Dart判断运算符

void main() {
  bool flag = true;
  var val = flag ? 'true' : 'false';
  print(val); //val为true

  int count;
  count ??= 20; //count为空赋值为20
  print(count);//count为20

  int a;
  int b = a ?? 30; //a为空赋值为30
  print(b);//b为30
}