build_runner/lib/src/watcher/collect_changes.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:build/build.dart';
import 'package:build_runner/src/watcher/asset_change.dart';
import 'package:watcher/watcher.dart';

/// Merges [AssetChange] events.
///
/// For example, if an asset was added then immediately deleted, no event will
/// be recorded for the given asset.
Map<AssetId, ChangeType> collectChanges(List<List<AssetChange>> changes) {
  var changeMap = <AssetId, ChangeType>{};
  for (var change in changes.expand((l) => l)) {
    var originalChangeType = changeMap[change.id];
    if (originalChangeType != null) {
      switch (originalChangeType) {
        case ChangeType.ADD:
          if (change.type == ChangeType.REMOVE) {
            // ADD followed by REMOVE, just remove the change.
            changeMap.remove(change.id);
          }
          break;
        case ChangeType.REMOVE:
          if (change.type == ChangeType.ADD) {
            // REMOVE followed by ADD, convert to a MODIFY
            changeMap[change.id] = ChangeType.MODIFY;
          } else if (change.type == ChangeType.MODIFY) {
            // REMOVE followed by MODIFY isn't sensible, just throw.
            throw StateError(
                'Internal error, got REMOVE event followed by MODIFY event for '
                '${change.id}.');
          }
          break;
        case ChangeType.MODIFY:
          if (change.type == ChangeType.REMOVE) {
            // MODIFY followed by REMOVE, convert to REMOVE
            changeMap[change.id] = change.type;
          } else if (change.type == ChangeType.ADD) {
            // MODIFY followed by ADD isn't sensible, just throw.
            throw StateError(
                'Internal error, got MODIFY event followed by ADD event for '
                '${change.id}.');
          }
          break;
      }
    } else {
      changeMap[change.id] = change.type;
    }
  }
  return changeMap;
}