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# Null safety Flutter example
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This folder contains a demonstration of the upcoming Dart feature, sound null
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safety. For additional information about this feature, see
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https://dart.dev/null-safety.
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The folder contains two sample Flutter apps:
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* `null_unsafe_app`: This contains a small weather app, which has three null
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errors in its rendering code as a result of not using null safety.
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Try running the app (`flutter run` in a terminal, or the Run action in
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VSCode/Android Studio), and see if the UI works. The null errors are
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sporadic, so try hot reloading a bunch of times and experience the app
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sometimes working and sometimes failing.
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* `null_safe_app`: This contains the same weather app, but this time using
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null safety. There are two copies of the source code:
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`lib/badapp.dart`: This is the same source code as in the unsafe app. Try
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uncommenting the two commented lines of null-unsafe code, and notice how the
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Dart analyzer with null safety immediately flags the lines as unsafe.
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`lib/goodapp.dart`: This a safe version of the unsafe app, with the null
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errors fixed. Read through the comments to see illustrations of how to resolve
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null errors with null safety.
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## Running the example code
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Null safety is still in technical preview. As a result you need to use a
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dev-channel Flutter build, and you have to pass an [experiment
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flag](https://dart.dev/tools/experiment-flags).
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### Flutter SDK requirements
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This sample is
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[experimental](https://github.com/flutter/samples/tree/master/experimental), and
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requires using a **dev channel** Flutter SDK. You can change to the dev channel
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with `flutter channel dev` followed by `flutter upgrade` (don't forget to change
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back to `flutter channel stable` for production use.
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### Running from the terminal/command-prompt
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Because null safety is still in tech preview, we need to pass a so-called
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_experiment flag_ when invoking any Flutter command in the terminal, which looks
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like this: `--enable-experiment=non-nullable`.
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To run the main app, type these commands in the terminal/command-prompt:
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- Windows:
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- `cd <folder with samples repo>\experimental\null_safety\null_safe_app\`
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- `flutter pub get`
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- `flutter run --enable-experiment=non-nullable`
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- macOS/Linux:
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- `cd <folder with samples repo>/experimental/null_safety/null_safe_app/`
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- `flutter pub get`
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- `flutter run --enable-experiment=non-nullable`
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### Running from VSCode
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This example contains a launch configuration for VSCode that passes the
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experimental flag, so to run the sample in VSCode:
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1. Invoke **File > Open**, and select the `null_safety/null_safe_app` folder.
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1. Select **Run > Run** and the project should run and print a message in the
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Debug Console.
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### Running from Android Studio
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1. Start Android Studio.
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1. Select Open Project, and select the `null_safety/null_safe_app` folder.
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1. Open the file `lib/main.dart` in the code editor.
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1. Select Run > Run and the project should run and print a message in the Run
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pane.
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