Rune Persistence

A .rune file (app/RuneFile.h/.cpp) is plain JSON. RuneConfig is the in-memory shape; saveRuneFile()/loadRuneFile() convert it to and from disk.

{
  "channelDisplayName": "...",
  "busConfig": { "nominalBitrateBps": 500000, "fd": false, "nominalTiming": {...}, "dataTiming": {...} },
  "displayMode": "Waterfall",
  "displayRateMs": 33,
  "dbcPath": "...",
  "graph": { "windows": [ { "axes": [...] } ] },
  "transmitMessages": [ { "id": 0, "extended": false, "fd": false, "dlc": 8, "data": "hex...", "cycleTimeMs": 0, ... } ],
  "traceHeaderState": "hex-encoded QHeaderView::saveState() bytes",
  "listenOnly": false
}

No version field, by design

Rather than a version number gating what gets parsed, every field just tolerates its own absence. QJsonObject::operator[] on a missing key returns a null QJsonValue, and QJsonValue::toBool(default)/toString()/ toArray() etc. all convert that null value to a sensible default rather than erroring. So a rune saved before a given field existed just loads with that field at its default - listenOnly (added for a .rune predating it) defaults to false, matching the behavior that already existed before the field did.

config.listenOnly = root["listenOnly"].toBool(false);

graphWindows: the one field that needed a real migration

Everything above is a simple "missing key → default" story. Graph layout is different, because its shape itself changed (single-window → up to 6 windows), not just whether a key exists:

if (graph.contains("windows")) {
    // current: array of window objects, each with its own "axes" array
} else if (graph.contains("axes")) {
    // pre-multi-window: one flat "axes" array directly under "graph" -
    // load it as a single window rather than rejecting the file or
    // silently dropping the layout
}

This is the pattern to reach for if a future field needs to change shape rather than just start existing: branch on which shape is actually present (contains()), don't bump a global version number for one field.

Byte-for-byte persistence: traceHeaderState

Trace view column order/widths are Qt's own opaque QHeaderView::saveState() byte blob, not hand-rolled per-column JSON - stored as a hex string the same way TransmitMessage::data (frame payload bytes) already was, and restored via QHeaderView::restoreState(), which is a documented no-op on an empty/invalid byte array. That's what makes an old rune with no saved header state safe to load: restoreState() on the empty default just leaves the header exactly as it currently is.