init method
Implementation
Future<void> init(int jobCount) async {
_jobCount = jobCount;
_receivePort.listen((dynamic message) => _listen(message));
_isolate =
await Isolate.spawn(_downloadIsolateRoutine, _receivePort.sendPort);
_tasks = <int, DownloadTask>{};
_taskTotalSizes = <int, int>{};
_taskCountSizes = <int, int>{};
_appendedTask = HashSet<int>();
_completedTask = HashSet<int>();
_erroredTask = HashSet<int>();
_canceledTask = HashSet<int>();
_errorContent = <int, IsolateDownloaderErrorUnit>{};
_taskTotalCount = 0;
}