Scope

Builds off prior art:

• SF Perf Experiment: JS polling and tab visibility #3125

Implementation

• Basically rewrites the JS logic into a Phoenix client hook.
  • On component mount, assigns the visibilitychange event listener. It pushes the document visibilityState to the server, which can be handled in the parent LiveView via handle_event.
  • On mount also immediately sends the current visibilityState
  • Reconnections also trigger immediately sending the current visibilityState
  • On component destroyed() we remove the event listener.

Then I edited the Schedule Finder to respond to the changed value:

• Toggles a new assign, :should_refresh?, which if true will schedule (ahem) the next upcoming departure update.

How to test

I put an IO.inspect() in assign_upcoming_departures/1 to see how frequently it's being called after changing the tab visibility state! I'll note it's not a perfect/immediate transition - for example, if the next upcoming departure update is already queued when the tab changes to a hidden state, that next update will still happen.