createApplicationCatalog takes an optional placement provider. Its
layer(kind) hands Flights, Military Flights, AIS Vessels, Transit and Rail
a place(id, wallMs), and optionally observe(id, fix) and forget(id). While
live, each layer tells it every fix it accepts and draws a contact where it
places it: position, height (the layer's own when none is given), course
and speed. A null answer, a provider that throws, a kind it leaves out, or
no provider at all keeps the layer's own motion unchanged, and following
the world clock the layers never ask it. A transit vehicle or train it
holds still lets the render loop idle and is asked again each second;
vessels ask for frames only while it places one, and stop when the layer
turns off.
Tests show the layers draw and reconcile exactly as before without a
provider and with one that places nothing, draw where a provider places
them, and idle while it holds them still.
A live source may now declare a timeline: it can answer a snapshot at a
given time (source.timeline = { coverage(), stepMs, subscribe? }, read by
readSnapshotTimeline). While such a layer is on it follows the world clock
and registers the coverage. At the live edge it polls exactly as before;
away from it the layer asks for getSnapshot({ ...query, atMs }) at the step
at or before the clock time and shows that snapshot instead of polling.
src/time/snapshotTimeline.js owns the following: a seek, time going
backwards or leaving live replaces every contact with the snapshot at the
new time (play waits on it, for at most 8 s); playing on keeps one
snapshot ahead and gates play on it, applying those snapshots as polls so
contacts move smoothly; outside coverage the layer is empty; back at live
it is cleared and polls at once. One load at a time, and a load the
source drops for a newer one is not a failure.
While following, aircraft are drawn at the clock time with no render
delay, records and absences are judged at the clock, track requests carry
atMs, and vessels move between fixes along the great circle. The world
clock counts jumps (seeks and returns to live) in getState().
A source with no timeline never follows the clock, and every layer stays
live, unchanged.
Sameh Khamis