A coroutine example: Streaming XML parsing using xml_parser

<?php

 

error_reporting(E_ALL);

 

/* Data can be send to coroutines using `$coroutine->send($data)`. The sent data will then

* be the result of the `yield` expression. Thus it can be received using a code like

* `$data = yield;`.

*/

 

/* What we're building in this script is a coroutine-based streaming XML parser. The PHP

* extension for parsing streamed XML is xml_parser. It is used by defining a set of

* callback functions for various events (like start tag, end tag, content).

*

* This event model makes the parsing process very complicated, because you basically

* have to implement your own state machine (which is a lot of boilerplate code the

* more complicated the XML gets).

*

* To solve this problem, we build a wrapper (the following function), which redirects

* the events to a coroutine ($target). This is done simply using

* `$target->send([$eventName, $data])`.

*/

function streamingXMLParser($target) {

$xmlParser = xml_parser_create();

xml_set_element_handler(

$xmlParser,

function ($xmlParser, $name, array $attributes) use ($target) {

$target->send(['start', [$name, $attributes]]);

},

function ($xmlParser, $name) use ($target) {

$target->send(['end', $name]);

}

);

xml_set_character_data_handler(

$xmlParser,

function ($xmlParser, $text) use ($target) {

$target->send(['text', $text]);

}

);

 

while ($data = yield) {

if (!xml_parse($xmlParser, $data)) {

throw new Exception(sprintf(

'XML error "%s" on line %d',

xml_error_string(xml_get_error_code($xmlParser)),

xml_get_current_line_number($xmlParser)

));

}

}

 

xml_parser_free($xmlParser);

}

 

/* Inside the target coroutine the actual parsing happens. The events are received

* using `list($event, $data) = yield`. The main advantage that coroutines bring

* here is that you can fetch the events in nested loops. This way you are implicitly

* building a state machine (but the state is managed by PHP, not you!)

*

* This particular coroutine parses bus location data (for samples scroll down). The

* result is passed to another $target coroutine.

*/

function busXMLParser($target) {

while (true) {

list($event, $data) = yield;

if ($event == 'start' && $data[0] == 'BUS') {

$dict = [];

$content = '';

while (true) {

list($event, $data) = yield;

if ($event == 'start') {

$content = '';

} elseif ($event == 'text') {

$content .= $data;

} elseif ($event == 'end') {

if ($data == 'BUS') {

$target->send($dict);

break;

}

 

$dict[strtolower($data)] = $content;

}

}

}

}

}

 

/* This coroutine prints out the info it receives from the bus XML parser. */

function busLocationPrinter() {

while (true) {

$data = yield;

echo "Bus $data[id] is currently at $data[latitude]/$data[longitude]\n";

}

}

 

/* Here we are building up a coroutine pipeline. You should read this as:

* The streaming XML parser is passing data to the bus XML parser, which

* is passing data to the bus location printer.

*/

$parser = streamingXMLParser(busXMLParser(busLocationPrinter()));

 

/* I don't have access to a real bus location API, so I'll just stream some

* fictional sample data */

$parser->send('<?xml version="1.0"?><buses>');

while (true) {

sleep(1);

$parser->send(sprintf(

'<bus><id>%d</id><latitude>%f</latitude><longitude>%f</longitude></bus>',

mt_rand(1, 1000), lcg_value(), lcg_value()

));

}

 

/* If your head is buzzing now, that's a good thing :P */