
return self::$instance;
}
protected function _db() {
if (!$this->objects['db']) {
$this->objects['db'] = new DatabaseAdapter(DB_HOST, DB_NAME, DB_USER, DB_PASSWORD);
}
return $this->objects['db'];
}
protected function _redis() {
if (!$this->objects['redis']) {
$this->objects['redis'] = new Redis(REDIS_HOST, REDIS_DB, REDIS_USER, REDIS_PASSWORD);
}
return $this->objects['redis'];
}
public static function db() {
return self::getInstance()->_db();
}
public static function redis() {
return self::getInstance()->_redis();
}
}
?>
As you can see, DatabaseAdapter() or Redis() will be created only in we'll request them. Everything seems to be ok, but as you can see it's not a standalone class because _db(), _redis() methods contains connection constants etc.
How to avoid it? How can I define registry method within registry class to separate Registy class and objects inside it?
I'm really sorry about my English, but I hope it is clear for you.
Thank you.
PS: All code above was written 1 min. ago and wasn't tested.
If you use global constants you will always have a dependency on the global scope. It doesnt matter where it is. Also, even if you do not use constants, you still have the dependency on the Database class inside the Registry. If you want to dissolve those dependencies, you could use Factory methods on the to be created classes:
public function get($service)
{
if( !this->_data[$service] ) {
// requires PHP 5.2.3
this->_data[$service] = call_user_func($service .'::create');
}
return this->_data[$service];
}
So if you do get('DB'), the code would try to call the static DB::create() method inside the class you intend to create. But like I said, if you use global Constants for the configuration, you would just move the problem into another class.
Your db class could look like this:
class DB
{
protected static $_config;
public static setConfig(array $config)
{
self::_config = $config;
}
public static create()
{
return new self(
self::config['host'],
self::config['db'],
self::config['user'],
self::config['pass']);
}
}
The configuration can be stored inside an external configuration file, which you load and set to the DB class during bootstrap, e.g.
DB::setConfig(parse_ini_file('/path/to/db-config.ini'));
The disadvantage of this is, you have to add create() methods all over the place and all classes must be able to store their own configuration. You could centralize these responsibilities into a Builder pattern. But if you do this, you are half way to implementing an IoC Container anyways, so check out the following resources:
Fabien Potencier: What is Dependency Injection
Martin Fowler: Inversion of Control Containers and the Dependency Injection pattern
Design pattern – Inversion of control and Dependency injection
Note: You are using a "static" modifier for $objects - as you are working with an instance, this is probaby not necessary.
How can I define registry method within registry class to separate Registy class and objects inside it?
They are always separate: Each object inside the registry class is just a reference to the (independent) object. But if this question is about including the appropriate class definition (?) you may use the class_exists() function to load the class as soon as required.
BurninLeo