看PHP在内部迭代的动作

以下我们来了解怎样实现一个自己定义的迭代器，然后再開始慢慢理解迭代器的内部工作原理。先来看一个官方的样例：

<?

php
class myIterator implements Iterator {
private $position = 0;
private $array = array(
"first_element",
"second_element",
"last_element",
);

public function __construct() {
$this->position = 0;
}

function rewind() {
var_dump(__METHOD__);
$this->position = 0;
}

function current() {
var_dump(__METHOD__);
return $this->array[$this->position];
}

function key() {
var_dump(__METHOD__);
return $this->position;
}

function next() {
var_dump(__METHOD__);
++$this->position;
}

function valid() {
var_dump(__METHOD__);
return isset($this->array[$this->position]);
}
}

$it = new myIterator;

foreach($it as $key => $value) {
echo '输出键值：';
var_dump($key, $value);
//echo $key;
echo "\n";
}

程序执行输出：

string(18) "myIterator::rewind"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
输出键值：int(0)
string(13) "first_element"

string(16) "myIterator::next"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
输出键值：int(1)
string(14) "second_element"

string(16) "myIterator::next"
string(17) "myIterator::valid"
string(19) "myIterator::current"
string(15) "myIterator::key"
输出键值：int(2)
string(12) "last_element"

string(16) "myIterator::next"
string(17) "myIterator::valid"

一般的迭代器内部须要以下的方法：

Iterator::current — Return the current element 返回当前元素
Iterator::key — Return the key of the current element 返回当前元素的键
Iterator::next — Move forward to next element 移向下一个元素
Iterator::rewind — Rewind the Iterator to the first element 又一次回到第一个元素
Iterator::valid — Checks if current position is valid 检查当前位置的有效性

假设不是非常清楚迭代器的内容工作流程。能够查看以下的迭代器对数组的遍历过程：

<?

php
/**
* @author nicesunboy@gmail.com
*/
class MyIterator implements Iterator
{
private $var = array();

public function __construct($array)
{
if (is_array($array)) {
$this->var = $array;
}
}

public function rewind() {
echo "倒回第一个元素\n";
reset($this->var);
}

public function current() {
$var = current($this->var);
echo "当前元素: $var\n";
return $var;
}

public function key() {
$var = key($this->var);
echo "当前元素的键: $var\n";
return $var;
}

public function next() {
$var = next($this->var);
echo "移向下一个元素: $var\n";
return $var;
}

public function valid() {
$var = $this->current() !== false;
echo "检查有效性: {$var}\n";
return $var;
}
}

$values = array(1,2,3);
$it = new MyIterator($values);

foreach ($it as $k => $v) {
print "此时键值对 -- key $k: value $v\n\n";
}

程序执行结果：

倒回第一个元素
当前元素: 1
检查有效性: 1
当前元素: 1
当前元素的键: 0
此时键值对 -- key 0: value 1

移向下一个元素: 2
当前元素: 2
检查有效性: 1
当前元素: 2
当前元素的键: 1
此时键值对 -- key 1: value 2

移向下一个元素: 3
当前元素: 3
检查有效性: 1
当前元素: 3
当前元素的键: 2
此时键值对 -- key 2: value 3

移向下一个元素:
当前元素:
检查有效性:

今天，它是很清楚？