HardWire ⚡

Crystal CI

A Compile-time Dependency Injection system for Crystal.


  1. Add the dependency to your shard.yml:
    github: rwnx/hardwire
  1. Run shards install


require "hardwire"

Hardwire is designed to operate inside a container object. Since the resolution is compile-time (Using Macros), normally this will be a module.

Creating a container 📦

# To create a new container, include `HardWire::Container`
# This will add the macros you need to register and resolve wiring
module Container
  include HardWire::Container

  # use transient/singleton to wire different lifecycles
  # singleton dependencies will be memoized
  # dependencies for the constructor will be resolved from the constructor automatically
  transient Dependency
  singleton NeedsDependency
  scoped Webservice

  # you can also register dependencies with a block instead of inspecting the constructor
  # Your block MUST return an instance of the class you are registering
  singleton NeedsDependency {
    NeedsDependency.new( self.resolve Dependency )

Hardwire tries to operate with minimal modifications to other classes (unless required). "simple" classes, e.g.

If your classes match this signature, you can wire up in the container without adding anything to the classes.

For everything else, there's:

Multiple Constructors 🚧

Hardwire needs to know which constuctor function to use.

Annotate your "Injectable" constructor with the Hardwire::Inject annotation.

class MultipleInits
  def initialize(input: String)
    # register will inspect this method's arguments
    # [...]

  def initialize
    # will not be used for injection
    # [...]

Tags 🏷

To differentiate between registrations of the same type, use the HardWire::Tags annotation. Tags allow you to attach additional metadata to the signature. Tags themselves are string-based, simple identifiers (/\w+/) that allow you to resolve a different registration of the same class.

# [...]

# registering a transient dependency with tag "secret"
transient String, "secret" {
  "a secret string"

# registering a singleton
# When no tags are set, it is considered the "default" registration
singleton DbService

# registering a different singleton with a tag
singleton DbService, "primary"

# Resolving Dependencies
class Resolving
  @[Hardwire::Tags(input: "secret", primary_db: "primary")]
  def initialize(input : String, primary_db : DbService, default_db : DbService)

Lifecycles ♽

There are 3 lifecycles available for registrations:

Scopes 🔭

To managed scoped instances, you should create a scope object with the .scope macro.

# This example will init a database DatabaseConnection for each http request
# but all the databases will recieve the same instance of config (singleton)
# the ScopedLogging dependency will also be instantiated once for each scope resolution
require "kemal"
class Config; end
class ScopedLogging; end
class DatabaseConnection
  def initialize(@config : Config, @logging : ScopedLogging)

module Container
  include HardWire::Container

  singleton Config
  scoped ScopedLogging
  scoped DatabaseConnection

get "/" do
  # create a unique scope
  scope = Container.scope

  logger = scope.resolve ScopedLogging
  db = scope.resolve DatabaseConnection

  logger.log("I share a logger with the database in scope!")


Resolving Manually 🔨

You can resolve dependencies manually using the .resolve macro. This allows you to resolve dependencies manually with the tag string.

module Container
  include HardWire::Container

  transient SecretService, "primary"
  singleton DatabaseThing

service = Container.resolve SecretService, "primary"
db = Container.resolve DatabaseThing

Runtime Interrogation 👀

Hardwire can tell you information about the registrations at runtime, but the dependencies are HardWired (See what I did there?), so they can't be changed.

module Container
  include HardWire::Container

  singleton DbService

Container.registered?(DbService) # true
Container.registered?(DbService, "tagged") # false
Container.registered?(String) # false


  1. Fork it (https://github.com/rwnx/hardwire/fork)
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request