I was told that 25th line of code contains an issue where setState() with random color being used. Help identify an issue - flutter

Question

Original task sounded like:
The application should: display the text "Hey there" in the middle of
the screen and after tapping anywhere on the screen a background color
should be changed to a random color. You can also add any other
feature to the app - that adds bonus points Please do not use any
external libraries for color generation
My solution (GitHub):
import 'dart:math';
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
home: RandomBackgroundColorWidget(),
);
}
}
class RandomBackgroundColorWidget extends StatefulWidget {
#override
_RandomBackgroundColorWidget createState() => _RandomBackgroundColorWidget();
}
class _RandomBackgroundColorWidget extends State<RandomBackgroundColorWidget> {
int _colorIndex = 0xFF42A5F5;
void _randomColorIndexGenerator() {
final _rng = new Random();
setState(() => {_colorIndex = (_rng.nextInt(0xFFFFFF) + 0xFF000000)});
}
#override
Widget build(BuildContext context) {
return Stack(children: [
Material(
color: Color(_colorIndex),
child: Center(
child: Text("Hey there"),
),
),
GestureDetector(
onTap: () => _randomColorIndexGenerator(),
),
]);
}
}
While reviewing my test task interviewer said that 25th line of code contains an issue.
setState(() => {_colorIndex = (_rng.nextInt(0xFFFFFF) + 0xFF000000)});
And he commented:
"It is working in a way that is not intended by you."
Help to identify an issue in 25th line of code.

Answer 1

You are accidentally combining the two ways to declare a function in Dart: the arrow operator => and curly braces {}.
Line 25 should be:
setState(() => _colorIndex = _rng.nextInt(0xFFFFFF) + 0xFF000000);
with no extra curly braces.

Answer 2

The issue is a syntax error. When using setState() =>, you dont need the {}
setState(() {_colorIndex = (_rng.nextInt(0xFFFFFF) + 0xFF000000)});
or
setState(() => _colorIndex = (_rng.nextInt(0xFFFFFF) + 0xFF000000));

Answer 3

I couldn't find the error you mention, however I recommend that you always use init state when assigning default values.
Here other way
import 'dart:math';
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
home: RandomBackgroundColorWidget(),
);
}
}
class RandomBackgroundColorWidget extends StatefulWidget {
#override
_RandomBackgroundColorWidget createState() => _RandomBackgroundColorWidget();
}
class _RandomBackgroundColorWidget extends State<RandomBackgroundColorWidget> {
Color _color;
#override
void initState() {
_color = Colors.white;
super.initState();
}
void getrandomColor() {
setState(() {
_color = Colors.primaries[Random().nextInt(Colors.primaries.length)];
});
}
#override
Widget build(BuildContext context) {
return Stack(children: [
Material(
color: _color,
child: Center(
child: Text("Hey there"),
),
),
GestureDetector(
onTap: getrandomColor,
),
]);
}
}

Answer 4

Issue is in nextInt function.
Flutter accepts color codes for generation up to 0xFFFFFFFF, where first pair of numbers is needed for opacity level and other pairs needed for RGB levels
nextInt function generates random number in range up to, but not including, passed number. For example, nextInt(3) will generate randomly 0,1 or 2, but not 3.
So original app was generating all random colors (from 0x0 to 0xFFFFFE) except last one - 0xFFFFFF
Therefore, 25th line should look like this in order to generate every possible color.
setState(() => _colorIndex = (_rng.nextInt(0x1000000) + 0xFF000000));