PHP: Detecting when a variables value has been cha

Question

I was wondering if there is a way to add something like a change listener to a variable. The simplest example of what I mean would work something along these lines;

// Start with a variable
$variable = "some value";

// Define a listener
function myChangeListener($variable) {
    // encode with json_encode and send in cookie
}

// Add my listener to the variable
addListenerToVariable(&$variable, 'myChangeListener');

// Change the variables value, triggering my listener
$variable = "new value";

Now you're probably asking why in the world I would even need to bother with this approach, why not just make a function to set the cookie. The answer is I've got a &__get($var) magic method that returns a reference to a multi-dimensional array element, and I am hoping to use something like this to detect when/if the array element or one of its children has been edited, then send a cookie if it has. The hoped for result would work something like this;

$cookies->testArray["test"] = "newValue";
// This would send the cookie 'testArray' with the value '{"test":"newValue"}'

I honestly expect this to be completely impossible, and I apologize if I'm correct. But I just learned how to use references correctly yesterday, so I thought I'd ask before I completely write off the idea.

Thanks for any responses I get, be they what I was hoping for or what I expected.

EDIT:

For added clarity, here's an example class for what I'm trying to accomplish;

class MyCookies {
    private $cookies = array();
    private $cookieTag = "MyTag";

    public function __construct() {
        foreach($_COOKIE as $cookie => $value) {
            if(strlen($cookie)>strlen($this->cookieTag."_")&&substr($cookie,0,strlen($this->cookieTag."_"))==$this->cookieTag."_") {
                $cookieVar = substr($cookie,strlen($this->cookieTag."_"));
                $this->cookies[$cookieVar]['value'] = json_decode($value);
            }
        }
    }

    // This works great for $cookies->testArray = array("testKey" => "testValue");
    // but never gets called when you do $cookies->testArray['testKey'] = "testValue";
    public function __set($var, $value) {
        if(isset($value)) {
            $this->cookies[$var]['value'] = $value;
            setcookie($this->cookieTag.'_'.$var,json_encode($value),(isset($this->cookies[$var]['expires'])?$this->cookies[$var]['expires']:(time()+2592000)),'/','');
        } else {
            unset($this->cookies[$var]);
            setcookie($this->cookieTag.'_'.$var,'',(time()-(172800)),'/','');
        }
        return $value;
    }

    // This gets called when you do $cookies->testArray['testKey'] = "testValue";
    public function &__get($var) {
        // I want to add a variable change listener here, that gets triggered
        // when the references value has been changed.

        // addListener(&$this->config[$var]['value'], array(&$this, 'changeListener'));

        return $this->config[$var]['value'];
    }

    /*
    public function changeListener(&$reference) {
        // scan $this->cookies, find the variable that $reference is the reference to (don't know how to do that ether)
        // send cookie
    }
    */

    public function __isset($var) {
        return isset($this->cookies[$var]);
    }

    public function __unset($var) {
        unset($this->cookies[$var]);
        setcookie($this->cookieTag.'_'.$var,'',(time()-(172800)),'/','');
    }

    public function setCookieExpire($var, $value, $expire=null) {
        if(!isset($expire)) {
            $expire = $value;
            $value = null;
        }
        if($expire<time()) $expire = time() + $expire;
        if(isset($value)) $this->cookies[$var]['value'] = $value;
        $this->cookies[$var]['expires'] = $expire;
        setcookie($this->cookieTag.'_'.$var,json_encode((isset($value)?$value:(isset($this->cookies[$var]['value'])?$this->cookies[$var]['value']:''))),$expire,'/','');
    }
}

As for why I don't want to have an update function, it's really just personal preference. This is going to be used in a framework that other people can expand upon, and I think having them be able to treat the cookies as just variables in single lines of code feels slicker.

Answer 1

I think you have some error in your thinking because they only way a variable can change its value in php is if you change it. Therefore, when you change your variable, run your code.

If you need a generic way of doing this, I would suggest encapsulating either the array or its values into a class. But there really is no good reason you can't just run sendCookie() after each value change.

Change listeners are for multi-threaded programming I think.

Answer 2

How about never changing the variable at all.

How about only accessing the variable through a function (or class method) that can handle whatever "listening" tasks you wanted to perform?

Why must you do: $variable = 5; ?

When you could do: setMyVariable(5) ?

function setMyVariable($new_value) {
  $variable = $new_value;
  // do other stuff

}

Answer 3

If you want to implement own event handlers / triggers in array or anything else, then you might use class wrapper.

For one variable __get() and __set() magic methods are enough, as you noticed. For array there is a better handler: ArrayAccess. It allows to emulate array actions with class methods and common array semantics (like collections in some other languages).

<?php
header('Content-Type: text/plain');

// ArrayAccess for Array events
class TriggerableArray implements ArrayAccess {
    protected $array;      // container for elements
    protected $triggers;   // callables to actions

    public function __construct(){
        $this->array    = array();

        // predefined actions, which are availible for this class:
        $this->triggers = array(
            'get'    => null,     // when get element value
            'set'    => null,     // when set existing element's value
            'add'    => null,     // when add new element
            'exists' => null,     // when check element with isset()
            'unset'  => null      // when remove element with unset()
        );
    }

    public function __destruct(){
        unset($this->array, $this->triggers);
    }

    public function offsetGet($offset){
        $result = isset($this->array[$offset]) ? $this->array[$offset] : null;

        // fire "get" trigger
        $this->trigger('get', $offset, $result);

        return $result;
    }

    public function offsetSet($offset, $value){
        // if no offset provided
        if(is_null($offset)){
            // fire "add" trigger
            $this->trigger('add', $offset, $value);

            // add element
            $this->array[] = $value;
        } else {
            // if offset exists, then it is changing, else it is new offset
            $trigger = isset($this->array[$offset]) ? 'set' : 'add';

            // fire trigger ("set" or "add")
            $this->trigger($trigger, $offset, $value);

            // add or change element
            $this->array[$offset] = $value;
        }
    }

    public function offsetExists($offset){
        // fire "exists" trigger
        $this->trigger('exists', $offset);

        // return value
        return isset($this->array[$offset]);
    }

    public function offsetUnset($offset){
        // fire "unset" trigger
        $this->trigger('unset', $offset);

        // unset element
        unset($this->array[$offset]);
    }

    public function addTrigger($trigger, $handler){
        // if action is not availible and not proper handler provided then quit
        if(!(array_key_exists($trigger, $this->triggers) && is_callable($handler)))return false;

        // assign new trigger
        $this->triggers[$trigger] = $handler;

        return true;
    }

    public function removeTrigger($trigger){
        // if wrong trigger name provided then quit
        if(!array_key_exists($trigger, $this->triggers))return false;

        // remove trigger
        $this->triggers[$trigger] = null;

        return true;
    }

    // call trigger method:
    // first arg  - trigger name
    // other args - trigger arguments
    protected function trigger(){
        // if no args supplied then quit
        if(!func_num_args())return false;

        // here is supplied args
        $arguments  = func_get_args();

        // extract trigger name
        $trigger    = array_shift($arguments);

        // if trigger handler was assigned then fire it or quit
        return is_callable($this->triggers[$trigger]) ? call_user_func_array($this->triggers[$trigger], $arguments) : false;
    }
}

function check_trigger(){
    print_r(func_get_args());
    print_r(PHP_EOL);
}

function check_add(){
    print_r('"add" trigger'. PHP_EOL);
    call_user_func_array('check_trigger', func_get_args());
}

function check_get(){
    print_r('"get" trigger'. PHP_EOL);
    call_user_func_array('check_trigger', func_get_args());
}

function check_set(){
    print_r('"set" trigger'. PHP_EOL);
    call_user_func_array('check_trigger', func_get_args());
}

function check_exists(){
    print_r('"exists" trigger'. PHP_EOL);
    call_user_func_array('check_trigger', func_get_args());
}

function check_unset(){
    print_r('"unset" trigger'. PHP_EOL);
    call_user_func_array('check_trigger', func_get_args());
}

$victim = new TriggerableArray();

$victim->addTrigger('get', 'check_get');
$victim->addTrigger('set', 'check_set');
$victim->addTrigger('add', 'check_add');
$victim->addTrigger('exists', 'check_exists');
$victim->addTrigger('unset', 'check_unset');

$victim['check'] = 'a';
$victim['check'] = 'b';

$result = $victim['check'];

isset($victim['check']);
unset($victim['check']);

var_dump($result);
?>

Shows:

"add" trigger
Array
(
    [0] => check
    [1] => a
)

"set" trigger
Array
(
    [0] => check
    [1] => b
)

"get" trigger
Array
(
    [0] => check
    [1] => b
)

"exists" trigger
Array
(
    [0] => check
)

"unset" trigger
Array
(
    [0] => check
)

string(1) "b"

---

I assume, that you might modify constructor in this code with array reference argument to be able to pass there superglobal array, like $_COOKIE. Another way is to replace $this->array with $_COOKIE directly within class. Also, there is a way to replace callables with anonymous functions.