我正在检查一些PHP 5.3.0的特性，在网站上看到了一些看起来很有趣的代码:
public function getTotal($tax)
{
$total = 0.00;
$callback =
/* This line here: */
function ($quantity, $product) use ($tax, &$total)
{
$pricePerItem = constant(__CLASS__ . "::PRICE_" .
strtoupper($product));
$total += ($pricePerItem * $quantity) * ($tax + 1.0);
};
array_walk($this->products, $callback);
return round($total, 2);
}
作为匿名函数的例子之一。
有人知道吗?文档吗?它看起来很邪恶，应该被使用吗?
Zupa很好地解释了带“use”的闭包，以及EarlyBinding和引用“used”变量之间的区别。
所以我做了一个早期绑定变量(=复制)的代码示例:
<?php
$a = 1;
$b = 2;
$closureExampleEarlyBinding = function() use ($a, $b){
$a++;
$b++;
echo "Inside \$closureExampleEarlyBinding() \$a = ".$a."<br />";
echo "Inside \$closureExampleEarlyBinding() \$b = ".$b."<br />";
};
echo "Before executing \$closureExampleEarlyBinding() \$a = ".$a."<br />";
echo "Before executing \$closureExampleEarlyBinding() \$b = ".$b."<br />";
$closureExampleEarlyBinding();
echo "After executing \$closureExampleEarlyBinding() \$a = ".$a."<br />";
echo "After executing \$closureExampleEarlyBinding() \$b = ".$b."<br />";
/* this will output:
Before executing $closureExampleEarlyBinding() $a = 1
Before executing $closureExampleEarlyBinding() $b = 2
Inside $closureExampleEarlyBinding() $a = 2
Inside $closureExampleEarlyBinding() $b = 3
After executing $closureExampleEarlyBinding() $a = 1
After executing $closureExampleEarlyBinding() $b = 2
*/
?>
引用变量的示例(注意变量前面的'&'字符);
<?php
$a = 1;
$b = 2;
$closureExampleReferencing = function() use (&$a, &$b){
$a++;
$b++;
echo "Inside \$closureExampleReferencing() \$a = ".$a."<br />";
echo "Inside \$closureExampleReferencing() \$b = ".$b."<br />";
};
echo "Before executing \$closureExampleReferencing() \$a = ".$a."<br />";
echo "Before executing \$closureExampleReferencing() \$b = ".$b."<br />";
$closureExampleReferencing();
echo "After executing \$closureExampleReferencing() \$a = ".$a."<br />";
echo "After executing \$closureExampleReferencing() \$b = ".$b."<br />";
/* this will output:
Before executing $closureExampleReferencing() $a = 1
Before executing $closureExampleReferencing() $b = 2
Inside $closureExampleReferencing() $a = 2
Inside $closureExampleReferencing() $b = 3
After executing $closureExampleReferencing() $a = 2
After executing $closureExampleReferencing() $b = 3
*/
?>
Zupa很好地解释了带“use”的闭包，以及EarlyBinding和引用“used”变量之间的区别。
所以我做了一个早期绑定变量(=复制)的代码示例:
<?php
$a = 1;
$b = 2;
$closureExampleEarlyBinding = function() use ($a, $b){
$a++;
$b++;
echo "Inside \$closureExampleEarlyBinding() \$a = ".$a."<br />";
echo "Inside \$closureExampleEarlyBinding() \$b = ".$b."<br />";
};
echo "Before executing \$closureExampleEarlyBinding() \$a = ".$a."<br />";
echo "Before executing \$closureExampleEarlyBinding() \$b = ".$b."<br />";
$closureExampleEarlyBinding();
echo "After executing \$closureExampleEarlyBinding() \$a = ".$a."<br />";
echo "After executing \$closureExampleEarlyBinding() \$b = ".$b."<br />";
/* this will output:
Before executing $closureExampleEarlyBinding() $a = 1
Before executing $closureExampleEarlyBinding() $b = 2
Inside $closureExampleEarlyBinding() $a = 2
Inside $closureExampleEarlyBinding() $b = 3
After executing $closureExampleEarlyBinding() $a = 1
After executing $closureExampleEarlyBinding() $b = 2
*/
?>
引用变量的示例(注意变量前面的'&'字符);
<?php
$a = 1;
$b = 2;
$closureExampleReferencing = function() use (&$a, &$b){
$a++;
$b++;
echo "Inside \$closureExampleReferencing() \$a = ".$a."<br />";
echo "Inside \$closureExampleReferencing() \$b = ".$b."<br />";
};
echo "Before executing \$closureExampleReferencing() \$a = ".$a."<br />";
echo "Before executing \$closureExampleReferencing() \$b = ".$b."<br />";
$closureExampleReferencing();
echo "After executing \$closureExampleReferencing() \$a = ".$a."<br />";
echo "After executing \$closureExampleReferencing() \$b = ".$b."<br />";
/* this will output:
Before executing $closureExampleReferencing() $a = 1
Before executing $closureExampleReferencing() $b = 2
Inside $closureExampleReferencing() $a = 2
Inside $closureExampleReferencing() $b = 3
After executing $closureExampleReferencing() $a = 2
After executing $closureExampleReferencing() $b = 3
*/
?>
函数()use(){}类似于PHP的闭包。
如果不使用，函数不能访问父作用域变量
$s = "hello";
$f = function () {
echo $s;
};
$f(); // Notice: Undefined variable: s
$s = "hello";
$f = function () use ($s) {
echo $s;
};
$f(); // hello
use变量的值是函数定义时的值，而不是调用时的值
$s = "hello";
$f = function () use ($s) {
echo $s;
};
$s = "how are you?";
$f(); // hello
使用变量引用&
$s = "hello";
$f = function () use (&$s) {
echo $s;
};
$s = "how are you?";
$f(); // how are you?
闭包很漂亮!它们解决了许多匿名函数带来的问题，并使真正优雅的代码成为可能(至少只要我们谈论php)。
Javascript程序员一直在使用闭包，有时甚至不知道它，因为绑定变量没有显式定义——这就是php中“use”的含义。
现实世界中有比上面的例子更好的例子。假设你必须按子值对多维数组排序，但是键值改变了。
<?php
function generateComparisonFunctionForKey($key) {
return function ($left, $right) use ($key) {
if ($left[$key] == $right[$key])
return 0;
else
return ($left[$key] < $right[$key]) ? -1 : 1;
};
}
$myArray = array(
array('name' => 'Alex', 'age' => 70),
array('name' => 'Enrico', 'age' => 25)
);
$sortByName = generateComparisonFunctionForKey('name');
$sortByAge = generateComparisonFunctionForKey('age');
usort($myArray, $sortByName);
usort($myArray, $sortByAge);
?>
警告:未经测试的代码(我没有php5.3安装atm)，但它应该看起来像这样。
这有一个缺点:如果您让许多PHP开发人员面对闭包，他们可能会有点束手无策。
为了更好地理解闭包的优点，我将给你另一个例子——这次是用javascript。其中一个问题是作用域和浏览器固有的异步性。尤其是window.setTimeout();(或间隔)。所以，你传递一个函数给setTimeout，但你不能给出任何参数，因为提供参数会执行代码!
function getFunctionTextInASecond(value) {
return function () {
document.getElementsByName('body')[0].innerHTML = value; // "value" is the bound variable!
}
}
var textToDisplay = prompt('text to show in a second', 'foo bar');
// this returns a function that sets the bodys innerHTML to the prompted value
var myFunction = getFunctionTextInASecond(textToDisplay);
window.setTimeout(myFunction, 1000);
myFunction返回一个带有预定义参数的函数!
说实话，自5.3以来我更喜欢PHP和匿名函数/闭包。名称空间可能更重要，但它们没有那么吸引人。
