生成器函数看起来像普通函数——不同的是普通函数返回一个值，而生成器可以 yield 生成多个想要的值。 任何包含 yield 的函数都是一个生成器函数。
当一个生成器被调用的时候，它返回一个可以被遍历的对象.当你遍历这个对象的时候(例如通过一个foreach循环)，PHP 将会在每次需要值的时候调用对象的遍历方法，并在产生一个值之后保存生成器的状态，这样它就可以在需要产生下一个值的时候恢复调用状态。
一旦不再需要产生更多的值，生成器可以简单退出，而调用生成器的代码还可以继续执行，就像一个数组已经被遍历完了。
Note:
生成器能够返回多个值，通过 Generator::getReturn() 可以获取到。
生成器函数的核心是yield关键字。它最简单的调用形式看起来像一个return申明，不同之处在于普通return会返回值并终止函数的执行，而yield会返回一个值给循环调用此生成器的代码并且只是暂停执行生成器函数。
Example #1 一个简单的生成值的例子
<?php
function gen_one_to_three() {
for ($i = 1; $i <= 3; $i++) {
//注意变量$i的值在不同的yield之间是保持传递的。
yield $i;
}
}
$generator = gen_one_to_three();
foreach ($generator as $value) {
echo "$value\n";
}
?>
以上例程会输出：
1 2 3
Note:
在内部会为生成的值配对连续的整型索引，就像一个非关联的数组。
传入 Generator::send() 的值会被赋值到 $data，
或者直接调用 Generator::next() 时，赋的值将是 null。
PHP的数组支持关联键值对数组，生成器也一样支持。所以除了生成简单的值，你也可以在生成值的时候指定键名。
如下所示，生成一个键值对与定义一个关联数组十分相似。
Example #2 生成一个键值对
<?php
/*
* 下面每一行是用分号分割的字段组合，第一个字段将被用作键名。
*/
$input = <<<'EOF'
1;PHP;Likes dollar signs
2;Python;Likes whitespace
3;Ruby;Likes blocks
EOF;
function input_parser($input) {
foreach (explode("\n", $input) as $line) {
$fields = explode(';', $line);
$id = array_shift($fields);
yield $id => $fields;
}
}
foreach (input_parser($input) as $id => $fields) {
echo "$id:\n";
echo " $fields[0]\n";
echo " $fields[1]\n";
}
?>
以上例程会输出：
1:
PHP
Likes dollar signs
2:
Python
Likes whitespace
3:
Ruby
Likes blocks
和之前生成简单值类型一样，在一个表达式上下文中生成键值对也需要使用圆括号进行包围：
$data = (yield $key => $value);
Yield 可以在没有参数传入的情况下被调用来生成一个 null 值并配对一个自动的键名。
Example #3 生成nulls
<?php
function gen_three_nulls() {
foreach (range(1, 3) as $i) {
yield;
}
}
var_dump(iterator_to_array(gen_three_nulls()));
?>
以上例程会输出：
array(3) {
[0]=>
NULL
[1]=>
NULL
[2]=>
NULL
}
生成函数可以像使用值一样来使用引用生成。这个和从函数返回一个引用一样：通过在函数名前面加一个引用符号。
Example #4 使用引用来生成值
<?php
function &gen_reference() {
$value = 3;
while ($value > 0) {
yield $value;
}
}
/*
* 我们可以在循环中修改 $number 的值，而生成器是使用的引用值来生成，所以 gen_reference() 内部的 $value 值也会跟着变化。
*/
foreach (gen_reference() as &$number) {
echo (--$number).'... ';
}
?>
以上例程会输出：
2... 1... 0...
Generator delegation allows you to yield values from another generator, Traversable object, or array by using the yield from keyword. The outer generator will then yield all values from the inner generator, object, or array until that is no longer valid, after which execution will continue in the outer generator.
If a generator is used with yield from, the yield from expression will also return any value returned by the inner generator.
yield from does not reset the keys. It preserves the keys returned by the Traversable object, or array. Thus some values may share a common key with another yield or yield from, which, upon insertion into an array, will overwrite former values with that key.
A common case where this matters is iterator_to_array()
returning a keyed array by default, leading to possibly unexpected results.
iterator_to_array() has a second parameter
use_keys which can be set to false to collect
all the values while ignoring the keys returned by the Generator.
Example #5 yield from with iterator_to_array()
<?php
function inner() {
yield 1; // key 0
yield 2; // key 1
yield 3; // key 2
}
function gen() {
yield 0; // key 0
yield from inner(); // keys 0-2
yield 4; // key 1
}
// pass false as second parameter to get an array [0, 1, 2, 3, 4]
var_dump(iterator_to_array(gen()));
?>
以上例程会输出：
array(3) {
[0]=>
int(1)
[1]=>
int(4)
[2]=>
int(3)
}
Example #6 yield from 的基本用法
<?php
function count_to_ten() {
yield 1;
yield 2;
yield from [3, 4];
yield from new ArrayIterator([5, 6]);
yield from seven_eight();
yield 9;
yield 10;
}
function seven_eight() {
yield 7;
yield from eight();
}
function eight() {
yield 8;
}
foreach (count_to_ten() as $num) {
echo "$num ";
}
?>
以上例程会输出：
1 2 3 4 5 6 7 8 9 10
Example #7 yield from 并返回多个值
<?php
function count_to_ten() {
yield 1;
yield 2;
yield from [3, 4];
yield from new ArrayIterator([5, 6]);
yield from seven_eight();
return yield from nine_ten();
}
function seven_eight() {
yield 7;
yield from eight();
}
function eight() {
yield 8;
}
function nine_ten() {
yield 9;
return 10;
}
$gen = count_to_ten();
foreach ($gen as $num) {
echo "$num ";
}
echo $gen->getReturn();
?>
以上例程会输出：
1 2 3 4 5 6 7 8 9 10
Module list of a number from 1 to 100.
<?php
function list_of_modulo(){
for($i = 1; $i <= 100; $i++){
if(($i % 2) == 0){
yield $i;
}
}
}
$modulos = list_of_modulo();
foreach($modulos as $value){
echo "$value\n";
}
?>
If for some strange reason you need a generator that doesn't yield anything, an empty function doesn't work; the function needs a yield statement to be recognised as a generator.
<?php
function gndn()
{
}
foreach(gndn() as $it)
{
echo 'FNORD';
}
?>
But it's enough to have the yield syntactically present even if it's not reachable:
<?php
function gndn()
{
if(false) { yield; }
}
foreach(gndn() as $it)
{
echo 'FNORD';
}
?>
Do not call generator functions directly, that won't work.
<?php
function my_transform($value) {
var_dump($value);
return $value * 2;
}
function my_function(array $values) {
foreach ($values as $value) {
yield my_transform($value);
}
}
$data = [1, 5, 10];
// my_transform() won't be called inside my_function()
my_function($data);
# my_transform() will be called.
foreach (my_function($data) as $val) {
// ...
}
?>
That is a simple fibonacci generator.
<?php
function fibonacci($item) {
$a = 0;
$b = 1;
for ($i = 0; $i < $item; $i++) {
yield $a;
$a = $b - $a;
$b = $a + $b;
}
}
# give me the first ten fibonacci numbers
$fibo = fibonacci(10);
foreach ($fibo as $value) {
echo "$value\n";
}
?>
[This comment replaces my previous comment]
You can use generators to do lazy loading of lists. You only compute the items that are actually used. However, when you want to load more items, how to cache the ones already loaded?
Here is how to do cached lazy loading with a generator:
<?php
class CachedGenerator {
protected $cache = [];
protected $generator = null;
public function __construct($generator) {
$this->generator = $generator;
}
public function generator() {
foreach($this->cache as $item) yield $item;
while( $this->generator->valid() ) {
$this->cache[] = $current = $this->generator->current();
$this->generator->next();
yield $current;
}
}
}
class Foobar {
protected $loader = null;
protected function loadItems() {
foreach(range(0,10) as $i) {
usleep(200000);
yield $i;
}
}
public function getItems() {
$this->loader = $this->loader ?: new CachedGenerator($this->loadItems());
return $this->loader->generator();
}
}
$f = new Foobar;
# First
print "First\n";
foreach($f->getItems() as $i) {
print $i . "\n";
if( $i == 5 ) {
break;
}
}
# Second (items 1-5 are cached, 6-10 are loaded)
print "Second\n";
foreach($f->getItems() as $i) {
print $i . "\n";
}
# Third (all items are cached and returned instantly)
print "Third\n";
foreach($f->getItems() as $i) {
print $i . "\n";
}
?>
<?php
//Example of class implementing IteratorAggregate using generator
class ValueCollection implements IteratorAggregate
{
private $items = array();
public function addValue($item)
{
$this->items[] = $item;
return $this;
}
public function getIterator()
{
foreach ($this->items as $item) {
yield $item;
}
}
}
//Initializes a collection
$collection = new ValueCollection();
$collection
->addValue('A string')
->addValue(new stdClass())
->addValue(NULL);
foreach ($collection as $item) {
var_dump($item);
}
Note that you can't use count() on generators.
/**
* @return integer[]
*/
function xrange() {
for ($a = 0; $a < 10; $a++)
{
yield $a;
}
}
function mycount(Traversable $traversable)
{
$skip = 0;
foreach($traversable as $skip)
{
$skip++;
}
return $skip;
}
echo "Count:" . count(xrange()). PHP_EOL;
echo "Count:" . mycount(xrange()). PHP_EOL;
This is little example of using generators with recursion. Used version of php is 5.5.5
[php]
<?php
define ("DS", DIRECTORY_SEPARATOR);
define ("ZERO_DEPTH", 0);
define ("DEPTHLESS", -1);
define ("OPEN_SUCCESS", True);
define ("END_OF_LIST", False);
define ("CURRENT_DIR", ".");
define ("PARENT_DIR", "..");
function DirTreeTraversal($DirName, $MaxDepth = DEPTHLESS, $CurrDepth = ZERO_DEPTH)
{
if (($MaxDepth === DEPTHLESS) || ($CurrDepth < $MaxDepth)) {
$DirHandle = opendir($DirName);
if ($DirHandle !== OPEN_SUCCESS) {
try{
while (($FileName = readdir($DirHandle)) !== END_OF_LIST) { //read all file in directory
if (($FileName != CURRENT_DIR) && ($FileName != PARENT_DIR)) {
$FullName = $DirName.$FileName;
yield $FullName;
if(is_dir($FullName)) { //include sub files and directories
$SubTrav = DirTreeTraversal($FullName.DS, $MaxDepth, ($CurrDepth + 1));
foreach($SubTrav as $SubItem) yield $SubItem;
}
}
}
} finally {
closedir($DirHandle);
}
}
}
}
$PathTrav = DirTreeTraversal("C:".DS, 2);
print "<pre>";
foreach($PathTrav as $FileName) printf("%s\n", $FileName);
print "</pre>";
[/php]
For example yield keyword with Fibonacci:
function getFibonacci()
{
$i = 0;
$k = 1; //first fibonacci value
yield $k;
while(true)
{
$k = $i + $k;
$i = $k - $i;
yield $k;
}
}
$y = 0;
foreach(getFibonacci() as $fibonacci)
{
echo $fibonacci . "\n";
$y++;
if($y > 30)
{
break; // infinite loop prevent
}
}