什么是flutter/Dart中的factory和formJson Model class？

Question

To access data we need to pass response to a dart model. what is the purpose behind this? i tried reading the dart model code where "factory" and other keyword like "formJson" are there. what is the implementation of this?

simple model class code below:

factory Posts.fromJson(Map<String, dynamic> json) => Posts(
        userId: json["userId"],
        id: json["id"],
        title: json["title"],
        body: json["body"],
    );

Answer 1

In Dart, we use the factory keyword to identify a default or named constructor. We use the factory keyword to implement constructors that do not produce new instances of an existing class.

Syntax

class Class_Name {
  factory Class_Name() {
    // TODO: return Class_name instance
  }
}

We must follow some rules when using the factory constructor.

The return keyword is used. It does not have access to the this keyword. Return value A factory constructor can return a value from a cache or a sub-type instance.

Example The following code shows how to use the factory keyword in Dart:

// create Class Car
class Car {
    //class properties
    String name;
    String color;

    //constructor
    Car({ this.name, this.color});

    // factory constructor that returns a new instance
    factory Car.fromJson(Map json) {
    return Car(name : json['name'], 
    color : json['color']);
    }
}

void main(){
    // create a map
    Map myCar = {'name': 'Mercedes-Benz', 'color': 'blue'};
    // assign to Car instance
    Car car = Car.fromJson(myCar);
    //display result
    print(car.name);
    print(car.color);
}