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Wait for a specific element before capturing in Ruby with Net::HTTP

A fixed delay is a guess: too short and you capture the spinner, too long and every render pays for the worst case. wait_for_selector replaces the guess with a condition — pass a CSS selector and the render proceeds the moment a matching element is visible. The dashboard's chart, the map's tiles, the price table: name the thing you are actually waiting for, and wait for that.

Below is a complete, runnable Ruby program using Net::HTTP, no third-party dependency needed. It reads your API key from the SNAPDOK_KEY environment variable — free keys take about thirty seconds and need no card.

require "json"
require "net/http"
require "uri"

uri = URI("https://snapdok.io/v1/render")

res = Net::HTTP.start(uri.host, uri.port, use_ssl: true, read_timeout: 90) do |http|
  req = Net::HTTP::Post.new(uri)
  req["Authorization"] = "Bearer #{ENV.fetch("SNAPDOK_KEY")}"
  req["Content-Type"] = "application/json"
  req.body = JSON.generate({
    url: "https://your-app.com/report/42",
    format: "png",
    wait_for_selector: "#chart-ready",
    wait_until: "networkidle",
})
  http.request(req)
end

raise "snapdok error #{res.code}: #{res.body}" unless res.is_a?(Net::HTTPSuccess)

File.binwrite("ready.png", res.body)
puts "#{res["X-Cache"]}  # the settled render is cached like any other"

Selector choice is the whole game. Pick an element that appears last in your page's load sequence — a #chart-ready marker your own code adds after drawing, the final list item, the element a skeleton placeholder is replaced by. An element that exists in the initial HTML matches immediately and waits for nothing; visibility is required precisely so a pre-rendered-but-hidden node does not count as ready.

The failure mode is the feature: if the element never becomes visible within timeout, the render fails with 504 SELECTOR_TIMEOUT — not metered, and unambiguous in your logs — instead of silently shipping a half-rendered image the way an expired delay does. The sequencing composes with the other waits: navigation finishes (wait_until), then delay runs, then the selector wait begins. It works on every format, pdf_forms included.

Same endpoint, one more trick: if the page you are rendering has a form on it, adding "pdf_forms": true to a PDF render brings it back with real, fillable AcroForm fields — a PDF people can type into, not a picture of one. How that works.

Notes for Net::HTTP

Stdlib only — no gems. The traps Net::HTTP is famous for, all handled in the sample: TLS is not automatic (set use_ssl: true or you will POST in the clear to port 80 and get a redirect); the default read_timeout of 60 s is close enough to real render times that raising it is wise; and the response body is a binary-ready String, but write it with File.binwrite — plain File.write on Windows would mangle line-ending bytes in the PDF. Check is_a?(Net::HTTPSuccess): Net::HTTP never raises on HTTP error statuses.

Response headers worth reading

Header	Meaning
`X-Cache`	HIT when an identical request rendered within 24h.
`X-RateLimit-Remaining`	Requests left in the current second.

Full parameter reference: the docs. Hard numbers on caps and timeouts: limits.

Wait for a specific element before capturing in Ruby with Net::HTTP

Notes for Net::HTTP

Response headers worth reading

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