Colon after Constructor in dart

Question

This code is from flutter gallery and i\'m trying to understanding and adapting it. I would know what this syntax means:

class DemoItem {
  DemoItem         


        

Answer 1

To elaborate on other answers and to complete the syntax, it is also possible to have a real body for the constructor along with initializer code

NonNegativePoint(this.x, this.y) : assert(x >= 0), assert(y >= 0) {
    print('I just made a NonNegativePoint: ($x, $y)');
}

^ Here the assertions happen before the execution of the body

Another use case is to assign values to final fields before body executes

final String x;
final String y;

Point.fromJson(Map json) : x = json['x'], y = json['y'] {
    print('In Point.fromJson(): ($x, $y)'); 
}