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# EventLoop Component
Event loop abstraction layer that libraries can use for evented I/O.
In order for async based libraries to be interoperable, they need to use the
same event loop. This component provides a common `LoopInterface` that any
library can target. This allows them to be used in the same loop, with one
single `run` call that is controlled by the user.
In addition to the interface there are some implementations provided:
* `StreamSelectLoop`: This is the only implementation which works out of the
box with PHP. It does a simple `select` system call. It's not the most
performant of loops, but still does the job quite well.
* `LibEventLoop`: This uses the `libevent` pecl extension. `libevent` itself
supports a number of system-specific backends (epoll, kqueue).
* `LibEvLoop`: This uses the `libev` pecl extension
([github](https://github.com/m4rw3r/php-libev)). It supports the same
backends as libevent.
All of the loops support these features:
* File descriptor polling
* One-off timers
* Periodic timers
## Usage
Here is an async HTTP server built with just the event loop.
$loop = React\EventLoop\Factory::create();
$server = stream_socket_server('tcp://127.0.0.1:8080');
stream_set_blocking($server, 0);
$loop->addReadStream($server, function ($server) use ($loop) {
$conn = stream_socket_accept($server);
$data = "HTTP/1.1 200 OK\r\nContent-Length: 3\r\n\r\nHi\n";
$loop->addWriteStream($conn, function ($conn) use (&$data, $loop) {
$written = fwrite($conn, $data);
if ($written === strlen($data)) {
fclose($conn);
$loop->removeStream($conn);
} else {
$data = substr($data, 0, $written);
}
});
});
$loop->addPeriodicTimer(5, function () {
$memory = memory_get_usage() / 1024;
$formatted = number_format($memory, 3).'K';
echo "Current memory usage: {$formatted}\n";
});
$loop->run();
**Note:** The factory is just for convenience. It tries to pick the best
available implementation. Libraries `SHOULD` allow the user to inject an
instance of the loop. They `MAY` use the factory when the user did not supply
a loop.
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