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PHP 单例模式讲解和代码示例
单例是一种创建型设计模式， 让你能够保证一个类只有一个实例， 并提供一个访问该实例的全局节点。
单例拥有与全局变量相同的优缺点。 尽管它们非常有用， 但却会破坏代码的模块化特性。
在某些其他上下文中， 你不能使用依赖于单例的类。 你也将必须使用单例类。 绝大多数情况下， 该限制会在创建单元测试时出现。
复杂度：******
流行度：******
使用示例： 许多开发者将单例模式视为一种反模式。 因此它在 PHP 代码中的使用频率正在逐步减少。
识别方法： 单例可以通过返回相同缓存对象的静态构建方法来识别。
概念示例
本例说明了单例设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
了解该模式的结构后， 你可以更容易地理解下面基于真实世界的 PHP 应用案例。
** index.php: 概念示例
<?php
namespace RefactoringGuru\Singleton\Conceptual;
/**
* The Singleton class defines the `GetInstance` method that serves as an
* alternative to constructor and lets clients access the same instance of this
* class over and over.
*/
class Singleton
{
/**
* The Singleton's instance is stored in a static field. This field is an
* array, because we'll allow our Singleton to have subclasses. Each item in
* this array will be an instance of a specific Singleton's subclass. You'll
* see how this works in a moment.
*/
private static $instances = [];
/**
* The Singleton's constructor should always be private to prevent direct
* construction calls with the `new` operator.
*/
protected function __construct() { }
/**
* Singletons should not be cloneable.
*/
protected function __clone() { }
/**
* Singletons should not be restorable from strings.
*/
public function __wakeup()
{
throw new \Exception("Cannot unserialize a singleton.");
}
/**
* This is the static method that controls the access to the singleton
* instance. On the first run, it creates a singleton object and places it
* into the static field. On subsequent runs, it returns the client existing
* object stored in the static field.
*
* This implementation lets you subclass the Singleton class while keeping
* just one instance of each subclass around.
*/
public static function getInstance(): Singleton
{
$cls = static::class;
if (!isset(self::$instances[$cls])) {
self::$instances[$cls] = new static();
}
return self::$instances[$cls];
}
/**
* Finally, any singleton should define some business logic, which can be
* executed on its instance.
*/
public function someBusinessLogic()
{
// ...
}
}
/**
* The client code.
*/
function clientCode()
{
$s1 = Singleton::getInstance();
$s2 = Singleton::getInstance();
if ($s1 === $s2) {
echo "Singleton works, both variables contain the same instance.";
} else {
echo "Singleton failed, variables contain different instances.";
}
}
clientCode();
** Output.txt: 执行结果
Singleton works, both variables contain the same instance.
