Dart append to file when i use transform in read text from file - dart

Question

in this simple code i can show all fetched ids when finished reading file and get id from text file, but i want to append this fetched id inside JsonObjectTransformer class, not finished reading file
Future<void> main() async {
final ids = await File('sample.json')
.openRead()
.transform(const Utf8Decoder())
.transform<dynamic>(JsonObjectTransformer())
.map((dynamic json) => json['id'] as String)
.toList();
print(ids); // [#123456, #123456]
}
class JsonObjectTransformer extends StreamTransformerBase<String, dynamic> {
static final _openingBracketChar = '{'.codeUnitAt(0);
static final _closingBracketChar = '}'.codeUnitAt(0);
#override
Stream<dynamic> bind(Stream<String> stream) async* {
final sb = StringBuffer();
var bracketsCount = 0;
await for (final string in stream) {
for (var i = 0; i < string.length; i++) {
final current = string.codeUnitAt(i);
sb.writeCharCode(current);
if (current == _openingBracketChar) {
bracketsCount++;
}
if (current == _closingBracketChar && --bracketsCount == 0) {
yield json.decode(sb.toString());
sb.clear();
}
}
}
/*for example this line*/
//new File('test.txt').writeAsStringSync(sb.toString(), mode: FileMode.APPEND);
}
}
how can i do that?

Answer 1

There are multiple ways to do this but a simple way is to change the JsonObjectTransformer like this:
class JsonObjectTransformer extends StreamTransformerBase<String, dynamic> {
static final _openingBracketChar = '{'.codeUnitAt(0);
static final _closingBracketChar = '}'.codeUnitAt(0);
#override
Stream<dynamic> bind(Stream<String> stream) async* {
final sb = StringBuffer();
var bracketsCount = 0;
final ioSink = File('test.txt').openWrite(mode: FileMode.append);
await for (final string in stream) {
for (var i = 0; i < string.length; i++) {
final current = string.codeUnitAt(i);
sb.writeCharCode(current);
if (current == _openingBracketChar) {
bracketsCount++;
}
if (current == _closingBracketChar && --bracketsCount == 0) {
final dynamic jsonObject = json.decode(sb.toString());
ioSink.writeln(jsonObject['id'] as String);
yield jsonObject;
sb.clear();
}
}
}
await ioSink.flush();
await ioSink.close();
}
}
A more clean solution (since we want some separate of concern) would be to make use of the Stream in your main to write the ID's as each object are parsed. An example how to do that would be:
Future<void> main() async {
final file = File('test.txt').openWrite(mode: FileMode.append);
final ids = <String>[];
await File('sample.json')
.openRead()
.transform(const Utf8Decoder())
.transform<dynamic>(JsonObjectTransformer())
.map((dynamic json) => json['id'] as String)
.forEach((id) {
file.writeln(id);
ids.add(id);
});
await file.flush();
await file.close();
print(ids); // [#123456, #123456]
}