# 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](https://dart.dev/tools/experiment-flags).

### Flutter SDK requirements

This sample is
[experimental](https://github.com/flutter/samples/tree/master/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.

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


### Running from Android Studio

  1. Start Android Studio.

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

  1. Open the file `lib/main.dart` in the code editor.

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