PHP面向对象中的重要知识点(二)

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:
coffee: processing
     count:
     high price reached: 

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。