What is the proper way of structuring a Dart Object? - flutter

Question

I'm having a hard-time understanding the structure of dart objects. Can someone explain what is the rightway to structure dart objects?
In the first code snippet, the constructor is called before the initiation of the variables with a final tag.
class _VideoDescription extends StatelessWidget {
const _VideoDescription({
Key key,
this.title,
this.user,
this.viewCount,
}) : super(key: key);
final String title;
final String user;
final int viewCount;
...
}
While in the second snippet, I am getting an error when I place the variable initiation after the constructor and give it a type of final. Hence this structure.
class Category {
String imgUrl;
String name;
Category(name, imgUrl) {
this.imgUrl = imgUrl;
this.name = name;
}
}
Another thing, what is the use of : super(key: key); at the end of the constructor on the first snippet?

Answer 1

From the docs:
By default, a constructor in a subclass calls the superclass’s unnamed, no-argument constructor. The superclass’s constructor is called at the beginning of the constructor body. If an initializer list is also being used, it executes before the superclass is called. In summary, the order of execution is as follows:
initializer list
superclass’s no-arg constructor
main class’s no-arg constructor
Regarding final variables you can do the following two ways:
class Category {
final String imgUrl;
final String name;
Category(this.imgUrl,this.name);
}
or:
class Category {
final String imgUrl;
final String name;
Category(imgUrl,name)
: imgUrl = imgUrl,
name = name;
}
Basically the final variables need to be initialized before the constructor body is executed, since the final variable cannot change.
Regarding the super check the following:
void main() {
var students = Students(1, "peter");
print(students.name);
}
class Students extends Person {
final int id;
Students(id, age)
: id = id,
super(age) {
print("I'm the constructor body");
}
}
class Person {
final String name;
Person(name) : name = name {
print("I'm the super constructor");
}
}
This will print:
I'm the super constructor
I'm the constructor body
peter
So here since you have final variables then you need to create an initializer list to initialize these variables and since Students extends Person and since the super class Person doesnt have a no-argument constructor, then you must call super(age). The order of execution will be the same as explained above.