Description

This is a basic prototype for 3 phase macros using package:build.

The general idea is that macros run in 3 different phases, and each phase has different capabilities available to it in terms of both introspection and code generation power.

Macros are applied as annotations on declarations. They are currently supported on classes, methods, and fields.

This phases approach allows us to provide consistent, correct results for the introspection apis that are available, while simultaneously providing a lot of power in the form of adding entirely new declarations to the program.

Evaluation Guidelines

This prototype is not intended to reflect the final dev experience. Specifically, the IDE and codegen experience you see here is not a reflection of the final expected product. The actual feature would be built into the compilation pipelines you already use, and you would not see any generated files in your source directory.

We would like feedback focused on any other area though, such as:

• Does this provide enough functionality for you to do everything you want to?
• General feedback on the macro apis (anything exported by lib/definition.dart).
• General feedback on the multi-phased approach.
• Any other feedback unrelated to build_runner or the specific code generation process used in this prototype.

Note that if you do implement some macros, we would love for you to contribute them to the repo so we have more concrete examples of the use cases!

Intro to the Macro interfaces

Each type of macro has it's own interface, each of which have a single method that you must implement. These methods each take two arguments, the first argument is the object you use to introspect on the object that was annotated with the macro, and the second argument is a builder object used to modify the program.

For example, lets take a look at the ClassDeclarationMacro interface:

/// The interface for [DeclarationMacro]s that can be applied to classes.
abstract class ClassDeclarationMacro implements DeclarationMacro {
  void visitClassDeclaration(
      ClassDeclaration declaration, ClassDeclarationBuilder builder);
}

This macro is given a ClassDeclaration object as the first argument, which gives you all the reflective information available for a class, in the "declaration" phase (phase 2).

The second argument is a ClassDeclarationBuilder, which has an addToClass(Declaration declaration) method you can use to add new declarations to the class. A ClassDeclarationBuilder is also a DeclarationBuilder, which gives you the ability to add top level declarations with the addToLibrary(Declaration declaration) method.

Implementing Multiple Macro Interfaces

A single macro class is allowed to implement multiple macro interfaces, which allows it to run in several phases. As an example of this you can look at the example/macros/json.dart macro, which implements three different macro interfaces. First, is the ClassDeclarationMacro, which it uses to define the interface only of the fromJson constructor and toJson methods.

It then also implements the ConstructorDefinitionMacro and MethodDefinitionMacro interfaces which it uses to fill in those declarations with the full implementations.

Note that a macro can provide a full definition in the declaration phase, but the json macro needs more reflective information than is available to it in that phase, so it waits until the later phase where it can fully introspect on the program to fill in the implementations.

Phase 1 - Type Macros

These macros have almost no introspection capability, but are allowed to introduce entirely new classes to the application.

To make a macro run in this phase you should implement either ClassTypeMacro, FieldTypeMacro, or MethodTypeMacro, depending on which type of declaration your macro supports running on.

Phase 2 - Declaration Macros

These macros can introspect on the declaration they annotate, but cannot recursively introspect on the types referenced in those declarations. They are however allowed to ask questions about type relationships for any types they see, through the isSubtypeOf api.

These macros are allowed to introduce new public declarations to classes, as well as the current library, but not new types.

To make a macro run in this phase you should implement either ClassDeclarationMacro, FieldDeclarationMacro, or MethodDeclarationMacro, depending on which type of declaration your macro supports running on.

Phase 3 - Definition Macros

These macros can introspect fully on the declaration they annotate, including recursively introspecting on the types referenced in those declarations.

In exchange for this introspection power, these macros are only allowed to implement existing declarations. No new declarations can be added in this phase, so the static shape of the program is fully complete before these macros run.

To make a macro run in this phase you should implement either ClassDefinitionMacro, FieldDefinitionMacro, or MethodDefinitionMacro, depending on which type of declaration your macro supports running on.

Example Macros, Wiring Up a New Macro

VM example

You can see some examples under example/macros.

Once you create a macro you will need to add it to all the phases in which it should run, see the typesBuilder, declarationsBuilder, and definitionsBuilder methods in example/builders.dart, and pass in your new macro to the corresponding constructor for the phase in which it should run.

Flutter example

You can see flutter examples under flutter_example/macros.

Once you create a macro you will need to add it to all the phases in which it should run, see the typesBuilder, declarationsBuilder, and definitionsBuilder methods in flutter_example/lib/builders.dart, and pass in your new macro to the corresponding constructor for the phase in which it should run.

Using a Macro

For this prototype you need to create your example files that use your macros with the extension .gen.dart. We will run codegen on only those files, applying the macros and generating a file for the output at each phase.

The final phase will create a regular .dart file which is what you should import. If you want to import other files which also use codegen, you should do so by importing the .gen.dart files, this will ensure they are visible to the build of your library (and only the appropriate info should be available).

Running macros

You will use the build_runner package to run builds, which you can run with the pub run build_runner build (or flutter pub run build_runner build) command. There is also a watch command you may want to use to get fast rebuilds.

For vm apps if you want to build and run an app in a single command, you can use the run command: pub run build_runner run example/main.dart.

For flutter apps you will need to run build_runner separately, and then launch the flutter app as normal.