samples/experimental/null_safety/README.md

Null safety Flutter example

This folder contains a demonstration of the upcoming Dart feature, sound null safety. For additional information about this feature, see https://dart.dev/null-safety.

The folder contains two sample Flutter apps:

• null_unsafe_app: This contains a small weather app, which has three null errors in its rendering code as a result of not using null safety.

  Try running the app (flutter run in a terminal, or the Run action in VSCode/Android Studio), and see if the UI works. The null errors are sporadic, so try hot reloading a bunch of times and experience the app sometimes working and sometimes failing.

• null_safe_app: This contains the same weather app, but this time using null safety. There are two copies of the source code:

  lib/badapp.dart: This is the same source code as in the unsafe app. Try uncommenting the two commented lines of null-unsafe code, and notice how the Dart analyzer with null safety immediately flags the lines as unsafe.

  lib/goodapp.dart: This a safe version of the unsafe app, with the null errors fixed. Read through the comments to see illustrations of how to resolve null errors with null safety.

Running the example code

Null safety is still in technical preview. As a result you need to use a dev-channel Flutter build, and you have to pass an experiment flag.

Flutter SDK requirements

This sample is experimental, and requires using a dev channel Flutter SDK. You can change to the dev channel with flutter channel dev followed by flutter upgrade (don't forget to change back to flutter channel stable for production use.

Running from the terminal/command-prompt

Because null safety is still in tech preview, we need to pass a so-called experiment flag when invoking any Flutter command in the terminal, which looks like this: --enable-experiment=non-nullable.

To run the main app, type these commands in the terminal/command-prompt:

• Windows:
  • cd <folder with samples repo>\experimental\null_safety\null_safe_app\
  • flutter pub get
  • flutter run --enable-experiment=non-nullable
• macOS/Linux:
  • cd <folder with samples repo>/experimental/null_safety/null_safe_app/
  • flutter pub get
  • flutter run --enable-experiment=non-nullable

Running from VSCode

This example contains a launch configuration for VSCode that passes the experimental flag, so to run the sample in VSCode:

1. Invoke File > Open, and select the null_safety/null_safe_app folder.

2. Select Run > Run and the project should run and print a message in the Debug Console.

Running from Android Studio

1. Start Android Studio.

2. Select Open Project, and select the null_safety/null_safe_app folder.

3. Open the file lib/main.dart in the code editor.

4. Select Run > Run and the project should run and print a message in the Run pane.