Catalyst Builder

A dependency injection provider builder for dart.

Background

Since Catalyst is only for Dart and Flutter Catalyst supports Flutter, but a mess to configure I decided to do something cooler.

Catalyst Builder is a dependency injection provider builder for both, Dart and Flutter. It's easy to use and dependency injection is almost done automatically. You only have to decorate your services with @Service and the build_runner will create a service provider for you.

Installation

Warning, I am offering this package at an early stage. It may not perform as expected.

Add this to your pubspec.yaml and run pub get or flutter pub get:

dependencies:
  catalyst_builder: ^0.0.4

dev_dependencies:
  build_runner: ^2.0.1

Usage

Decorate your services with @Service:

@Service()
class MyService {}

Then run pub run build_runner build or flutter pub run build_runner build.
You can also run pub run build_runner watch or flutter pub run build_runner watch to update the provider automatically as you perform changes.

You should see a new file service_provider.dart after the build. Import it to use the service provider.

import 'default_service_provider.dart';

void main()
{
  var provider = DefaultServiceProvider();
  provider.boot();
  
  var myService1 = provider.resolve<MyService>();
  
  // Inferred types are also supported
  MyService myService2 = provider.resolve();
}

Advanced usage

Service lifetime

You can specify the lifetime of the service in the annotation.

@Service(
  lifetime: ServiceLifetime.singleton, // default
)
class SingletonService {}

@Service(
  lifetime: ServiceLifetime.transient,
)
class TransientService {}

Lifetime	Description
Singleton	The instance is stored in the provider. You'll always receive the same instance.
Transient	Everytime you call resolve or tryResolve you'll receive a fresh instance.

Exposing

You can expose the service with another type in the service provider. This is useful if you want to depend on an interface instead of an implementation.

abstract class Transport {
  void transferData(String data);
}

@Service(exposeAs: Transport)
class ConsoleTransport implements Transport {
  @override
  void transferData(String data) {}
}

abstract class ChatProvider {
  Transport transport;
  Future<void> sendChatMessage(String message);
}

@Service(exposeAs: ChatProvider)
class CoolChatProvider implements ChatProvider {
  @override
  Transport transport;

  CoolChatProvider(this.transport);

  @override
  Future<void> sendChatMessage(String message) {}
}

void main() {
  var provider = DefaultServiceProvider();

  var chatService = provider.resolve<ChatProvider>();

  print(chatService is CoolChatProvider); // true
  print(chatService.transport is ConsoleTransport); // true
}

Preloading services

By default, a service will be instantiated when it's requested. In some cases you need to create an instance after booting the service provider. For example a database connection or a background service that checks the connectivity.

To preload services you can use the @Preload annotation. This annotation is only available for singleton services. Example:

@Service()
@Preload()
class MyService {
  MyService(){
    print('Service was created');
  }
}

void main() {
  ServiceProvider provider;
  provider.boot(); // prints "Service was created" 
  provider.resolve<MyService>(); // Nothing printed
}

Inject parameters by name

The service provider will try to lookup values for non-existent services in the parameters map. By default, the lookup is done based on the name of the parameter. For example:

@Service()
class MyService {
  String username;
  MyService(this.username);
}

void main() {
  ServiceProvider provider;
  provider['username'] = 'Test';
  provider.boot();
  
  var myService = provider.resolve<MyService>();
  
  print(myService.username); // Test 
}

In many cases you've generic terms like 'key' or 'name'. If you've many services with the same name, you'll get in trouble.

You can use the @Parameter('param name') annotation to solve this problem:

@Service()
class MyService {
  String username;
  MyService(@Parameter('senderUserName') this.username);
}

void main() {
  ServiceProvider provider;
  provider['senderUserName'] = 'Test 2';
  provider.boot();
  
  var myService = provider.resolve<MyService>();
  
  print(myService.username); // Test 2 
}

Configuration

To customize the builder, create a build.yaml beside your pubsepc.yaml with this content:

targets:
  $default:
    auto_apply_builders: true
    builders:
      catalyst_builder|buildServiceProvider:
        options:
            providerClassName: 'DefaultServiceProvider' # class name of the provider
            outputName: 'default_service_provider.dart' # file name of the provider. (Can also contain /)