[cynthiajoan]

Description

Fixes an off-by-one bug in _childKeysFromValue (packages/firebase_database/firebase_database_platform_interface/lib/src/method_channel/method_channel_data_snapshot.dart, line 107) when constructing a nested MethodChannelDataSnapshot whose value is a List.

Why this is needed

• For a top-level DataSnapshot, the native Android/iOS plugin populates _data['childKeys'] (for array-valued nodes, '0', '1', '2', ...).
• For a nested DataSnapshot created in Dart via snapshot.child(...) (line 65) or while iterating snapshot.children (line 93), Dart computes _data['childKeys'] locally via _childKeysFromValue(childValue):

  } else if (value is List) {
    childChildKeys = List<String>.generate(
      value.length,
      (int index) => '${index - 1}',
    );
  }

• Because List.generate passes a 0-based index (0 .. value.length - 1), '${index - 1}' produced ['-1', '0', '1', ..., '${length - 2}'].
• Accessing .children on that nested List snapshot then executes (value! as List)[int.parse(childKey)] (line 83), which indexes list[-1] on the first element and throws RangeError (length): Invalid value: Not in inclusive range 0..2: -1.

Why this does not break existing SDK behavior

• _childKeysFromValue is a file-private helper in method_channel_data_snapshot.dart called only when constructing child snapshots in child() (line 65) and children (line 93).
• Map and scalar/null values do not enter the else if (value is List) branch.
• For non-empty List values, iterating .children on a nested snapshot previously threw RangeError unconditionally on index 0 (int.parse('-1')), whereas snapshot.child('0') (line 26) parses the index directly from childPath without reading childKeys. Returning '$index' ('0', '1', ...) matches the 0-based keys emitted by the native Android/iOS SDKs and firebase_database_web.

Verification

• Red (before fix, on 387c707b2 with only the new test): flutter test --no-pub test/method_channel_test.dart fails with:

  RangeError (length): Invalid value: Not in inclusive range 0..2: -1 (package:firebase_database_platform_interface/src/method_channel/method_channel_data_snapshot.dart:83:40)
  

• Green (with fix, at d61349f6e):
  • dart format --output=none --set-exit-if-changed . (0 changes)
  • dart analyze --fatal-infos . (0 issues)
  • flutter test --no-pub in firebase_database_platform_interface (74 passed, 0 failed)
  • Added integration test children returns the children for a nested list snapshot in packages/firebase_database/firebase_database/example/integration_test/data_snapshot_e2e.dart to exercise s.child('items').children against the Realtime Database emulator across all platforms.

Related Issues

N/A

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• I signed the CLA.
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

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