嵌套类与其他类的关系与外部类略有不同。以Java为例：
非静态嵌套类可以访问封闭类的其他成员，即使它们被声明为私有的也是如此。同样，非静态嵌套类要求实例化父类的实例。
OuterClass outerObj = new OuterClass(arguments);
outerObj.InnerClass innerObj = outerObj.new InnerClass(arguments);
使用它们有几个令人信服的原因：
如果一个类仅对其他一个类有用，那么将其关联并嵌入该类并将两者保持在一起是合乎逻辑的。
考虑两个顶级类A和B，其中B需要访问A的成员，否则将其声明为私有。通过将类B隐藏在类A中，可以将A的成员声明为私有，而B可以访问它们。另外，B本身可以对外界隐藏。
如果没有嵌套类，则在PHP中可以具有 类似的 行为。
如果您要实现的只是结构/组织，如Package.OuterClass.InnerClass，则PHP名称空间可能就足够了。您甚至可以在同一文件中声明多个名称空间（尽管由于标准的自动加载功能，可能不建议这样做）。
namespace;
class OuterClass {}
namespace OuterClass;
class InnerClass {}
如果您希望模拟其他特征，例如成员可见性，则需要花费更多的精力。
namespace {
class Package {
/* protect constructor so that objects can't be instantiated from outside
* Since all classes inherit from Package class, they can instantiate eachother
* simulating protected InnerClasses
*/
protected function __construct() {}
/* This magic method is called everytime an inaccessible method is called
* (either by visibility contrains or it doesn't exist)
* Here we are simulating shared protected methods across "package" classes
* This method is inherited by all child classes of Package
*/
public function __call($method, $args) {
//class name
$class = get_class($this);
/* we check if a method exists, if not we throw an exception
* similar to the default error
*/
if (method_exists($this, $method)) {
/* The method exists so Now we want to kNow if the
* caller is a child of our Package class. If not we throw an exception
* Note: This is a kind of a dirty way of finding out who's
* calling the method by using debug_backtrace and reflection
*/
$trace = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 3);
if (isset($trace[2])) {
$ref = new ReflectionClass($trace[2]['class']);
if ($ref->isSubclassOf(__CLASS__)) {
return $this->$method($args);
}
}
throw new \Exception("Call to private method $class::$method()");
} else {
throw new \Exception("Call to undefined method $class::$method()");
}
}
}
}
namespace Package {
class MyParent extends \Package {
public $publicChild;
protected $protectedChild;
public function __construct() {
//instantiate public child inside parent
$this->publicChild = new \Package\MyParent\PublicChild();
//instantiate protected child inside parent
$this->protectedChild = new \Package\MyParent\ProtectedChild();
}
public function test() {
echo "Call from parent -> ";
$this->publicChild->protectedMethod();
$this->protectedChild->protectedMethod();
echo "<br>Siblings<br>";
$this->publicChild->callSibling($this->protectedChild);
}
}
}
namespace Package\MyParent
{
class PublicChild extends \Package {
//Makes the constructor public, hence callable from outside
public function __construct() {}
protected function protectedMethod() {
echo "I'm ".get_class($this)." protected method<br>";
}
protected function callSibling($sibling) {
echo "Call from " . get_class($this) . " -> ";
$sibling->protectedMethod();
}
}
class ProtectedChild extends \Package {
protected function protectedMethod() {
echo "I'm ".get_class($this)." protected method<br>";
}
protected function callSibling($sibling) {
echo "Call from " . get_class($this) . " -> ";
$sibling->protectedMethod();
}
}
}
$parent = new Package\MyParent();
$parent->test();
$pubChild = new Package\MyParent\PublicChild();//create new public child (possible)
$protChild = new Package\MyParent\ProtectedChild(); //create new protected child (ERROR)
输出：
Call from parent -> I'm Package protected method
I'm Package protected method
Siblings
Call from Package -> I'm Package protected method
Fatal error: Call to protected Package::__construct() from invalid context
我真的不认为尝试在PHP中模拟innerClasses是一个好主意。我认为代码不太清晰易读。另外，可能还有其他方法可以使用完善的模式来获得相似的结果，例如观察者，装饰或合并模式。有时，即使是简单的继承也已足够。
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