PHP面向对象深入研究之【高级特性】

静态属性

<?php
class StaticExample {
    static public $aNum = 0; // 静态共有属性
    static public function sayHello() { // 静态共有方法
        print "hello";
    }
}

print StaticExample::$aNum;
StaticExample::sayHello();
?>
输出：0	hello

点评：静态属性和方法，可以通过类直接调用。

SELF

<?php

class StaticExample {
    static public $aNum = 0;
    static public function sayHello() { // 这里的static 和 public的顺序可以颠倒
        self::$aNum++;
        print "hello (".self::$aNum.")
"; // self 指向当前类, $this指向当前对象。
    }
}

StaticExample::sayHello();
StaticExample::sayHello();
StaticExample::sayHello();
?>
输出：
hello (1)
hello (2)
hello (3)

点评：self 指向当前类, (this指向当前对象。self可以调用当前类的静态属性和方法。)this可以调用当前类的正常属性和方法。

常量属性

<?php
class ShopProduct {
    const AVAILABLE      = 0; // 只能用大写字母命名常量
    const OUT_OF_STOCK   = 1;
    public $status;
}
print ShopProduct::AVAILABLE;
?>
输出：0

点评：常量只能用大写字母，并且可以通过类直接调用。

接口

<?php
interface Chargeable { // 接口,抽象类是介于基类与接口之间的东西
    public function getPrice();
}

class ShopProduct implements Chargeable {
    // ...
    protected $price;
    // ...

    public function getPrice() {
        return $this->price;
    }
    // ...

}

$product = new ShopProduct();

?>

如果没有实现getPrice方法，将会报错。
Fatal error: Class ShopProduct contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Chargeable::getPrice)

继承类与接口

<?php
class TimedService{ }
interface Bookable{ }
interface Chargeable{ }

class Consultancy extends TimedService implements Bookable, Chargeable { // 继承类与接口
    // ...
}
?>

抽象类

先来看一段代码

<?php

abstract class DomainObject {

}

class User extends DomainObject {
    public static function create() {
        return new User();        
    }
}

class Document extends DomainObject {
    public static function create() {
        return new Document();        
    }
}

$document = Document::create();
print_r( $document );
?>
输出：
Document Object
(
)

静态方法

<?php
abstract class DomainObject {
    private $group; // 私有属性group
    public function __construct() {
        $this->group = static::getGroup();//static 静态类
    }

    public static function create() {
        return new static();        
    }

    static function getGroup() { // 静态方法
        return "default"; 
    }
}

class User extends DomainObject {
}

class Document extends DomainObject {
    static function getGroup() { // 改变了内容
        return "document"; 
    }
}

class SpreadSheet extends Document { // 继承之后，group也就与document相同了
}

print_r(User::create());
print_r(SpreadSheet::create());
?>
输出：
User Object
(
    [group:DomainObject:private] => default
)
SpreadSheet Object
(
    [group:DomainObject:private] => document
)

final字段

使类无法被继承，用的不多

<?php
final class Checkout { // 终止类的继承
    // ...
}

class IllegalCheckout extends Checkout {
    // ...
}

$checkout = new Checkout();

?>
输出：
Fatal error: Class IllegalCheckout may not inherit from final class (Checkout)

final方法不能够被重写

<?php
class Checkout {
    final function totalize() {
        // calculate bill
    }
}


class IllegalCheckout extends Checkout {
    function totalize() { // 不能重写final方法
        // change bill calculation
    }
}

$checkout = new Checkout();

?>
输出：
Fatal error: Cannot override final method Checkout::totalize() 

析构函数

<?php
class Person {
    protected $name;    
    private $age;    
    private $id;    

    function __construct( $name, $age ) {
        $this->name = $name;
        $this->age  = $age;
    }

    function setId( $id ) {
        $this->id = $id;
    }
    
    function __destruct() { // 析构函数
        if ( ! empty( $this->id ) ) {
            // save Person data
            print "saving person
";
        }

        if ( empty( $this->id ) ) {
            // save Person data
            print "do nothing
";
        }
    }
}

$person = new Person( "bob", 44 );
$person->setId( 343 );
$person->setId( '' ); // 最后执行析构函数，使用完之后执行

?>
输出：
do nothing

__clone方法

克隆的时候执行

<?php
class Person {
    private $name;    
    private $age;    
    private $id;    

    function __construct( $name, $age ) {
        $this->name = $name;
        $this->age = $age;
    }

    function setId( $id ) {
        $this->id = $id;
    }
    
    function __clone() { // 克隆时候执行
        $this->id = 0;
    }
}

$person = new Person( "bob", 44 );
$person->setId( 343 );
$person2 = clone $person;
print_r( $person );
print_r( $person2 );

?>
输出：
Person Object
(
    [name:Person:private] => bob
    [age:Person:private] => 44
    [id:Person:private] => 343
)
Person Object
(
    [name:Person:private] => bob
    [age:Person:private] => 44
    [id:Person:private] => 0
)

再看一个例子

<?php
class Account { // 账户类
    public $balance; // 余额
    function __construct( $balance ) {
        $this->balance = $balance;
    }
}

class Person {
    private $name;
    private $age;
    private $id;
    public $account;

    function __construct( $name, $age, Account $account ) {
        $this->name = $name;
        $this->age  = $age;
        $this->account = $account;
    }

    function setId( $id ) {
        $this->id = $id;
    }

    function __clone() {
        $this->id   = 0;
    }
}

$person = new Person( "bob", 44, new Account( 200 ) ); // 以类对象作为参数
$person->setId( 343 );
$person2 = clone $person;

// give $person some money
$person->account->balance += 10;
// $person2 sees the credit too
print $person2->account->balance; // person的属性account也是一个类，他的属性balance的值是210

// output:
// 210

?>

点评：学习还是能够开拓大脑的，今天终于明白为什么有多个箭头的概念了$person->account->balance。这里的account属性是一个对象。

__toString

<?php

class Person {
    function getName()  { return "Bob"; }
    function getAge() { return 44; }
    function __toString() {
        $desc  = $this->getName()." (age ";
        $desc .= $this->getAge().")";
        return $desc;
    }
}

$person = new Person();
print $person; // 打印时候集中处理
// Bob (age 44)
?>

点评：必须是print或echo时才有效，print_r就输出对象。
Person Object()