flutter空安全问题，平时用到的数据一定要注意

参考
涉及数据，都要考虑一下是否可以为空，为空的时候显示什么
可空（?）类型，数据是可以为空的，在json解析的范类里面基本是？，因为服务器可能会返回空值

class BottleModel {
  BottleModel({
    String? appId,
    int?    downTime,
    int?    downUserId,
    int?    driftBottleId,
    int?    level,
    int?    noReadNum,
    int?    status,
    String? text,
    int?    upTime,
    int?    upUserId,
  }) {
    _appId = appId;
    _downTime = downTime;
    _downUserId = downUserId;
    _driftBottleId = driftBottleId;
    _level = level;
    _noReadNum = noReadNum;
    _status = status;
    _text = text;
    _upTime = upTime;
    _upUserId = upUserId;
  }

  BottleModel.fromJson(dynamic json) {
    _appId = json['app_id'];
    _downTime = json['down_time'];
    _downUserId = json['down_user_id'];
    _driftBottleId = json['drift_bottle_id'];
    _level = json['level'];
    _noReadNum = json['no_read_num'];
    _status = json['status'];
    _text = json['text'];
    _upTime = json['up_time'];
    _upUserId = json['up_user_id'];
  }

  String? _appId;
  int? _downTime;
  int? _downUserId;
  int? _driftBottleId;
  int? _level;
  int? _noReadNum;
  int? _status;
  String? _text;
  int? _upTime;
  int? _upUserId;

  BottleModel copyWith({
    String? appId,
    int? downTime,
    int? downUserId,
    int? driftBottleId,
    int? level,
    int? noReadNum,
    int? status,
    String? text,
    int? upTime,
    int? upUserId,
  }) =>
      BottleModel(
        appId: appId ?? _appId,
        downTime: downTime ?? _downTime,
        downUserId: downUserId ?? _downUserId,
        driftBottleId: driftBottleId ?? _driftBottleId,
        level: level ?? _level,
        noReadNum: noReadNum ?? _noReadNum,
        status: status ?? _status,
        text: text ?? _text,
        upTime: upTime ?? _upTime,
        upUserId: upUserId ?? _upUserId,
      );

  String? get appId => _appId;

  int? get downTime => _downTime;

  int? get downUserId => _downUserId;

  int? get driftBottleId => _driftBottleId;

  int? get level => _level;

  int? get noReadNum => _noReadNum;

  int? get status => _status;

  String? get text => _text;

  int? get upTime => _upTime;

  int? get upUserId => _upUserId;

  Map<String, dynamic> toJson() {
    final map = <String, dynamic>{};
    map['app_id'] = _appId;
    map['down_time'] = _downTime;
    map['down_user_id'] = _downUserId;
    map['drift_bottle_id'] = _driftBottleId;
    map['level'] = _level;
    map['no_read_num'] = _noReadNum;
    map['status'] = _status;
    map['text'] = _text;
    map['up_time'] = _upTime;
    map['up_user_id'] = _upUserId;
    return map;
  }
}
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class Person{
   String name = "Tony"
}

var userName ;
late Person p = Person();
userName = p?.name;
print(userName)
userName = p?.name; 等同于

if(p == null){
  userName = null;//就是可空的意思
}else {
  userName = p.name;
}
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?? 如果是空的就赋后面那个值

var expr1;
var expr2 = 'b';
expr1 = expr1 ?? expr2;
如果expr1为null,那么将expr2赋值给expr1, 反之如果不为null,就将expr1赋值给expr1.等同于:

expr1 ??= expr2;
等同于:

if(expr1 == null){
  expr1  = expr2
}
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延迟初始化（late)
可以不使用?.，可以让开发者自己选择初始化的时机，比如在initState进行初始化

空值断言操作符（!）
赋！的一定是不能为null
处理方式：

  String getName() {
    String name = "";//如果为空值，保底赋值为“”
    if (replyList != null) {
      if (replyList!.otherInfo != null) {
        if (replyList!.otherInfo!.nickname != null) {//全部都不为空的时候才赋值
          name = replyList!.otherInfo!.nickname!;
        }
      }
    }
    return name;
  }
  
if (getName() != "")//如果不是“”，下面的内容才可以显示
 UserExInfoWidget(
                    nickName: getName(),//调用方法，在方法里面判空
                    nickFontSize: 17.0,
                    nickFontColor: CommonColors.getColor1A1A1A,
                  ),
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如果是列表，就判断是否为空，如果不为空才可以正常显示，如果是空的就返回一个Container

       (currentMessageList == null || currentMessageList.length == 0)
                  ? [Container()]: replyList == null
                  ? [Container()]
                  : currentMessageList.map((e) => MsgContainer(replyList: replyList!,downList: e,)).toList(),
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