Add Swift Package Manager support.

[BrianDoig]

This implements #753 by adding a Package.swift file.

[codecov-io]

Codecov Report

Merging #2950 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2950   +/-   ##
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  Coverage   19.64%   19.64%           
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  Files         113      113           
  Lines       13792    13792           
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  Hits         2709     2709           
  Misses      11083    11083

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[liuxuan30]

seems fine comparing to https://swift.org/package-manager/#example-usage. @petester42 or swift experts?

[doofyus]

Could this please be merged in.

[liuxuan30]

I will merge.

[danielgindi]

Cool :-)

[pmairoldi]

What about the dependency on UIKit/AppKit? Is there a way to identify that this will only work on Mac and if you have the sdk installed?

[pmairoldi]

@doofyus could you elaborate what you needed this for? I was asked a question about spm support and from what I know about spm it can’t be used with apps right now.

[doofyus]

It's true that SPM is a bit limited currently, but it can be used to keep the dependencies up to date and with some workarounds, it's possible to use them in an app as well (i use SPM in a couple of them).