Just recently, Flutter 1.17 was released and it brings a lot of cool features. You can read more about it here. It comes with lots of performance improvements, new Widgets, and more. But, besides all that, this version of Flutter also got one small, but a very useful feature: compile-time variables. Yes, you heard me, starting from 1.17 you can build your Android and iOS application with predefined, compile-time variables that can be used in all layers of your applications.
Let’s take a look closer
If you are working with Flutter Web, you may know the Flutter Tool argument –dart-define. Primarily it was used to enable Skia for the Flutter Web like this –dart-define=FLUTTER_WEB_USE_SKIA=true . But what you may not know, is that starting from 1.17 you can pass multiple custom key/value pairs there.
For example, if you run the flutter app with the following arguments
flutter run –dart-define=SOME_VAR=SOME_VALUE –dart-define=OTHER_VAR=OTHER_VALUE
then you can get these values in your Dart code like this
And use this class as your Environment specific config all over the project. This means that now you can get rid of packages like environment_config.
If you don’t know what this package does, you can read this article on how to configure Flutter environment in the proper way.
Can I use these variables in native?
hort answer — yes, you can use these variables in native too but… it’s a little bit trickier.
If you are curious how Flutter passes Dart Defines into the native layers, you can take a look at the Flutter Tools package. It treats Dart Defines in a slightly different way for each platform but, I will show you examples for iOS and Android builds.
Also, keep in mind that starting from Flutter 1.20, Flutter Tool changed the way in which format Dart Defines are passed to the native layer.
To do so, I created a Flutter project that defines the Application Name and Application Suffix ID for Android and iOS based on Dart Defines variables. master branch is compatible with Flutter 1.20, and flutter-1.17 — compatible with Flutter .1.17
Let’s explore this application
This app works with just two compile-time variables which are DEFINEEXAMPLE_APP_NAME and DEFINEEXAMPLE_APP_SUFFIX. I would recommend adding a prefix to every compile-time variable just to avoid conflicts with variables that Flutter uses. In this example, I’m using DEFINEEXAMPLE_*.
The application is quite simple. If you open main.dart you will see that it defines EnvironmentConfig class, and prints it’s value.
You can notice that I used awesomeApp as a default value for DEFINEEXAMPLE_APP_NAME. We’re going to use this default value in all layers, including native. As for APP_SUFFIX — we will get an actual Application ID (packageName) in the FutureBuilder Widget from package_info plugin. And if DEFINEEXAMPLE_APP_SUFFIX is defined, you will see the Application ID on your screen.
One important note: ensure that you are assigning environment variables to the const fields. Currently, Flutter has an issue if a non-const variable was used.
Ok, so to pass compile-time variables into Android we will need to use Flavors.
No worries! No Flavors, no tons of iOS Schemes, no multiple entries, and no copy/paste… well, except for default values for different layers.
So, for Android, Flutter Tool passes all Dart Defines as a single string value, joined with a comma. In Flutter 1.17 in your Gradle, you will get a String value like: DEFINEEXAMPLE_APP_NAME=awesomeApp1,DEFINEEXAMPLE_APP_SUFFIX=.dev
And in Flutter 1.20 value will be like this: DEFINEEXAMPLE_APP_NAME%3Dawesome1,DEFINEEXAMPLE_APP_SUFFIX%3D.dev
The case here that starting from Flutter 1.19 Flutter Tool encodes URI symbols before Dart Defines are passed to the compiler, including = that is why instead of this symbol you see %3D in the Dart Define item.
So we will need to parse it before we can use each key-value pair. Full gradle code can be found here. Flutter passes Dart Defines in the project properties with dart-defines key. For Flutter 1.17 parse logic will look like this:
After we defined String resource it can be used for example in your android/app/src/main/AndroidManifest.xml to specify Application Name. But basically, you can use this set of values anywhere in your project.
And now if you take a look at your application on the Android device, you can notice that the application name was successfully defined
During iOS build, Flutter Tool creates a couple of files, including Generated.xcconfig and flutter_export_environment.sh. They can be found in ios/Flutter. Those files are in gitignore by default, so GitHub project won’t contain them. But after the build, you will find Dart Defines in those files, defined like this for Flutter 1.17
- Create Defineexample-defaults.xcconfig file with defaults
- Import it in Debug and Release configurations
- Edit your plist file and define Bundle name
- Edit Schema
- Add Pre-Action
Same as with Android, now you can use this set of values anywhere in your project.
That’s pretty much it 🙂 The example project can be found here. Example in master branch is built for Flutter 1.20, flutter-1.17 — for Flutter 1.17.