面向对象魔术常量
Posted 一阙梅曲香素笺
tags:
篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了面向对象魔术常量相关的知识，希望对你有一定的参考价值。
1. __construct:
内置构造函数，在对象被创建时自动调用。见如下代码：
<?php class ConstructTest { private $arg1; private $arg2; public function __construct($arg1, $arg2) { $this->arg1 = $arg1; $this->arg2 = $arg2; print "__construct is called...\n"; } public function printAttributes() { print ‘$arg1 = ‘.$this->arg1.‘ $arg2 = ‘.$this->arg2."\n"; } } $testObject = new ConstructTest("arg1","arg2"); $testObject->printAttributes();
运行结果如下：
Stephens-Air:Desktop$ php Test.php __construct is called... $arg1 = arg1 $arg2 = arg2
2. parent:
用于在子类中直接调用父类中的方法，功能等同于Java中的super。
<?php
class BaseClass {
protected $arg1;
protected $arg2;
function __construct($arg1, $arg2) {
$this->arg1 = $arg1;
$this->arg2 = $arg2;
print "__construct is called...\n";
}
function getAttributes() {
return ‘$arg1 = ‘.$this->arg1.‘ $arg2 = ‘.$this->arg2;
}
}
class SubClass extends BaseClass {
protected $arg3;
function __construct($baseArg1, $baseArg2, $subArg3) {
parent::__construct($baseArg1, $baseArg2);
$this->arg3 = $subArg3;
}
function getAttributes() {
return parent::getAttributes().‘ $arg3 = ‘.$this->arg3;
}
}
$testObject = new SubClass("arg1","arg2","arg3");
print $testObject->getAttributes()."\n";
运行结果如下：
Stephens-Air:Desktop$ php Test.php __construct is called... $arg1 = arg1 $arg2 = arg2 $arg3 = arg3
3. self:
在类内调用该类静态成员和静态方法的前缀修饰，对于非静态成员变量和函数则使用this。
<?php
class StaticExample {
static public $arg1 = "Hello, This is static field.\n";
static public function sayHello() {
print self::$arg1;
}
}
print StaticExample::$arg1;
StaticExample::sayHello();
运行结果如下：
Stephens-Air:Desktop$ php Test.php Hello, This is static field. Hello, This is static field.
4. static:
这里介绍的static关键字主要用于PHP 5.3以上版本新增的延迟静态绑定功能。请看一下代码和关键性注释。
<?php
abstract class Base {
public static function getInstance() {
//这里的new static()实例化的是调用该静态方法的当前类。
return new static();
}
abstract public function printSelf();
}
class SubA extends Base {
public function printSelf() {
print "This is SubA::printSelf.\n";
}
}
class SubB extends Base {
public function printSelf() {
print "This is SubB::printSelf.\n";
}
}
SubA::getInstance()->printSelf();
SubB::getInstance()->printSelf();
运行结果如下：
Stephens-Air:Desktop$ php Test.php This is SubA::printSelf. This is SubB::printSelf.
static关键字不仅仅可以用于实例化。和self和parent一样，static还可以作为静态方法调用的标识符，甚至是从非静态上下文中调用。在该场景下，self仍然表示的是当前方法所在的类。见如下代码：
<?php
abstract class Base {
private $ownedGroup;
public function __construct() {
//这里的static和上面的例子一样，表示当前调用该方法的实际类。
//需要另外说明的是，这里的getGroup方法即便不是静态方法，也会得到相同的结果。然而倘若
//getGroup真的只是普通类方法，那么这里还是建议使用$this。
$this->ownedGroup = static::getGroup();
}
public function printGroup() {
print "My Group is ".$this->ownedGroup."\n";
}
public static function getInstance() {
return new static();
}
public static function getGroup() {
return "default";
}
}
class SubA extends Base {
}
class SubB extends Base {
public static function getGroup() {
return "SubB";
}
}
SubA::getInstance()->printGroup();
SubB::getInstance()->printGroup();
运行结果如下：
Stephens-Air:Desktop$ php Test.php My Group is default My Group is SubB
5. __destruct:
析构方法的作用和构造方法__construct刚好相反，它只是在对象被垃圾收集器收集之前自动调用，我们可以利用该方法做一些必要的清理工作。
<?php
class TestClass {
function __destruct() {
print "TestClass destructor is called.\n";
}
}
$testObj = new TestClass();
unset($testObj);
print "Application will exit.\n";
运行结果如下：
Stephens-Air:Desktop$ php Test.php TestClass destructor is called. Application will exit.
6. __clone:
在PHP 5之后的版本中，对象之间的赋值为引用赋值，即赋值后的两个对象将指向同一地址空间，如果想基于对象赋值，可以使用PHP提供的clone方法。该方法将当前对象浅拷贝之后的副本返回，如果想在clone的过程中完成一些特殊的操作，如深拷贝，则需要在当前类的声明中实现__clone方法，该方法在执行clone的过程中会被隐式调用。另外需要格外注意的是，__clone方法是作用再被拷贝的对象上，即赋值后的对象上执行。
<?php
class InnerClass {
public $id = 10;
public function printSelf() {
print ‘$id = ‘.$this->id."\n";
}
}
class OuterClass {
public $innerClass;
public function __construct() {
$this->innerClass = new InnerClass();
}
public function __clone() {
$this->innerClass = clone $this->innerClass;
print "__clone is called.\n";
}
}
$outerA = new OuterClass();
print "Before calling to clone.\n";
$outerB = clone $outerA;
print "After calling to clone.\n";
$outerA->innerClass->id = 20;
print "In outerA: ";
$outerA->innerClass->printSelf();
print "In outerB: ";
$outerB->innerClass->printSelf();
运行结果如下：
Stephens-Air:Desktop$ php Test.php Before calling to clone. __clone is called. After calling to clone. In outerA: $id = 20 In outerB: $id = 10
7. const:
PHP5可以在类中定义常量属性。和全局常量一样，一旦定义就不能改变。常量属性不需要像普通属性那样以$开头，按照惯例，只能用大写字母来命名常量。另外和静态属性一样，只能通过类而不能通过类的实例访问常量属性，引用常量时同样也不需要以$符号作为前导符。另外常量只能被赋值为基础类型，如整型，而不能指向任何对象类型。
<?php
class TestClass {
const AVAILABLE = 0;
}
print "TestClass::AVAILABLE = ".TestClass::AVAILABLE."\n";
运行结果如下：
0Stephens-Air:Desktop$ php Test.php TestClass::AVAILABLE = 0
注：该Blog中记录的知识点，是在我学习PHP的过程中，遇到的一些PHP和其他面向对象语言相比比较特殊的地方，或者是对我本人而言确实需要簿记下来以备后查的知识点。虽然谈不上什么深度，但还是希望能与大家分享。
1. __toString:
当对象被打印时，如果该类定义了该方法，则打印该方法的返回值，否则将按照PHP的缺省行为输出打印结果。该方法类似于Java中的toString()。
<?php
class TestClass {
public function __toString() {
return "This is TestClass::__toString.\n";
}
}
$testObj = new TestClass();
print $testObj;
运行结果如下：
Stephens-Air:Desktop$ php Test.php This is TestClass::__toString.
2. __get和__set:
这两个方法用于处理类中未声明的属性访问。当对象使用者试图访问未声明的对象属性时，__get()会被调用，并带有一个包含要访问的属性名称字符串作为参数。无论从__get()方法返回什么，都会直接返回给调用者，就如同带有该值的属性存在一样。另外需要注意的是，如果属性存在，但是其访问可见性为private或protected，那么这两个拦截方法同样会被调用，反之，如果属性存在切可访问，那么直接访问属性即可，这两个方法将不再会被调用。以下为__get()拦截方法的示例代码：
<?php
class TestClass {
private $privateField;
public $publicField;
public function __construct() {
$this->privateField = "This is a private Field.\n";
$this->publicField = "This is a public Field.\n";
}
public function __get($property) {
print "__get() is called.\n";
$method = "get${property}";
if (method_exists($this, $method)) {
return $this->$method();
}
return "This is undefined field.\n";
}
public function getPrivateField() {
return $this->privateField;
}
}
$testObj = new TestClass();
print $testObj->privateField;
print $testObj->undefinedField;
print $testObj->publicField;
运行结果如下：
Stephens-Air:Desktop$ php Test.php __get() is called. This is a private Field. __get() is called. This is undefined field. This is a public Field.
__set()方法被调用的规则和__get()基本相同，差别是用于拦截未定义或不可见类属性的赋值操作。另外，该方法接收两个参数，分别是属性名称和要设定的值。见如下代码示例：
<?php
class TestClass {
private $privateField;
public $publicField;
public function __construct() {
$this->privateField = "This is a private Field.\n";
$this->publicField = "This is a public Field.\n";
}
public function __get($property) {
print "__get() is called.\n";
$method = "get${property}";
if (method_exists($this, $method)) {
return $this->$method();
}
return "This is an undefined field.\n";
}
public function __set($property, $value) {
print "__set is called.\n";
$method = "set${property}";
if (method_exists($this, $method)) {
$this->$method($value);
} else {
print "This is an undefined field.\n";
}
}
public function getPrivateField() {
return $this->privateField;
}
public function setPrivateField($value) {
$this->privateField = $value;
}
}
$testObj = new TestClass();
$testObj->privateField = "This is a private Field after set.\n";
$testObj->undefinedField = "This is a undefined Field after set.\n";
$testObj->publicField = "This is a public Field after set.\n";
print $testObj->privateField;
print $testObj->undefinedField;
print $testObj->publicField;
运行结果如下：
Stephens-Air:Desktop$ php Test.php __set is called. __set is called. This is an undefined field. __get() is called. This is a private Field after set. __get() is called. This is an undefined field. This is a public Field after set.
3. __isset和__unset:
这两个拦截方法被调用的规则和__get()和__set()非常类似，只是用于类中不存在或不可见属性被isset()和unset()两个全局方法应用时才会被分别触发。
<?php
class TestClass {
private $privateField;
public $publicField;
public function __construct() {
$this->privateField = "Defined private field";
$this->publicField = "Defined public field";
}
public function __isset($property) {
print "__isset is called.\n";
return isset($this->$property);
}
public function __unset($property) {
print "__unset is called.\n";
if (isset($this->$property)) {
unset($this->$property);
}
}
}
$testObj = new TestClass();
print ‘isset($testObj->privateField) is ‘.(isset($testObj->privateField) ? "true" : "false")."\n";
print ‘isset($testObj->undefinedField) is ‘.(isset($testObj->undefinedField) ? "true" : "false")."\n";
print ‘isset($testObj->publicField) is ‘.(isset($testObj->publicField) ? "true" : "false")."\n";
print "After unset......\n";
//下面两个函数调用后，$testObj的两个对象属性均会变为不可用。
//另外从输出结果来看，__unset方法仅仅被调用一次，因为publicField为可见属性，所以__unset不会因该属性而被调用。
unset($testObj->privateField);
unset($testObj->publicField);
print ‘isset($testObj->privateField) is ‘.(isset($testObj->privateField) ? "true" : "false")."\n";
print ‘isset($testObj->publicField) is ‘.(isset($testObj->publicField) ? "true" : "false")."\n";
运行结果如下：
Stephens-Air:Desktop$ php Test.php __isset is called. isset($testObj->privateField) is true __isset is called. isset($testObj->undefinedField) is false isset($testObj->publicField) is true After unset...... __unset is called. __isset is called. isset($testObj->privateField) is false __isset is called. isset($testObj->publicField) is false
4. __call:
__call()方法是一个非常有用但又非常容易被滥用的拦截方法。当对象使用者试图访问当前对象未定义的成员函数时，__call()会被自动调用，同时传递两个参数，分别为函数名称和传递给调用函数的所有参数(数组)。__call方法返回的任何值都会返回给函数调用者，就如同该成员函数真实存在一样。下面给出一个非常有用的委托示例。
<?php
class DelegateClass {
function printMessage($arg1, $arg2) {
print "DelegateClass:delegatedMethod is called.\n";
print ‘$arg1 = ‘.$arg1.‘and $arg2 = ‘.$arg2."\n";
}
}
class TestClass {
private $delegateObj;
public function __construct() {
$this->delegateObj = new DelegateClass();
}
public function __call($method, $args) {
$this->delegateObj->$method($args[0],$args[1]);
}
}
$testObj = new TestClass();
$testObj->printMessage("hello","world");
运行结果如下：
Stephens-Air:Desktop$ php Test.php DelegateClass:delegatedMethod is called. $arg1 = helloand $arg2 = world
从以上示例可以看出，TestClass并未声明printMessage成员方法，但是通过__call()方法的巧妙桥接直接传递给了委托对象。个人认为该技巧为双刃剑，切勿过度使用。
5. 回调函数：
回调函数的应用场景无须多述，在C/C++中充斥着无数的回调函数典型用例。 这里只是简单给出PHP中回调函数的使用规则。见如下示例代码和关键性注释：
<?php
class Product {
public $name;
public $price;
public function __construct($name, $price) {
$this->name = $name;
$this->price = $price;
}
}
class ProcessSale {
private $callbacks;
function registerCallback($cb) {
if (!is_callable($cb)) {
throw new Exception("callback not callable.");
}
$this->callbacks[] = $cb;
}
function sale($product) {
print "{$product->name}: processing \n";
foreach ($this->callbacks as $cb) {
//以下两种调用方式均可。
call_user_func($cb, $product);
$cb($product);
}
}
}
$logger = function($product) {
print " logging ({$product->name})\n";
};
$processor = new ProcessSale();
$processor->registerCallback($logger);
$processor->sale(new Product("shoes",6));
print "\n";
$processor->sale(new Product("coffee",6));
运行结果如下：
Stephens-Air:Desktop$ php Test.php
shoes: processing
logging (shoes)
logging (shoes)
coffee: processing
logging (coffee)
logging (coffee)
6. use(闭包):
在javascript中存在大量的闭包应用，PHP中的闭包则是通过use关键字来完成的。对于闭包这个概念本身而言，简要的说就是函数内的代码可以访问其父作用域中的变量。见如下示例代码和关键性注释：
<?php
class Product {
public $name;
public $price;
public function __construct($name, $price) {
$this->name = $name;
$this->price = $price;
}
}
class ProcessSale {
private $callbacks;
function registerCallback($cb) {
if (!is_callable($cb)) {
throw new Exception("callback not callable.");
}
$this->callbacks[] = $cb;
}
function sale($product) {
print "{$product->name}: processing \n";
foreach ($this->callbacks as $cb) {
$cb($product);
}
}
}
class Totalizer {
static function warnAmount($amt) {
$count = 0;
//注意这里的$amt和$count均为闭包变量，其中&$count是以引用的形式传递的，即一旦函数内部修改了该变量的值，
//那么下次再访问该闭包变量时，$count将为之前调用中修改后的值。
return function($product) use($amt, &$count) {
$count += $product->price;
print " count: $count\n";
if ($count > $amt) {
print " high price reached: {$count}\n";
}
};
}
}
$processor = new ProcessSale();
$processor->registerCallback(Totalizer::warnAmount(8));
$processor->sale(new Product("shoes",6));
$processor->sale(new Product("coffee",6));
运行结果如下：
shoes: processing
count: 6
coffee: processing
count: 12
high price reached: 12
注：该Blog中记录的知识点，是在我学习PHP的过程中，遇到的一些PHP和其他面向对象语言相比比较独特的地方，或者是对我本人而言确实需要簿记下来以备后查的知识点。虽然谈不上什么深度，但是还是希望能与大家分享。
以上是关于面向对象魔术常量的主要内容，如果未能解决你的问题，请参考以下文章