Important knowledge points in PHP object-oriented (2)

1. __toString:

When the object is printed, if the class defines this method, the return value of the method will be printed, otherwise the printing result will be output according to PHP's default behavior. This method is similar to toString() in Java.

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class TestClass {

public function __toString() {

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

}

$testObj = new TestClass();

print $testObj;

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The running results are as follows:

Stephens-Air:Desktop$ php Test.php

This is TestClass::__toString.

2. __get and __set:

These two methods are used to handle undeclared attribute access in the class. When an object user attempts to access an undeclared object property, __get() is called with a string containing the name of the property to be accessed as an argument. Whatever is returned from the __get() method is returned directly to the caller, as if a property with that value existed. In addition, it should be noted that if the attribute exists but its access visibility is private or protected, then these two interception methods will also be called. On the contrary, if the attribute exists and is accessible, then the attribute can be accessed directly. These two The method will no longer be called. The following is a sample code for the __get() interception method:

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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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The running results are as follows:

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.

The rules for calling the __set() method are basically the same as __get(). The difference is that it is used to intercept assignment operations of undefined or invisible class attributes. In addition, this method receives two parameters, namely the attribute name and the value to be set. See the following code example:

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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:

The __call() method is a very useful but very easy to abuse interception method. When an object user attempts to access an undefined member function of the current object, __call() will be automatically called, passing two parameters, namely the function name and all parameters (array) passed to the calling function. Any value returned by the __call method will be returned to the function caller as if the member function actually existed. A very useful delegate example is given below.

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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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The running results are as follows:

Stephens-Air:Desktop$ php Test.php

DelegateClass:delegatedMethod is called.

$arg1 = helloand $arg2 = world

As can be seen from the above example, TestClass does not declare the printMessage member method, but it is passed directly to the delegate object through the clever bridging of the __call() method. I personally think this technique is a double-edged sword and should not be overused.

5. Callback function:

There is no need to describe the application scenarios of callback functions. There are countless typical use cases of callback functions in C/C++. Here we simply give the rules for using callback functions in PHP. See the following sample code and key comments:

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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) {

//The following two calling methods are available.

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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The running results are as follows:

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Stephens-Air:Desktop$ php Test.php

shoes: processing

logging (shoes)

coffee: processing

logging (coffee)

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6. use(closure):

There are a large number of closure applications in Javascript, and closures in PHP are implemented through the use keyword. As for the concept of closure itself, simply speaking, code within a function can access variables in its parent scope. See the following sample code and key comments:

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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: $countn";

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。

Related labels：

php object-oriented

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