Make StyleCollectionExtentions.ComputeDeclarations take a IEnumerable<ICssStyleRule>to allow for caching of the StyleCollection.

[The-Nutty]

Bug Report

Iterating through IWindow.GetStyleCollection is slow if doing on multiple elements, this could be because _sheets is deferred or just because GetEnumerator is doing lots of work.

Prerequisites

• Can you reproduce the problem in a MWE?
• Are you running the latest version of AngleSharp?
• Did you check the FAQs to see if that helps you?
• Are you reporting to the correct repository? (there are multiple AngleSharp libraries, e.g., AngleSharp.Css for CSS support)
• Did you perform a search in the issues?

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Description

This relates to #51 where i was hoping to be able to cache the StyleCollection returned by IWindow.GetStyleCollection to significantly speed up the process of calling StyleCollection.ComputeDeclarations on every element in a document. However it is still just as slow.

Possible Solution

1. Make ComputeDeclarations take a IEnumerable that could be pre calculated.
2. Optionally 1 but return a IEnumerable from IWindow.GetStyleCollection.

Im happy to implement the change if that would help as if possible i would appreciate it if we could get this out in a patch version, but wanted to run potential solutions by you before creating a PR.

[FlorianRappl]

Im happy to implement the change if that would help as if possible i would appreciate it if we could get this out in a patch version, but wanted to run potential solutions by you before creating a PR.

Sure just make a PR and adjust the version to 0.14.1!
(https://github.com/AngleSharp/AngleSharp.Css/blob/master/src/Directory.Build.props and in the CHANGELOG)

[FlorianRappl]

Thanks for the superb contribution! I'll close the issue the update should be out in a bit.