Replaced the regex-based transformation of CSS to XPath

[stof]

The library now uses the Symfony CssSelector component, which supports more cases than the custom logic used previously.
Closes #34

I updated the doc to mention composer as the recommended installation way, as this takes care of including the CssSelector component automatically.
Btw, don't forget to update the composer.json again when bumping the version after this change :)

[hacfi]

👍 Much simpler

[barryvdh]

This does seem to have some side-effects when using mediaqueries, probably because it handles them more strict?
Setting setExcludeMediaQueries(true) makes it behave as earlier :)
I tweaked the default behaviour for this, together with my PR to preserve mediaqueries in #54

[stof]

@barryvdh what is the issue you face with media queries ?

[barryvdh]

This example html: http://zurb.com/ink/downloads/templates/sidebar-hero.html

This php:

$inliner = new TijsVerkoyen\CssToInlineStyles\CssToInlineStyles($html);
$inliner->setStripOriginalStyleTags(true);
$inliner->setUseInlineStylesBlock(true);
$stripped = $inliner->convert();

Results in 2 different outputs: http://i.imgur.com/f6Y7d1C.png

Adding $inliner->setExcludeMediaQueries(true); makes it behave the same as before.
Not sure why you would want to include the mediaqueries, so not problem excluding it, but it's a bit strange. But probably because the old xpath didn't handle mediaqueries properly?

[stof]

anyway, excluding media queries is the sensible default when inlining, as media queries cannot get inlined.

but the difference is strange indeed. This make me think that some of the rules may have been transformed to the wrong XPath previously, and so not matching.

[barryvdh]

A think to note is that does hurt performance a bit. In the referenced template, converting 10 templates go from 550ms to 1100 ms, so it's about twice as slow in some cases. Not sure if there is anything we can do to speed it up?

[barryvdh]

The Zend class mentioned in the original issue, seems to bit a bit faster (when just copying the tokenize/csstoxpath function): https://github.com/zendframework/zf2/blob/master/library/Zend/Dom/Document/Query.php, around 700 ms with the same template. Not sure if that's less good or misses some cases.

[stof]

well, the Zend class is also based on regex transformations, which will handle less cases (and produce broken XPath queries for cases they don't support, leading to error messages which are impossible to understand)

[barryvdh]

Okay cool, as long as it's better I'm fine with losing some performance :)

[stof]

note that we may improve performance in future versions of Symfony (btw, if you can run your CssSelector benchmark with xhprof to identify the bottleneck, it might even happen faster).
The latest version of Symfony already improved the memory usage (letting PHP recover memory more easily at the end of the conversion by avoiding cyclic references).

@tijsverkoyen what do you think about this PR ?

[tijsverkoyen]

Hi @stof, first of all: big thanks. I will review the code soon.

[tijsverkoyen]

Merged, thx @stof!