Super Annotations

Metaprogramming as you dream it. Use code generation and custom annotations with ease.

  • Write your code generation functions naturally alongside your normal code.
  • Define and use custom annotations in the same file or project.

For the first time, this makes code generation applicable for all kinds of projects. No complex setup, no experience in writing builders needed.

Outline

This package is still in active development. If you have any feedback or feature requests, write me and issue on github.

Get Started

First, add super_annotations as a dependency, and build_runner as a dev_dependency.

flutter pub add super_annotations
flutter pub add build_runner --dev

Next create a new file with the .super.dart extension. This tells super_annotations to run the code generation step on this file. Define your custom annotation like this:

import 'package:super_annotations/super_annotations.dart';

/// Choose a name and extend [ClassAnnotation]
class MyAnnotation extends ClassAnnotation {
  /// You need a const constructor to be usable as an annotation
  const MyAnnotation();

  /// You have to implement the [apply] method, which will be
  /// executed during the build phase
  /// @param target: A formal description of the annotated class, e.g. its name and fields
  /// @param library: The library that will be generated as output of the build phase
  @override
  void apply(Class target, LibraryBuilder library) {
    // Your custom implementation here
  }
}

The target parameter will hold all the information about the annotated class, and the library parameter can be used to produce your code generation output in a formal way.

Access information about the class using target.name, target.fields and so on. Add code to the generation output using: library.body.add(...). You can add declarations like Class(...), Extension(...), Mixin(...) etc, or use raw code with Code(...).

After that, use your custom annotation as you like:

@MyAnnotation()
class MyClass {
  MyClass(this.data);

  final String data;
}

Finally, run flutter pub run build_runner build, which will generate a .g.dart file alongside your .super.dart file.

Generating code

This package leverages the code_builder package to easily specify your generation outputs, whether it's classes, mixins, extensions or other code. This enables you to just care about the semantics of your generation output, while we take care of generating the correct syntax as well as formatting the generated code.

To define your generation outputs, modify the provided LibraryBuilder inside your annotations apply() method or a generation hook. The most used way is to modify the library using library.body.add() or library.body.addAll(). For a list of all supported declarations, check out the api reference of the code_builder package.

Generation hooks

With custom annotations, you have the possibility to generate code for each annotated class. Besides this you might want to do other tasks during code-generation, such as add imports to your generated library.

To do this, use generation hooks. These are just top-level functions, that are annotated with @CodeGen.runBefore() or CodeGen.runAfter(). As the names suggest, the annotated function will the be run before everything else, or after everything else.

You can define multiple functions for each type of hook, but those will be executed in no particular order.

When it fails

There exists a common misuse, that lead to a failure of the code generation when executing build_runner build.

You have to make sure that the code executed during the code generation phase can be compiled. Therefore it is important that the libraries that contain your annotations are syntactically correct and can be compiled, otherwise code generation will fail.

However it is common that your target code - the classes using your annotations - is not compilable until the code generation is complete, for example when you are referring to mixins or classes that are later part of the generated code.

In those cases you have to place your annotations in separate files and import them from your target code. Make sure that your dart files containing the annotations are compilable even when the target files are not, especially that they do not import any code with errors. The builder is then smart enough to only import the needed files with the annotations.

Mastering Annotations

With super_annotations, your build and runtime environment share the same codebase. This enables a few unique perks, that you wouldn't normally get with normal code generation.

Annotation parameters

Since your annotations are just a normal class, you can define fields that are then used as annotation parameters.

Look at the following example:

@MyAnnotation("abc", 42)
class MyClass {}

class MyAnnotation extends ClassAnnotation {

  const MyAnnotation(this.id, this.myValue);

  final String id;
  final int myValue;

  @override
  void apply(Class clazz, LibraryBuilder library) {
    // read on
  }
}

In the apply method, you can now access this.id and this.myValue, which will hold the appropriate values from the actual annotation. When you use your annotation on multiple classes, the fields will always have the correct value matching the currently analyzed class.

Resolved Annotations

On top of annotation parameters, you can even access non-functional annotations (that are not super annotations) with ease:

@MyAnnotation()
class MyClass {
  @MyOtherAnnotation("important_label")
  void doSomething() {}
}

/// Just a regular annotation, nothing 'super'
class MyOtherAnnotation {
  final String label;
  const MyOtherAnnotation(this.label);
}

/// The real deal
class MyAnnotation extends ClassAnnotation {
  const MyAnnotation();

  @override
  void apply(Class clazz, LibraryBuilder library) {
    // use [resolvedAnnotations] on any element (e.g. method) to get the actual annotation objects
    var methodAnnotation = clazz.methods.first.resolvedAnnotations.first;
    if (methodAnnotation is MyOtherAnnotation) { // yes
      // do something with [methodAnnotation.label]
    }
  }
}

Combine these two methods and bring your code generation game to a whole new level.

Examples

We prepared a few examples, that showcase different things that you can do with this package.

Json serialization

Source: json_serialization_example

This example shows how to generate json serialization code, which is probably the most common use-case for code generation. It is inspired by and mimics the basic behavior of json_serializable

Sealed classes

Source: sealed_classes_example

This example shows how to generate sealed classes / union types. It is inspired by and mimics the basic behavior of freezed

Data classes

Source: data_class_example

This example shows how to generate utility methods for data classes. This will generate copyWith and toString methods for each annotated class.

Code-Gen packages with similar functionality:

How does it work?

I plan to publish a detailed article on the inner workings of this package, but here is a short rundown:

  1. The package defines a builder that automatically runs on all *.super.dart files
  2. It identifies custom annotations and analyzes the annotated classes
  3. It produces a new *.runner.g.dart file, that:
    • contains the analyzed classes
    • calls the custom annotations write methods
    • calls additional runBefore or runAfter methods
  4. It spawns the runner as a new isolate and receives the generation results via inter-process communication
  5. It kills the isolate and deletes the *.runner.g.dart file
  6. It writes the generation results to a *.g.dart file as the build output

Bonus: Why super?

Custom annotations are not only super great, but also super in terms of programming. As you would define superclasses with superconstructors, this package defines structures above annotations.

Or in other words: While other code generation packages use code that generates other code, this package uses code that generates other code that generates other code.

So super.

Libraries

builder
super_annotations