arbor 0.2.0

Modular and compile-time safe DI for Dart without fragility and magic.

arbor

---

Modular and compile-time safe DI for Dart without fragility and magic.

For Flutter integration, see flutter_arbor.

Index

• Index
• About
• Motivation
• Install
• Usage
  • Starting point
  • Dependency nodes
    • Module
    • Child
    • Lifecycle
    • Dependency injection
  • Observer
  • Modularity
    • Simpler
    • More modular

About

arbor offers a dependency injection solution for Dart that is compile-time safe and does not rely on magic – no code generation, no runtime type checking, no singletons, and no dynamic mutable Maps. arbor abstracts out a tree-like structure of dependencies, which can be resolved at compile-time. This allows for a more flexible and modular approach to dependency injection.

arbor is partially inspired by Needle and common Dart approach for dependency storages.

Motivation

Currently, there are two main approaches to dependency injection in Dart: constructor injection and singleton injection. Both approaches have their pros and cons. Constructor injection is a good fit for small projects, but it quickly becomes cumbersome to manage dependencies in larger projects. Singleton injection relieves the burden of managing dependencies, but it is not compile-time safe and relies on magic.

arbor is a middle ground between the two approaches. It is compile-time safe and does not rely on magic, but it is not as cumbersome as constructor injection to manually manage dependencies. arbor is also more flexible than singleton injection, as it allows for a more modular approach to dependency injection.

Install

Add arbor to your pubspec.yaml file:

dependencies:
  arbor: "current version"

Or do it via CLI.

For Flutter projects:

$ flutter pub add arbor

For Dart projects:

$ dart pub add arbor

Usage

arbor can be accustomed to both small projects that require a simple dependency injection solution and large projects that require a more modular approach to dependency injection. The following examples will demonstrate how to use arbor in both scenarios.

Starting point

Every arbor project requires a Tree to be created. A Tree is a tree-like structure of dependencies that can be resolved at compile-time. A Tree can be created by using the BaseTree class:

class Tree extends BaseTree<Tree> {
  Tree({super.observer});
}

Every other Tree class should extend the BaseTree class. The Tree class should also be passed as a type parameter to the BaseTree class. The Tree class should also have a named constructor that takes in an optional ArborObserver as a parameter. The Tree class should also have a named constructor that takes in an optional ArborObserver as a parameter. The ArborObserver is used to observe the state of the Tree and can be used for debugging purposes.

Dependency nodes

The created Tree class is a shared starting point for every node of dependencies, which can be created by using either module or child methods that accept a factory of ModuleNode or ChildNode respectively.

Both dependency nodes can have their descendants and can create a shared object that is bound to a specific lifecycle point which differs between the two dependency nodes via the shared method and creates new instances of objects via the instance method.

Both node types have the same set of lifecycle methods that can be overridden to perform actions at specific lifecycle points and both have their parent, passed as a type parameter.

Module

A module is a stateless node that acts as a namespace for dependencies and descendant dependency nodes.

Its lifecycle and any stateful operations, such as creating shared dependencies, are strictly bound and are delegated to the nearest stateful parent, which can be either a ChildNode or a BaseTree.

Usually, a module is used to group dependencies that are related to a specific feature or a specific part of the application, or a module can represent a set of features as a whole. Modules are lightweight and are cached in memory for the duration of the application's lifecycle in the root Tree node, so they can be used as needed.

Child

On the other hand, the child is a stateful node that acts as a container for dependencies and descendant dependency nodes. The child method returns an ObjectFactory<Child>, which essentially is a thunk – Child Function(), which must be called to create a new instance of the Child class.

Child nodes are used to describe a node with some ephemeral state bound to it which must be recreated several times through the lifecycle of the application.

Lifecycle

All nodes implement a set of lifecycle methods that can be overridden to perform actions at specific lifecycle points. The lifecycle methods are init and dispose, which are represented by the Lifecycle interface.

Nodes are created lazily, which means that the init method is called only when the node is resolved for the first time. The dispose method can be called from the outside, either by hand or by integrations, such as flutter_arbor.

Dependency injection

To actually perform dependency injection, a dependency node implements consumers dependency interfaces, utilizing the dependency inversion.

// consumer.dart

abstract class ConsumerDependencies {
  StreamController<String> get messagesController;
}

class Consumer {
  final ConsumerDependencies _dependencies;

  Consumer(this._dependencies);
}

// di.dart

class SomeFeatureNode 
    extends BaseChildNode<SomeFeatureNode, SomeFeatureParent> 
    implements ConsumerDependencies {
  SomeFeatureNode(super.parent);

  @override
  StreamController<String> get messagesController => shared(
      StreamController.broadcast,
      dispose: (controller) => controller.close(),
    );
}

// main.dart

void main() {
  final appDependencies = AppDependencies();
  final node = appDependencies.features.feature();
  final consumer = Consumer(node);
}

Three things are happening here:

1. The Consumer class depends on the ConsumerDependencies interface. It declares the dependencies it needs to function itself, utilizing the dependency inversion, and does not care about how the dependencies are resolved.
2. The SomeFeatureNode class implements the ConsumerDependencies interface. It provides the dependencies that the Consumer class needs to function itself.
3. The Consumer class is created by passing the SomeFeatureNode instance to its constructor. The Consumer class does not care about how the dependencies are resolved, it only cares that they are resolved, and depends only on the dependencies that it needs to function itself.

Observer

The Tree class can be observed by passing an ArborObserver to its constructor. The ArborObserver is a base class that can be extended to override specific methods that are called in specific lifecycle points of the Tree and its nodes.

An example that logs the lifecycle of the Tree and its nodes:

class PrintObserver extends ArborObserver {
  @override
  void onInit<A extends Lifecycle>() {
    super.onInit();
    print('Init $A');
  }

  @override
  void onDisposed<A extends Disposable>() {
    super.onDisposed();
    print('Disposed $A');
  }
}

To view the full list of methods that can be overridden, see the ArborObserver class.

Modularity

As discussed previously, arbor can be used in both small and large projects with consideration to tradeoffs between modularity and simplicity.

Simpler

A simple approach could look something like that - concrete parents and concrete children:

abstract class StringConsumerDependencies {
  String get veryImportantString;
  String get anotherImportantString;
  String get yetAnotherImportantString;
}

class AppDependencies extends BaseTree<AppDependencies> {
  String get veryImportantString => shared(() => 'hello');

  ExampleChildModule get exampleModule => module(ExampleChildModule.new);
}

class ExampleChildModule
    extends BaseChildNode<ExampleChildModule, AppDependencies> {
  ExampleChildModule(super.parent);

  String get anotherImportantString => shared(() => 'world');

  ObjectFactory<ExampleChildNode> get exampleChild => child(ExampleChildNode.new);
}

class ExampleChildNode
    extends BaseChildNode<ExampleChildNode, ExampleChildModule>
    implements StringConsumerDependencies {
  ExampleChildNode(super.parent);

  @override
  String get veryImportantString => parent.parent.veryImportantString;

  @override
  String get anotherImportantString => parent.anotherImportantString;

  @override
  String get yetAnotherImportantString => shared(() => '!');
}

This approach is pretty straightforward – each node declares its parent as a Type parameter, and each node has access to the whole tree since parents are concrete classes that are connected.

It relieves the developer from having to think about the structure of the tree, since the tree is a concrete class that is connected, and the developer can just focus on the dependencies themselves. The most common use case for this approach is a small project with a small number of dependencies.

More modular

One could spot that this approach has some drawbacks. The most obvious one is that the ExampleChildModule has access to the whole tree, which is not always desirable and breaks a few rules of "good code". Secondly, the children are hard-attached to their position in the tree, which makes it hard to reuse them in different parts of the tree. A more modular approach could look something like that - abstract parents and interfaces for children.

Firstly, to abstract the parents, an interface for said parents should be created. The interfaces will implement a set of common dependencies that are shared across all children:

abstract class ImportantStringDependency {
  String get veryImportantString;
}

abstract class AnotherImportantStringDependency {
  String get anotherImportantString;
}

abstract class StringConsumerDependencies
    implements ImportantStringDependency, AnotherImportantStringDependency {
  String get yetAnotherImportantString;
}

abstract class SharedParent<N extends SharedParent<N>>
    implements
        Node<N>,
        AnotherImportantStringDependency,
        ImportantStringDependency {}

The SharedParent interface is a common interface for all parents, and it implements the Node interface that makes it eligible as a parent for a child node.

After that, the children that previously declared their concrete parents as Type parameters, now declare their parents as vague implementers of the SharedParent interface:

class AppDependencies extends BaseTree<AppDependencies>
    implements SharedParent<AppDependencies> {
  @override
  String get veryImportantString => shared(() => 'hello');

  @override
  String get anotherImportantString => exampleModule.anotherImportantString;

  ExampleChildModule<AppDependencies> get exampleModule => module(
        ExampleChildModule.new,
      );
}

class ExampleChildModule<P extends SharedParent<P>>
    extends BaseModule<ExampleChildModule<P>, P>
    implements SharedParent<ExampleChildModule<P>> {
  ExampleChildModule(super.parent);

  @override
  String get veryImportantString => parent.veryImportantString;

  @override
  String get anotherImportantString => shared(() => 'world');

  ObjectFactory<ExampleChildNode<ExampleChildModule<P>>>
      get exampleModule => child(ExampleChildNode.new);
}

class ExampleChildNode<P extends SharedParent<P>>
    extends BaseChildNode<ExampleChildNode<P>, P>
    implements
        SharedParent<ExampleChildNode<P>>,
        StringConsumerDependencies {
  ExampleChildNode(super.parent);

  @override
  String get veryImportantString => parent.veryImportantString;

  @override
  String get anotherImportantString => parent.anotherImportantString;

  @override
  String get yetAnotherImportantString => shared(() => '!');
}

The naïve usage of SharedParent has resolved the main issues, now the children are not hard-attached to their position in the tree, and they have access only to the dependencies that they need. It may take some squinting to fight through type parameters, as this approach uses recursive generics.

But two new problems have emerged. Firstly, the code is not DRY anymore, since the SharedParent interface is implemented by both the AppDependencies and the ExampleChildModule classes, and secondly, new type parameter info leaked even more implementation details.

To solve the first problem, a mixin can be created that will retrieve the dependencies from the parent, and implement the SharedParent interface:

mixin SharedParentMixin<C extends SharedParentMixin<C, P>,
    P extends SharedParent<P>> on HasParent<P> implements SharedParent<C> {
  @override
  String get veryImportantString => parent.veryImportantString;

  @override
  String get anotherImportantString => parent.anotherImportantString;
}

class AppDependencies extends BaseTree<AppDependencies>
    implements SharedParent<AppDependencies> {
  @override
  String get veryImportantString => shared(() => 'hello');

  @override
  String get anotherImportantString => exampleModule.anotherImportantString;

  ExampleChildModule<AppDependencies> get exampleModule => module(
        ExampleChildModule.new,
      );
}

class ExampleChildModule<P extends SharedParent<P>>
    extends BaseModule<ExampleChildModule<P>, P>
    with SharedParentMixin<ExampleChildModule<P>, P> {
  ExampleChildModule(super.parent);

  @override
  String get anotherImportantString => shared(() => 'world');

  ObjectFactory<ExampleChildNode<ExampleChildModule<P>>>
      get exampleChild => child(ExampleChildNode.new);
}

class ExampleChildNode<P extends SharedParent<P>>
    extends BaseChildNode<ExampleChildNode<P>, P>
    with SharedParentMixin<ExampleChildNode<P>, P>
    implements StringConsumerDependencies {
  ExampleChildNode(super.parent);

  @override
  String get yetAnotherImportantString => shared(() => '!');
}

The mixin works as intended, retrieving the dependencies from the parent for us and implementing the SharedParent interface.

The second problem can also be easily solved by using good old interfaces that will describe the tree of dependencies:

abstract class IDependencies {
  IExampleModule get exampleModule;
}

abstract class IExampleModule {
  ObjectFactory<IExampleChild> get exampleChild;
}

abstract class IExampleChild implements StringConsumerDependencies {}

class AppDependencies extends BaseTree<AppDependencies>
    implements SharedParent<AppDependencies>, IDependencies {
  @override
  String get veryImportantString => shared(() => 'hello');

  @override
  String get anotherImportantString => exampleModule.anotherImportantString;

  @override
  ExampleChildModule<AppDependencies> get exampleModule => module(
        ExampleChildModule.new,
      );
}

class ExampleChildModule<P extends SharedParent<P>>
    extends BaseModule<ExampleChildModule<P>, P>
    with SharedParentMixin<ExampleChildModule<P>, P>
    implements IExampleModule {
  ExampleChildModule(super.parent);

  @override
  String get anotherImportantString => shared(() => 'world');

  @override
  ObjectFactory<IExampleChild> get exampleChild =>
      child<ExampleChildNode<ExampleChildModule<P>>>(
        ExampleChildNode.new,
      );
}

class ExampleChildNode<P extends SharedParent<P>>
    extends BaseChildNode<ExampleChildNode<P>, P>
    with SharedParentMixin<ExampleChildNode<P>, P>
    implements IExampleChild {
  ExampleChildNode(super.parent);

  @override
  String get yetAnotherImportantString => shared(() => '!');
}

For a complete example, see the example project