Error cannot run with sound null safety because the following dependencies

environment:
  sdk: '>=2.12.0-0 <3.0.0'

dependencies:
  flutter:
    sdk: flutter
  statsfl:

import 'package:flutter/material.dart';
// ignore: import_of_legacy_library_into_null_safe
import 'package:statsfl/statsfl.dart';

void main() {
  runApp(MaterialApp(
    builder: (_, __) => _MyPage(),
  ));
}

class _MyPage extends StatefulWidget {
  @override
  __MyPageState createState() => __MyPageState();
}

class __MyPageState extends State<_MyPage> {
  int i = 10;

  @override
  Widget build(BuildContext context) {
    return StatsFl(child: Container(color: Colors.orange));
  }
}

Error: Cannot run with sound null safety, because the following dependencies
don't support null safety:
 - package:statsfl

You can pass --no-sound-null-safety as parameter.

For example:

$ dart --no-sound-null-safety run
$ flutter run --no-sound-null-safety

However, this is not ideal. The ideal is to wait for all dependencies to migrate to null-safety.

Ah I see, so the runtime flag is required to build or run in this unsound mode. Thanks! Before I close this, is there any sort of project specific flag I can enable so I don’t need to pass this for the lifetime of my project? Or some guidance on how to configure tooling btns to pass these flags automatically?

A bit of an aside, Idealism runs up against pragmatism on a daily basis in the trenches Many libs have no active maintainers, and the last thing we want to do mid-production, is take over someone else’s migration. At the same time, it be very nice to to start writing our own NNBD code today.

We have existing apps with 50 deps, it’s just not feasible to worry about all of them, it’s also not a big win for the effort. Packages are generally not having null reference errors, and by far the biggest win for developer productivity with nnbd is within our own /lib code, so it’s a relatively huge lift, for no real rewards from devs POV.

Like in this case, I really could not care less if StatsFl is doing
StatsFl({required child}) or StatsFl({@required child} or StatsFl({child} : asset(child != null), it really is not my concern as an end user. Obviously the compile time errors are preferred, but neither is a big deal from a pragmatic standpoint. Lots of libs are like this, where they just take some child, and upgrading their public interface to NNBD is really fairly pointless, and certainly not something we would defer NNBD usage for if the lib has no active maintainer or they are busy.

@esDotDev In general, we don’t recommend using null safety until your dependencies are all migrated, but as long as you aren’t planning to publish the code on pub (which gets into semver issues) we do support running in this mode. Just be aware that at any point, upgrading your deps may break you (since they may now be opted in, causing new errors in your null safe code that was depending on previously non-null safe libraries). You can of course downgrade your deps again, or just fix the errors. We do recommend publishing null safe packages as a major version bump, so you can also use version constraints on your packages to mitigate this, assuming authors follow our recommendations.

To your question, the easiest way to avoid having to pass the --no-sound-null-safety flag explicitly is to opt out your entry point (only) out of null safety by marking the file which contains main with // @dart=2.9. This will cause the system to auto-detect that you are not fully migrated, and it will run in unsound mode.

Note: you will not be able to use null safety syntax in the file containing main, since it is opted out, so you will either need to «un-migrate» it, or else move your real main to a different file and make your entry point a dummy main file (opted out as described above) which does nothing but calls your real main, which can then be opted in:

real_main.dart

// This contains my real entry point, and is opted in
void realMain(...) {
   // Do real stuff, using null safety features 
};

main.dart

// This contains a dummy entry point, and is opted out
// @dart=2.9
import 'real_main.dart';
void main(...) { // Can't use null safety features in this file
  realMain(...);
}

the docs are not explicit enough, after reading them i understood that using // @Dart=2.9 in a dart file part of a flutter project it allows you to use «non null safe» packages in that file only. but it seems that in order to compile the project with «non null packages» you got to place that comment in the main file.
in the docs
«4. Add a language version comment to the top of any Dart files that you don’t want to consider during your current migration:»
source: https://dart.dev/null-safety/unsound-null-safety

How can I set —no-sound-null-safety in Xcode?

In general, we don’t recommend using null safety until your dependencies are all migrated,_

Why? This makes sense for plugin developers, because the plugin might be used in a pure null-safe project. But it doesn’t make any sense for app developers. Many times there are 5% of your plugins you can’t upgrade, and there are still loads of benefits in the meantime to upgrade your /lib code.

I’m really struggling to understand this guidance for app developers. There is basically zero risk to running in this mode, upgrading all your /lib code, and having 1 or 2 legacy deps.

Just be aware that at any point, upgrading your deps may break you (since they may now be opted in, causing new errors in your null safe code that was depending on previously non-null safe libraries).

How? nnbd are always under a major version, as long as we stick on legacy versions, there is no way anything can just break unless we do an explicit version bump, or have no versions specified.

Note: you will not be able to use null safety syntax in the file containing main,

Ah, this is where I got confused as the docs don’t make any distinction here. They say to use this param to enable opt out opf sound-safety, but when you do it, it changes your entire file to legacy which is not expected. Cool trick about moving main though.

Also, fwiw, you can set this up in Android Studio easily enough:

This comment has been minimized.

How can we set in android studio, not for flutter, is for native?

This is for run&debug configuration
how to do same thing for build configuration?

How can we set in xcode?

For Xcode, it will configure this automatically after you run flutter run once.
or run flutter build ios --debug --config-only --no-sound-null-safety

explain: flutter run added User-Defined Build Settings EXTRA_FRONT_END_OPTIONS=--no-sound-null-safety in Flutter/Generated.xcconfig

How can we set in android studio, not for flutter, is for native?

For Android Studio run native code when you open android folder with Android Studio
add extra-front-end-options=--no-sound-null-safety in your gradle.properties file

explain: https://stackoverflow.com/a/67751484

How can we set in xcode?

For Xcode, it will configure this automatically after you run flutter run once.
or run flutter build ios --debug --config-only --no-sound-null-safety

explain: flutter run added User-Defined Build Settings EXTRA_FRONT_END_OPTIONS=--no-sound-null-safety in Flutter/Generated.xcconfig

How can we set in android studio, not for flutter, is for native?

For Android Studio run native code when you open android folder with Android Studio
add extra-front-end-options=--no-sound-null-safety in your gradle.properties file

explain: https://stackoverflow.com/a/67751484

thank solution. It’s work for me

Problem: I would like to write some NNBD code which relies on some legacy package, but I can’t seem to make it work.

In this example, statsfl is a legacy lib, we want to use NNBD in our own code, we don’t care that statsfl is legacy internally.

If I have a pubspec like this:

environment:
  sdk: '>=2.12.0-0 <3.0.0'

dependencies:
  flutter:
    sdk: flutter
  statsfl:

And some code like this:

import 'package:flutter/material.dart';
// ignore: import_of_legacy_library_into_null_safe
import 'package:statsfl/statsfl.dart';

void main() {
  runApp(MaterialApp(
    builder: (_, __) => _MyPage(),
  ));
}

class _MyPage extends StatefulWidget {
  @override
  __MyPageState createState() => __MyPageState();
}

class __MyPageState extends State<_MyPage> {
  int i = 10;

  @override
  Widget build(BuildContext context) {
    return StatsFl(child: Container(color: Colors.orange));
  }
}

We would expect it to build, but it doesn’t, throwing:

Error: Cannot run with sound null safety, because the following dependencies
don't support null safety:
 - package:statsfl

Note: I’m sure where exactly to log this bug as it straddles Flutter and Dart SDKs.

Only type // @Dart=2.9 in main.dart file. All problems related to null safety will be solved in other dart files also.