How to check 'late' variable is initialized in Dart - flutter

Question

In kotlin we can check if the 'late' type variables are initialized like below
lateinit var file: File
if (this::file.isInitialized) { ... }
Is it possible to do something similar to this in Dart..?

Answer 1

Unfortunately this is not possible.
From the docs:
AVOID late variables if you need to check whether they are initialized.
Dart offers no way to tell if a late variable has been initialized or
assigned to. If you access it, it either immediately runs the
initializer (if it has one) or throws an exception. Sometimes you have
some state that’s lazily initialized where late might be a good fit,
but you also need to be able to tell if the initialization has
happened yet.
Although you could detect initialization by storing the state in a
late variable and having a separate boolean field that tracks whether
the variable has been set, that’s redundant because Dart internally
maintains the initialized status of the late variable. Instead, it’s
usually clearer to make the variable non-late and nullable. Then you
can see if the variable has been initialized by checking for null.
Of course, if null is a valid initialized value for the variable, then
it probably does make sense to have a separate boolean field.
https://dart.dev/guides/language/effective-dart/usage#avoid-late-variables-if-you-need-to-check-whether-they-are-initialized

Answer 2

Some tips I came up with from advice of different dart maintainers, and my self-analysis:
late usage tips:
Do not use late modifier on variables if you are going to check them for initialization later.
Do not use late modifier for public-facing variables, only for private variables (prefixed with _). Responsibility of initialization should not be delegated to API users. EDIT: as Irhn mentioned, this rule makes sense for late final variables only with no initializer expression, they should not be public. Otherwise there are valid use cases for exposing late variables. Please see his descriptive comment!
Do make sure to initialize late variables in all constructors, exiting and emerging ones.
Do be cautious when initializing a late variable inside unreachable code scenarios. Examples:
late variable initialized in if clause but there's no initialization in else, and vice-versa.
Some control-flow short-circuit/early-exit preventing execution to reach the line where late variable is initialized.
Please point out any errors/additions to this.
Enjoy!
Sources:
eernstg's take
Hixie's take
lrhn's take
leafpetersen's final verdict as of 2021 10 22
Effective Dart
Self-analysis on how to approach this with some common-sense.

Answer 3

You can create a Late class and use extensions like below:
import 'dart:async';
import 'package:flutter/foundation.dart';
class Late<T> {
ValueNotifier<bool> _initialization = ValueNotifier(false);
late T _val;
Late([T? value]) {
if (value != null) {
this.val = value;
}
}
get isInitialized {
return _initialization.value;
}
T get val => _val;
set val(T val) => this
.._initialization.value = true
.._val = val;
}
extension LateExtension<T> on T {
Late<T> get late => Late<T>();
}
extension ExtLate on Late {
Future<bool> get wait {
Completer<bool> completer = Completer();
this._initialization.addListener(() async {
completer.complete(this._initialization.value);
});
return completer.future;
}
}
Create late variables with isInitialized property:
var lateString = "".late;
var lateInt = 0.late;
//or
Late<String> typedLateString = Late();
Late<int> typedLateInt = Late();
and use like this:
print(lateString.isInitialized)
print(lateString.val)
lateString.val = "initializing here";
Even you can wait for initialization with this class:
Late<String> lateVariable = Late();
lateTest() async {
if(!lateVariable.isInitialized) {
await lateVariable.wait;
}
//use lateVariable here, after initialization.
}

Answer 4

Someone may kill you if they encounter it down the road, but you can wrap it in a try/catch/finally to do the detection. I like it better than a separate boolean.
We have an instance where a widget is disposed if it fails to load and contains a late controller that populates on load. The dispose fails as the controller is null, but this is the only case where the controller can be null. We wrapped the dispose in a try catch to handle this case.

Answer 5

Use nullable instead of late:
File? file;
File myFile;
if (file == null) {
file = File();
}
myFile = file!;
Note the exclamation mark in myFile = file!; This converts File? to File.

Answer 6

I'm using boolean variable when I initiliaze late varible.
My case is :
I'm using audio player and I need streams in one dart file.
I'm sharing my code block this methodology easily implement with global boolean variables to projects.
My problem was the exception i got from dispose method when user open and close the page quickly