Flutter - Pass a dynamic Item as Key - dart

Question

The code below is displaying the following error when I run in flutter run debug mode:
type 'List' is not a subtype of type 'String' of 'value' where
When I run in release mode flutter run --release the error does not appear and I can identify the item I selected to delete.
The problem is on line 39:
key: new Key(i),
The key only accepts string as value:
Key(String value) → Key
But I'm passing a List that appears to be a dynamic item:
dynamic i
But I need to pass a List on the Key to be able to identify what the excluded item will be.
How could I solve this problem in debug mode?
And why is this behavior happening and in the release mode the problem does not occur?
import 'package:flutter/material.dart';
import 'dart:ui' as ui;
void main() {
runApp(new MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new MaterialApp(
home: new MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
#override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
List<Widget> tiles;
List foos = [];
#override
void initState() {
this.foos = [[0, 'foo1'], [1, 'foo2'], [2, 'foo3'], [3, 'foo4']];
this.tiles = buildTile(this.foos);
super.initState();
}
//function
List<Widget> buildTile(List list) {
var x = [];
for(var i in list) {
x.add(
new ItemCategory(
key: new Key(i),
category: i[1],
onPressed: () {
setState(() {
list.removeAt(i[0]);
var list2 = [];
for(var x = 0; x < list.length; x++) {
list2.add([ x, list[x][1] ]);
}
this.tiles = buildTile(list2);
});
},
)
);
}
return x;
}
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text('Categories'),
),
body: new ListView(
padding: new EdgeInsets.only(top: 8.0, right: 0.0, left: 0.0),
children: this.tiles
)
);
}
}
class ItemCategory extends StatefulWidget {
ItemCategory({ Key key, this.category, this.onPressed}) : super(key: key);
final String category;
final VoidCallback onPressed;
#override
ItemCategoryState createState() => new ItemCategoryState();
}
class ItemCategoryState extends State<ItemCategory> with TickerProviderStateMixin {
ItemCategoryState();
AnimationController _controller;
Animation<double> _animation;
double flingOpening;
bool startFling = true;
void initState() {
super.initState();
_controller = new AnimationController(duration:
const Duration(milliseconds: 246), vsync: this);
_animation = new CurvedAnimation(
parent: _controller,
curve: new Interval(0.0, 1.0, curve: Curves.linear),
);
}
void _move(DragUpdateDetails details) {
final double delta = details.primaryDelta / 304;
_controller.value -= delta;
}
void _settle(DragEndDetails details) {
if(this.startFling) {
_controller.fling(velocity: 1.0);
this.startFling = false;
} else if(!this.startFling){
_controller.fling(velocity: -1.0);
this.startFling = true;
}
}
#override
Widget build(BuildContext context) {
final ui.Size logicalSize = MediaQuery.of(context).size;
final double _width = logicalSize.width;
this.flingOpening = -(48.0/_width);
return new GestureDetector(
onHorizontalDragUpdate: _move,
onHorizontalDragEnd: _settle,
child: new Stack(
children: <Widget>[
new Positioned.fill(
child: new Row(
mainAxisAlignment: MainAxisAlignment.end,
children: <Widget>[
new Container(
decoration: new BoxDecoration(
color: new Color(0xFFE57373),
),
child: new IconButton(
icon: new Icon(Icons.delete),
color: new Color(0xFFFFFFFF),
onPressed: widget.onPressed
)
),
],
),
),
new SlideTransition(
position: new Tween<Offset>(
begin: Offset.zero,
end: new Offset(this.flingOpening, 0.0),
).animate(_animation),
child: new Container(
decoration: new BoxDecoration(
border: new Border(
top: new BorderSide(style: BorderStyle.solid, color: Colors.black26),
),
color: new Color(0xFFFFFFFF),
),
margin: new EdgeInsets.only(top: 0.0, bottom: 0.0),
child: new Row(
mainAxisAlignment: MainAxisAlignment.start,
crossAxisAlignment: CrossAxisAlignment.center,
children: <Widget>[
new Expanded(
child: new Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: <Widget>[
new Container(
margin: new EdgeInsets.only(left: 16.0),
padding: new EdgeInsets.only(right: 40.0, top: 4.5, bottom: 4.5),
child: new Row(
children: <Widget>[
new Container(
margin: new EdgeInsets.only(right: 16.0),
child: new Icon(
Icons.brightness_1,
color: Colors.black,
size: 35.0,
),
),
new Text(widget.category),
],
)
)
],
),
)
],
),
)
),
],
)
);
}
}

Answer 1

You should know that --release flag remove all assert and type checking from the app. Which is why your error disappear.
Asserts/Type Check are here only as tools for a better development to see potential errors without having to actually run the app. A release build don't need to be aware of these.
In your case, the problem occurs because you haven't specified the 'generic' type of list in List<Widget> buildTile(List list) ; which default list to List<dynamic>.
Consequence, the compiler don't know what type is an element of your list and therefore allows you to do new Key(i). Because i may be a String.
Modifying your function prototype to List<Widget> buildTile(List<List<String>> list) (which is the real type of your list here) will allow the compiler to alert you from a potential error.
And that error is List<String> can't be assigned to type String on new Key(i).
To fix that error, you can instead do new Key(i.toString(), which will serialize your list (thanks to being primitive objects).
Or use the ObjectKey that inherit from Key, but instead of using String as parameter, it takes anObject such as key: new ObjectKey(i)