dart null list

減少null判斷情況

https://stackoverflow.com/questions/51036758/flutter-the-method-map-was-called-on-null
https://juejin.im/post/5c6d38b1f265da2dcc7feb69
https://cloud.tencent.com/developer/article/1370380

?? => if null

https://blog.csdn.net/ffa_ijj/article/details/85051156

  
return qres?.map((value) {  
  LinkedHashMap result = json.decode(value["jsonitem"]);  
  return result;  
})?.first ?? new LinkedHashMap();

這個判斷方式有個問題，當查詢回來為[]，不是null情況下，在.first會出錯，因為 “No element"會出錯，但.toList()會正確，故改回

  
if (qres == null || qres.isEmpty) return new Member(); // 待確定是否出錯  
  
var resmap = qres.first;  
return new Member.fromJson(json.decode(resmap["jsonitem"]));