Prevent modifying Archive.files directly - doing this makes "files" a…

[aemelyanovff]

…nd "_fileMap" go out of sync

Problem

When you modify Archive.files directly, it does out of sync with _fileMap, and it results in errors or files disappearing mysteriously.

Example:

Future<void> main() async {
  final archive = Archive();
  archive.addFile(ArchiveFile('a.txt', 1, Uint8List.fromList([0])));
  archive.files.clear();
  archive.addFile(ArchiveFile('a.txt', 1, Uint8List.fromList([0])));
}

Output:

Unhandled exception:
RangeError (index): Invalid value: Valid value range is empty: 0
#0      List._setIndexed (dart:core-patch/growable_array.dart:273:49)
#1      List.[]= (dart:core-patch/growable_array.dart:268:5)
#2      Archive.addFile (package:archive/src/archive.dart:23:13)
#3      main (file:///Users/anton/dev/archive/example/example.dart:9:11)
#4      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:296:19)
#5      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:189:12)

It's even worse when it doesn't fail, but returns unexpected results:

Future<void> main() async {
  final archive = Archive();
  archive.addFile(ArchiveFile('a.txt', 1, Uint8List.fromList([0])));
  archive.files.clear();
  archive.addFile(ArchiveFile('b.txt', 1, Uint8List.fromList([0])));
  archive.addFile(ArchiveFile('a.txt', 1, Uint8List.fromList([0])));
  // Should be 2, but it prints 1
  print('Number of files in archive: ${archive.length}');
}

Solution

Since we can't keep files and _fileMap in sync when files is modified directly, Archive.files should return an unmodifiable view of the list of files. This way, attempts to modify Archive.files will fail right away with a clear message and force the user of the library to use methods like Archive.addFile() or Archive.clear().