readScenarioRunDrift function

Future<ScenarioRunDrift?> readScenarioRunDrift(String directory)

Reads the drift.json a run wrote beside its report — what moved between it and the run before it, with every step named rather than the first twenty a call hands back.

Null when the directory holds none — which means the run had no baseline to compare against, not that it compared and found nothing. Those are different answers and a gate needs to tell them apart: a run that did compare writes this file whether or not anything moved, so a clean one comes back non-null with ScenarioRunDrift.isEmpty true.

var drift = await readScenarioRunDrift('build/flutterware/screenshots');
for (var step in drift?.changed ?? const []) {
  if (step.isPixelsOnly) continue;
  print('$step moved where no screenshot could show it');
}

Implementation

Future<ScenarioRunDrift?> readScenarioRunDrift(String directory) async {
  var file = File('$directory${Platform.pathSeparator}$scenarioRunDriftFile');
  if (!file.existsSync()) return null;
  return ScenarioRunDrift.fromJson(switch (jsonDecode(
    await file.readAsString(),
  )) {
    Map json => json.cast<String, Object?>(),
    var other => throw FormatException(
      '${file.path} is ${other.runtimeType}, not an object.',
    ),
  });
}