if let url = URL(string: "https://…") {
    url.fetchOpenGraphData { (metadata) in
        print(metadata)
    }
}

// first, fetch data from the network
if let url = URL(string: "https://…") {
    // first fetch html that might have opengraph previews
  // The demo uses the built-in `URLSession`, but anything that can fetch data can be used here 
  let task = URLSession.shared.dataTask(with: url) { (data, response, error)
        // make sure we successfully completed a request
      if let response = response as? HTTPURLResponse, response.statusCode >= 200, response.statusCode < 300 {
            if let data = data, let html = String(data: data, encoding: .utf8) {
                parse(html: html)
            }
        }
    }

    task.resume()
}

// and then parse OpenGraph meta tags out of an html document
func parse(html: String) {
    // then create a parser that can tell us the contents of each html tag and any associated key/value properties it has
  // `Parser` is provided, but this can be substituted with anything else that can iterate through html tags 
  let parser = Parser()

    // and keep track of each <meta class="og:…"> tag as the parser encounters it
  // This could also be replaced with another component, but outside of testing purposes, there's less of an obvious need to do so than with the other steps of the process.
  let tagTracker = TagTracker()
    parser.onFind = { (tag, values) in
        if !tagTracker.track(tag, values: values) {
            print("refusing to track non-meta tag \(tag) with values \(values)")
        }
    }

    if parser.parse(html) {
        // - If we can parse html, map over our results to go from arrays of arrays of dictionaries (`[[String: OpenGraphType]]`)
      // to an array of OpenGraph objects.
      // - Note: OpenGraph can have multiple elements on a page (for example, an og:article, follwed by an og:author, followed by another og:author)
      let tags = tagTracker.metadatum.map(Metadata.from)
        print(tags)
    }
}