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Interface für externe Iteratoren oder Objekte, die selbstiterierend sind.
Beispiel #2 Basisnutzung
Dieses Beispiel demonstriert, in welcher Reihenfolge Methoden aufgerufen werden, wenn Sie foreach auf einem Iterator anwenden.
<?php
class meinIterator implements Iterator {
private $position = 0;
private $array = array(
"erstesElement",
"zweitesElement",
"letztesElement",
);
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]);
}
}
$it = new meinIterator;
foreach($it as $key => $value) {
var_dump($key, $value);
echo "\n";
}
?>
Das oben gezeigte Beispiel erzeugt eine ähnliche Ausgabe wie:
string(18) "meinIterator::rewind" string(17) "meinIterator::valid" string(19) "meinIterator::current" string(15) "meinIterator::key" int(0) string(12) "erstesElement" string(16) "meinIterator::next" string(17) "meinIterator::valid" string(19) "meinIterator::current" string(15) "meinIterator::key" int(1) string(13) "zweitesElement" string(16) "meinIterator::next" string(17) "meinIterator::valid" string(19) "meinIterator::current" string(15) "meinIterator::key" int(2) string(11) "letztesElement" string(16) "meinIterator::next" string(17) "meinIterator::valid"
For Iterators implement database queries, what I've found is that if you want to chain multiple iterators together using a "MultipleIterator" then you *do not* want ::rewind() to actually execute your query, especially if it's expensive.
Instead, what I've done is implement that portion in "valid."
e.g.
<?php
class Database_Result_Iterator {
...
private $_db_resource = null;
private $_loaded = false;
private $_valid = false;
function rewind() {
if ($this->_db_resource) {
mysql_free($this->_db_resource);
$this->_db_resource = null;
}
$this->_loaded = false;
$this->_valid = false;
}
function valid() {
if ($this->_loaded) {
$this->load();
}
return $this->_valid;
}
private function load() {
$this->_db_resource = mysql_query(...);
$this->_loaded = true;
$this->next(); // Sets _valid
}
}
?>
That way if you chain multiple queries in a "MultipleIterator" together, the "rewind" call (which rewinds all iterators at once) does not execute every query at once.
In addition, I found that the MultipleIterator may now work best for other reasons, but still, the above is a good idea to postpone queries until the last possible moment they are needed.
Order of operations when using a foreach loop:
1. Before the first iteration of the loop, Iterator::rewind() is called.
2. Before each iteration of the loop, Iterator::valid() is called.
3a. It Iterator::valid() returns false, the loop is terminated.
3b. If Iterator::valid() returns true, Iterator::current() and
Iterator::key() are called.
4. The loop body is evaluated.
5. After each iteration of the loop, Iterator::next() is called and we repeat from step 2 above.
This is roughly equivalent to:
<?php
$it->rewind();
while ($it->valid())
{
$key = $it->key();
$value = $it->current();
// ...
$it->next();
}
?>
The loop isn't terminated until Iterator::valid() returns false or the body of the loop executes a break statement.
The only two methods that are always executed are Iterator::rewind() and Iterator::valid() (unless rewind throws an exception).
The Iterator::next() method need not return anything. It is defined as returning void. On the other hand, sometimes it is convenient for this method to return something, in which case you can do so if you want.
If your iterator is doing something expensive, like making a database query and iterating over the result set, the best place to make the query is probably in the Iterator::rewind() implementation.
In this case, the construction of the iterator itself can be cheap, and after construction you can continue to set the properties of the query all the way up to the beginning of the foreach loop since the
Iterator::rewind() method isn't called until then.
Things to keep in mind when making a database result set iterator:
* Make sure you close your cursor or otherwise clean up any previous query at the top of the rewind method. Otherwise your code will break if the same iterator is used in two consecutive foreach loops when the first loop terminates with a break statement before all the results are iterated over.
* Make sure your rewind() implementation tries to grab the first result so that the subsequent call to valid() will know whether or not the result set is empty. I do this by explicitly calling next() from the end of my rewind() implementation.
* For things like result set iterators, there really isn't always a "key" that you can return, unless you know you have a scalar primary key column in the query. Unfortunately, there will be cases where either the iterator doesn't know the primary key column because it isn't providing the query, the nature of the query is such that a primary key isn't applicable, the iterator is iterating over a table that doesn't have one, or the iterator is iterating over a table that has a compound primary key. In these cases, key() can return either:
the row index (based on a simple counter that you provide), or can simply return null.
Iterators can also be used to:
* iterate over the lines of a file or rows of a CSV file
* iterate over the characters of a string
* iterate over the tokens in an input stream
* iterate over the matches returned by an xpath expression
* iterate over the matches returned by a regexp
* iterate over the files in a folder
* etc...
Here's a Fibonacci example using the formula, rather than addition.
<?php
/**
* @author Anthony Sterling
*/
class FibonacciSequence implements Iterator
{
protected
$limit = 0;
protected
$key = 0;
public function __construct($limit = 0)
{
$this->limit = (integer)$limit;
}
public function current()
{
return round(
(pow(((1 + sqrt(5)) / 2), $this->key) - pow((-1 / (1 + sqrt(5)) / 2), $this->key)) / sqrt(5),
null
);
}
public function key()
{
return $this->key;
}
public function next()
{
$this->key++;
}
public function rewind()
{
$this->key = 0;
}
public function valid()
{
return $this->key < $this->limit;
}
}
foreach(new FibonacciSequence() as $number)
{
printf(
'%d<br />',
$number
);
}
/*
0
1
1
2
3
5
8
13
21
34
55
89
144
233
377
610
987
1597
2584
4181
6765
10946
17711
28657
46368
75025
121393
196418
317811
514229
*/
?>
Anthony.
So, playing around with iterators in PHP (coming from languages where I'm spoiled with generators to do things like this), I wrote a quick piece of code to give the Fibonacci sequence (to infinity, though only the first terms up to F_{10} are output).
<?php
class Fibonacci implements Iterator {
private $previous = 1;
private $current = 0;
private $key = 0;
public function current() {
return $this->current;
}
public function key() {
return $this->key;
}
public function next() {
$newprevious = $this->current;
$this->current += $this->previous;
$this->previous = $newprevious;
$this->key++;
}
public function rewind() {
$this->previous = 1;
$this->current = 0;
$this->key = 0;
}
public function valid() {
return true;
}
}
$seq = new Fibonacci;
$i = 0;
foreach ($seq as $f) {
echo "$f\n";
if ($i++ === 10) break;
}
?>
<?php
# - Here is an implementation of the Iterator interface for arrays
# which works with maps (key/value pairs)
# as well as traditional arrays
# (contiguous monotonically increasing indexes).
# Though it pretty much does what an array
# would normally do within foreach() loops,
# this class may be useful for using arrays
# with code that generically/only supports the
# Iterator interface.
# Another use of this class is to simply provide
# object methods with tightly controlling iteration of arrays.
class tIterator_array implements Iterator {
private $myArray;
public function __construct( $givenArray ) {
$this->myArray = $givenArray;
}
function rewind() {
return reset($this->myArray);
}
function current() {
return current($this->myArray);
}
function key() {
return key($this->myArray);
}
function next() {
return next($this->myArray);
}
function valid() {
return key($this->myArray) !== null;
}
}
?>
It's important to note that following won't work if you have null values.
<?php
function valid() {
var_dump(__METHOD__);
return isset($this->array[$this->position]);
}
?>
Other examples have shown the following which won't work if you have false values:
<?php
function valid() {
return $this->current() !== false;
}
?>
Instead use:
<?php
function valid() {
return array_key_exists($this->array, $this->position);
}
?>
Or the following if you do not store the position.
<?php
public function valid() {
return !is_null(key($this->array));
}
?>
Because the purpose of the above example is to illustrate the foreach statement's call sequence I made my own example of the Iterator interface just to give other user's some idea of possible solutions that might benefit from the interface.
<?php
class MysqlResult implements Iterator
{
private $result;
private $position;
private $row_data;
public function __construct ($result)
{
$this->result = $result;
$this->position = 0;
}
public function current ()
{
return $this->row_data;
}
public function key ()
{
return $this->position;
}
public function next ()
{
$this->position++;
$this->row_data = mysql_fetch_assoc($this->result);
}
public function rewind ()
{
$this->position = 0;
mysql_data_seek($this->result, 0);
/* The initial call to valid requires that data
pre-exists in $this->row_data
*/
$this->row_data = mysql_fetch_assoc($this->result);
}
public function valid ()
{
return (boolean) $this->row_data;
}
}
$link = mysql_connect("localhost", "user");
mysql_select_db('example_db', $link);
$result = new MysqlResult(mysql_query("SELECT * FROM `example_tbl`"));
foreach ($result as $pos => $row) {
var_dump($pos, $row);
}
?>
For anyone interested in how exactly does a iterator work in a foreach:
<?php
class SomeIterator implements Iterator {/*...implement...*/}
$it=new SomeIterator();
foreach($it as $key => $val) {
print "{$key}=>{$val}\n";
}
// works exactly the same as
$it=new SomeIterator();
for(
$it->rewind();
$it->valid();
$val=$it->current(), $key=$it->key(), $it->next()
) {
print "{$key}=>{$val}\n";
}
// and if someone forgot how for(;;) works...
$it=new SomeIterator();
$it->rewind;
while($it->valid()) {
$val=$it->current();
$key=$it->key();
$it->next();
print "{$key}=>{$val}\n";
}
?>
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