Why the property execute before dart constructor? - dart

Question

Why the List list = new List() execute before the constructor HomeContent?
class HomeContent extends StatelessWidget{
List list = new List(); // no matter I write this line code before or after `HomeContent()`
HomeContent() {
for(var i = 0; i < 21; i ++) {
this.list.add("I am item $i .");
}
}
Widget build(BuildContext context) {
return ListView.builder(
itemCount: this.list.length,
itemBuilder: (context, index) {
return ListTile(
title: Text(this.list[index]),
);
}
);
}
}

Answer 1

The order in which you add class members has no effect on the order of execution. When constructing a new object, the class's properties are initialised before the constructor is executed.

Answer 2

Dart object initialization is defined to work the way you observe.
When you write new Foo(...) then the language allocates space for a Foo object and then goes through the initialization process for the Foo class and all its superclasses.
The initialization of an instance of Foo happens as follow:
First evaluate instance field initializers declared in Foo in source order.
Then evaluate the initializer list of the Foo class.
Then recursively initialize the instance as Foo's superclass, as specified by the super invocation of the initializer list (defaults to super() if you write nothing).
When done with that, execute the Foo constructor body.
When this process hits the Object class, the object is considered properly initialized, and it's made available to the constructor bodies that are executed on the way back.
As an example:
Object log(Object value) {
print(value);
return value;
}
class Super /* extends Object */ {
var superField6 = log(6);
var superField9;
Super(this.superField5) : superField8 = log(8), superField9 = log(9) {
log(10);
}
var superField7 = log(7);
var superField8;
var superField5;
}
class Sub extends Super {
var subField1 = log(1);
var subField4;
Sub() : subField3 = log(3), subField4 = log(4), super(log(5)) {
log(11);
}
var subField2 = log(2);
var subField3;
}
main() {
new Sub();
}
If you run this code, you will see the log calls happening in the order of their arguments.