Map作為Flutter中的函式引數-有解無憂

錯誤：Non-nullable instance field id must be initialised.

來源：Picture.fromMap

import 'dart:typed_data'; // Uint8List

class Picture {
 int id;
 String title;
 Uint8List? picture;

 Picture({this.id = 0, this.title = "", this.picture });

 Picture.fromMap(Map map) {
   id = map[id];
   title = map[title];
   picture = map[picture];
 }

  Map<String, dynamic> toMap() => {
   "id": id,
   "title": title,
   "picture" : picture,
 };
}

傳遞給函式引數時的初始化方式是什么Map，或者如何解決這個錯誤？

uj5u.com熱心網友回復：

late當您知道它稍后將被初始化時使用關鍵字。否則 int id=0;對其他不可為空的欄位使用和類似作為默認初始化

class Picture {
 late int id;
 late String title;
 Uint8List? picture;

 Picture({this.id = 0, this.title = "", this.picture });

 Picture.fromMap(Map map) {
   id = map[id];
   title = map[title];
   picture = map[picture];
 }

  Map<String, dynamic> toMap() => {
   "id": id,
   "title": title,
   "picture" : picture,
 };
}

uj5u.com熱心網友回復：

因為你在Picture.fromMap構造函式體中初始化。試試下面的代碼。

import 'dart:typed_data'; // Uint8List

class Picture {
  int id;
  String title;
  Uint8List? picture;

  Picture({this.id = 0, this.title = "", this.picture});

  factory Picture.fromMap(Map map) {
    return Picture(
      id: map["id"],
      title: map["title"],
      picture: map["picture"],
    );
  }

  Map<String, dynamic> toMap() => {
        "id": id,
        "title": title,
        "picture": picture,
      };
}

或者

import 'dart:typed_data'; // Uint8List

class Picture {
  int id;
  String title;
  Uint8List? picture;

  Picture({this.id = 0, this.title = "", this.picture});

  Picture.fromMap(Map map)
      : id = map["id"],
        title = map["title"],
        picture = map["picture"];

  Map<String, dynamic> toMap() => {
        "id": id,
        "title": title,
        "picture": picture,
      };
}

uj5u.com熱心網友回復：

此類實體欄位分配有 2 個選項。

命名生成建構式
命名工廠建構式

生成式建構式： 您可以在 (:) 冒號后面使用初始化串列

工廠構造器： 您可以使用任何業務邏輯并在構造器主體中初始化您的實體欄位（變數），就像常規函式一樣。

示例：

class User {
  String name;
  int id;

   **// Named generative constructor**
   User.fromJson(Map<String,Object> json)
     : name = json['name'] as String,
       id = json['id'] as int;

   **// Factory constructor**
  factory User.fromJson(Map<String,Object> json) {
    final id = json['id'] as int;
    final name = json['name'] as String;
    return User(id:id, name:name);
    }`

uj5u.com熱心網友回復：

在您的代碼塊中，您沒有定義可為空的變數。您必須定義所有具有 null 安全性的變數。

int? id;
String? title;
Uint8List? picture;

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