PHP面向对象中的重要知识点

Posted 2022-12-06 脚本叔叔

转自:http://www.cnblogs.com/stephen-liu74/p/3498492.html

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