A Source knows how to list and fetch gems from a RubyGems marshal index.
There are other Source subclasses for installed gems, local gems, the bundler dependency API and so-forth.
Creates a new Source which will use the index
located at uri.
 
               # File rubygems/source.rb, line 30
def initialize(uri)
  begin
    unless uri.kind_of? URI
      uri = URI.parse(uri.to_s)
    end
  rescue URI::InvalidURIError
    raise if Gem::Source == self.class
  end
  @uri = uri
end
             
            Sources are ordered by installation preference.
 
               # File rubygems/source.rb, line 45
def <=>(other)
  case other
  when Gem::Source::Installed,
       Gem::Source::Local,
       Gem::Source::Lock,
       Gem::Source::SpecificFile,
       Gem::Source::Git,
       Gem::Source::Vendor then
    -1
  when Gem::Source then
    if !@uri
      return 0 unless other.uri
      return 1
    end
    return -1 if !other.uri
    # Returning 1 here ensures that when sorting a list of sources, the
    # original ordering of sources supplied by the user is preserved.
    return 1 unless @uri.to_s == other.uri.to_s
    0
  else
    nil
  end
end
             
            Returns the local directory to write uri to.
 
               # File rubygems/source.rb, line 107
def cache_dir(uri)
  # Correct for windows paths
  escaped_path = uri.path.sub(/^\/([a-z]):\//i, '/\1-/')
  escaped_path.tap(&Gem::UNTAINT)
  File.join Gem.spec_cache_dir, "#{uri.host}%#{uri.port}", File.dirname(escaped_path)
end
             
            Downloads spec and writes it to dir.  See also Gem::RemoteFetcher#download.
 
               # File rubygems/source.rb, line 205
def download(spec, dir=Dir.pwd)
  fetcher = Gem::RemoteFetcher.fetcher
  fetcher.download spec, uri.to_s, dir
end
             
            Fetches a specification for the given name_tuple.
 
               # File rubygems/source.rb, line 130
def fetch_spec(name_tuple)
  fetcher = Gem::RemoteFetcher.fetcher
  spec_file_name = name_tuple.spec_name
  source_uri = uri + "#{Gem::MARSHAL_SPEC_DIR}#{spec_file_name}"
  cache_dir = cache_dir source_uri
  local_spec = File.join cache_dir, spec_file_name
  if File.exist? local_spec
    spec = Gem.read_binary local_spec
    spec = Marshal.load(spec) rescue nil
    return spec if spec
  end
  source_uri.path << '.rz'
  spec = fetcher.fetch_path source_uri
  spec = Gem::Util.inflate spec
  if update_cache?
    FileUtils.mkdir_p cache_dir
    File.open local_spec, 'wb' do |io|
      io.write spec
    end
  end
  # TODO: Investigate setting Gem::Specification#loaded_from to a URI
  Marshal.load spec
end
             
            Loads type kind of specs fetching from +@uri+ if the on-disk
cache is out of date.
type is one of the following:
:released => Return the list of all released specs :latest => Return the list of only the highest version of each gem :prerelease => Return the list of all prerelease only specs
 
               # File rubygems/source.rb, line 175
def load_specs(type)
  file       = FILES[type]
  fetcher    = Gem::RemoteFetcher.fetcher
  file_name  = "#{file}.#{Gem.marshal_version}"
  spec_path  = uri + "#{file_name}.gz"
  cache_dir  = cache_dir spec_path
  local_file = File.join(cache_dir, file_name)
  retried    = false
  FileUtils.mkdir_p cache_dir if update_cache?
  spec_dump = fetcher.cache_update_path spec_path, local_file, update_cache?
  begin
    Gem::NameTuple.from_list Marshal.load(spec_dump)
  rescue ArgumentError
    if update_cache? && !retried
      FileUtils.rm local_file
      retried = true
      retry
    else
      raise Gem::Exception.new("Invalid spec cache file in #{local_file}")
    end
  end
end