生成器总览
(PHP 5 >= 5.5.0)
生成器提供了一种更容易的方法来实现简单的对象迭代，相比较定义类实现 Iterator 接口的方式，性能开销和复杂性大大降低。
生成器允许你在 foreach 代码块中写代码来迭代一组数据而不需要在内存中创建一个数组, 那会使你的内存达到上限，或者会占据可观的处理时间。相反，你可以写一个生成器函数，就像一个普通的自定义函数一样, 和普通函数只返回一次不同的是, 生成器可以根据需要 yield 多次，以便生成需要迭代的值。
一个简单的例子就是使用生成器来重新实现 range() 函数。 标准的 range() 函数需要在内存中生成一个数组包含每一个在它范围内的值，然后返回该数组, 结果就是会产生多个很大的数组。 比如，调用 range(0, 1000000) 将导致内存占用超过 100 MB。
做为一种替代方法, 我们可以实现一个 xrange() 生成器, 只需要足够的内存来创建 Iterator 对象并在内部跟踪生成器的当前状态，这样只需要不到1K字节的内存。
Example #1 将 range() 实现为生成器
<?php
function xrange ( $start , $limit , $step = 1 ) {
if ( $start < $limit ) {
if ( $step <= 0 ) {
throw new LogicException ( 'Step must be +ve' );
}
for ( $i = $start ; $i <= $limit ; $i += $step ) {
yield $i ;
}
} else {
if ( $step >= 0 ) {
throw new LogicException ( 'Step must be -ve' );
}
for ( $i = $start ; $i >= $limit ; $i += $step ) {
yield $i ;
}
}
}
echo 'Single digit odd numbers from range(): ' ;
foreach ( range ( 1 , 9 , 2 ) as $number ) {
echo " $number " ;
}
echo "\n" ;
echo 'Single digit odd numbers from xrange(): ' ;
foreach ( xrange ( 1 , 9 , 2 ) as $number ) {
echo " $number " ;
}
?> 以上例程会输出：
Single digit odd numbers from range(): 1 3 5 7 9 Single digit odd numbers from xrange(): 1 3 5 7 9
Generator objects
When a generator function is called for the first time, an object of the internal Generator class is returned. This object implements the Iterator interface in much the same way as a forward-only iterator object would, and provides methods that can be called to manipulate the state of the generator, including sending values to and returning values from it.
用户评论:
[#1] damirchilo at outlook dot com [2015-08-02 12:45:40]
You can also declare generator functions in a function scope.
<?php
function gen_n($n){
function gen_t($len){
for($i = 1; $i < $len; $i++)
yield $i;
}
foreach(gen_t($n) as $out)
printf("%d, ", $out);
printf("%d", ++$out);
}
gen_n(15);
?>
[#2] dc at libertyskull dot com [2014-02-17 14:59:27]
Same example, different results:
----------------------------------
| time | memory, mb |
----------------------------------
| not gen | 0.7589 | 146.75 |
|---------------------------------
| with gen | 0.7469 | 8.75 |
|---------------------------------
Time in results varying from 6.5 to 7.8 on both examples.
So no real drawbacks concerning processing speed.
[#3] lubaev [2014-01-15 11:46:01]
Abstract test.
<?php
$start_time=microtime(true);
$array = array();
$result = '';
for($count=1000000; $count--;)
{
$array[]=$count/2;
}
foreach($array as $val)
{
$val += 145.56;
$result .= $val;
}
$end_time=microtime(true);
echo "time: ", bcsub($end_time, $start_time, 4), "\n";
echo "memory (byte): ", memory_get_peak_usage(true), "\n";
?>
<?php
$start_time=microtime(true);
$result = '';
function it()
{
for($count=1000000; $count--;)
{
yield $count/2;
}
}
foreach(it() as $val)
{
$val += 145.56;
$result .= $val;
}
$end_time=microtime(true);
echo "time: ", bcsub($end_time, $start_time, 4), "\n";
echo "memory (byte): ", memory_get_peak_usage(true), "\n";
?>
Result:
----------------------------------
| time | memory, mb |
----------------------------------
| not gen | 2.1216 | 89.25 |
|---------------------------------
| with gen | 6.1963 | 8.75 |
|---------------------------------
| diff | < 192% | > 90% |
----------------------------------
[#4] bloodjazman at gmail dot com [2013-08-15 07:32:21]
for the protection from the leaking of resources
see RFC https://wiki.php.net/rfc/generators#closing_a_generator
and use finnaly
sample code
function getLines($file) {
$f = fopen($file, 'r');
try {
while ($line = fgets($f)) {
yield $line;
}
} finally {
fclose($f);
}
}
foreach (getLines("file.txt") as $n => $line) {
if ($n > 5) break;
echo $line;
}