PHP面向对象中的重要知识点(二)_PHP教程-PHP-Tutorial-php.cn

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()。

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class TestClass {

public function __toString() {

return "This is TestClass::__toString.\n";

}

$testObj = new TestClass();

print $testObj;

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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：

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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;

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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：

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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;

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运行结果如下：

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Stephens-Air:Desktop$ php Test.php

__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.

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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。

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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";

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运行结果如下：

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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

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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。

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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");

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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：

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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));

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运行结果如下：

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Stephens-Air:Desktop$ php Test.php

shoes: processing

logging (shoes)

coffee: processing

logging (coffee)

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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：

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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));

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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。