flutter: 用于空值的空检查运算符

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flutter: 用于空值的空检查运算符

我有一个商业模型。我在post api中使用了该模型来发布数据。

但是当我将数据存储在这个 Business? business = Business(); business.bank!.bankName = _selectedIdProof.toString(); 中时，我遇到了错误。

请帮帮我，我不知道如何解决这个问题。

以下是我的商业模型：

import 'package:evillage_app/models/business/bank/bank.dart';

import 'package:evillage_app/models/business/hours/hours.dart';

import 'package:evillage_app/models/business/proof/proof.dart';

import 'package:evillage_app/models/utils/attachment/attachment.dart';

import 'package:evillage_app/models/utils/map/map.dart';

import 'package:json_annotation/json_annotation.dart';

part 'business.g.dart';

@JsonSerializable()

class Business {

String? id;

String? name;

String? contact;

String? website;

String? address;

String? landmark;

String? area;

String? pincode;

String? country;

String? state;

String? district;

String? tehsil;

String? villagePanchayat;

MapModel? map;

String? category;

String? aboutShop;

String? minimumPurchase;

Proof? idProof;

Proof? panProof;

Proof? addressProof;

Proof? gst;

Proof? registrationCertificate;

Bank? bank;

List? upi;

List? hours;

List? media;

Attachment? logoMedia;

bool? status;

factory Business.fromJson(Map json) => _$BusinessFromJson(json);

Business({

this.id,

this.name,

this.contact,

this.website,

this.address,

this.landmark,

this.area,

this.pincode,

this.country,

this.state,

this.district,

this.tehsil,

this.villagePanchayat,

this.map,

this.category,

this.aboutShop,

this.minimumPurchase,

this.idProof,

this.panProof,

this.addressProof,

this.gst,

this.registrationCertificate,

this.bank,

this.upi,

this.hours,

this.media,

this.logoMedia,

this.status,

});

Map toJson() => _$BusinessToJson(this);

}

这是我的银行模型：

import 'package:json_annotation/json_annotation.dart';

part 'bank.g.dart';

@JsonSerializable()

class Bank {

String? bankName;

String? branchName;

String? type;

String? accountNumber;

String? ifscCode;

String? micrCode;

String? beneficiaryName;

factory Bank.fromJson(Map json) => _$BankFromJson(json);

Bank({

this.bankName,

this.branchName,

this.type,

this.accountNumber,

this.ifscCode,

this.micrCode,

this.beneficiaryName,

});

Map toJson() => _$BankToJson(this);

}

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1 答案

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在Flutter中出现(flutter: Null check operator used on a null value)这个问题是因为在对一个空值进行非空检查操作。Dart语言提供了null安全性特性来帮助我们找出这样的错误。当我们尝试在一个null值上进行操作时，Dart会提示这个问题的出现位置。

在你的情况下，问题出现在你的Business()对象上，它的一个属性Bank? bank被定义为可空（使用了?操作符）。在你的submit()方法中，你创建了一个空的Business()对象，默认情况下构造函数会将bank属性填充为null。当你在这里尝试访问它时：

business.bank!.bankName = _selectedIdProof.toString();

你强制Dart在bank上调用bankName属性，但是bank是null，所以Dart会抛出异常。

你可以通过提高代码质量来避免使用null值，除非对于你的业务逻辑来说是有意义的；或者避免使用非空检查操作符!，除非你百分之百确定该值不会为null。而是更倾向于使用null安全操作符?，例如?.、??、??=等。

因此，解决这个问题的方法有两种：一是提高代码质量，避免使用null值；二是避免使用非空检查操作符!，而是使用null安全操作符?。

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