spawnIsolate method
创建Isolate
Implementation
Future<IsolateMessage> spawnIsolate() async {
final receivePort = ReceivePort();
Isolate isolate = await Isolate.spawn(_worker, receivePort.sendPort);
// 获取工作线程的 SendPort
final sendPort = await receivePort.first;
return IsolateMessage(
sendPort: sendPort, receivePort: receivePort, isolate: isolate);
}