container

Container

GoLobby Container is a lightweight yet powerful IoC (dependency injection) container for Go projects. It's built neat, easy-to-use, and performance-in-mind to be your ultimate requirement.

Documentation

Required Go Versions

It requires Go v1.11 or newer versions.

Installation

To install this package, run the following command in your project directory.

go get github.com/golobby/container/v3

Introduction

GoLobby Container is used to bind abstractions to their implementations. Binding is the process of introducing appropriate concretes (implementations) of abstractions to an IoC container. In this process, you also determine the resolving type, singleton or transient. In singleton bindings, the container provides an instance once and returns it for all the requests. In transient bindings, the container always returns a brand-new instance for each request. After the binding process, you can ask the IoC container to make the appropriate implementation of the abstraction that your code needs. Then your code will depend on abstractions, not implementations!

Quick Start

The following example demonstrates a simple binding and resolving.

// Bind Config interface to JsonConfig struct
err := container.Singleton(func() Config {
    return &JsonConfig{...}
})

var c Config
err := container.Bind(&c)
// `c` will be the instance of JsonConfig

Typed Binding

Singleton

The following snippet expresses singleton binding.

err := container.Singleton(func() Abstraction {
  return Implementation
})

It takes a resolver (function) whose return type is the abstraction and the function body returns the concrete (implementation).

The example below shows a singleton binding.

err := container.Singleton(func() Database {
  return &MySQL{}
})

Transient

The example below shows a transient binding.

err := container.Transient(func() Shape {
  return &Rectangle{}
})

Named Bindings

You may have different concretes for an abstraction. In this case, you can use named bindings instead of typed bindings. Named bindings take the dependency name into account as well. The rest is similar to typed bindings. The following examples demonstrate some named bindings.

// Singleton
err := container.NamedSingleton("square" func() Shape {
  return &Rectangle{}
})
err := container.NamedSingleton("rounded" func() Shape {
    return &Circle{}
})

// Transient
err := container.NamedTransient("sql" func() Database {
    return &MySQL{}
})
err := container.NamedTransient("noSql" func() Database {
    return &MongoDB{}
})

Resolver Errors

The process of creating concrete (resolving) might face an error. In this case, you can return the error as the second return value like the example below.

err := container.Transient(func() (Shape, error) {
  return nil, errors.New("my-app: cannot create a Shaple implementation")
})

It could be applied to other binding types.

Resolving

Container resolves the dependencies with the Resolve(), Call(), and Fill() methods.

Using References

The Resolve() method takes reference of the abstraction type and fills it with the appropriate concrete.

var a Abstraction
err := container.Resolve(&a)
// `a` will be an implementation of the Abstraction

Example of resolving using references:

var m Mailer
err := container.Resolve(&m)
// `m` will be an implementation of the Mailer interface
m.Send("contact@miladrahimi.com", "Hello Milad!")

Example of named-resolving using references:

var s Shape
err := container.NamedResolve(&s, "rounded")
// `s` will be an implementation of the Shape that named rounded

Using Closures

The Call() method takes a receiver (function) with arguments of abstractions you need. It calls it with parameters of appropriate concretes.

err := container.Call(func(a Abstraction) {
    // `a` will be an implementation of the Abstraction
})

Example of resolving using closures:

err := container.Call(func(db Database) {
  // `db` will be an implementation of the Database interface
  db.Query("...")
})

You can also resolve multiple abstractions like the following example:

err := container.Call(func(db Database, s Shape) {
  db.Query("...")
  s.Area()
})

Caution: The Call() method does not support named bindings.

Using Structs

The Fill() method takes a struct (pointer) and resolves its fields.

The example below expresses how the Fill() method works.

type App struct {
    mailer Mailer   `container:"type"`
    sql Database    `container:"name"`
    noSql Database  `container:"name"`
    x int
}

myApp := App{}

err := container.Fill(&myApp)

// [Typed Bindings]
// `myApp.mailer` will be an implementation of the Mailer interface

// [Named Bindings]
// `myApp.sql` will be a sql implementation of the Database interface
// `myApp.noSql` will be a noSql implementation of the Database interface

// `myApp.x` will be ignored since it has no `container` tag

Binding time

You can resolve dependencies at the binding time if you need previous dependencies for the new one.

The following example shows resolving dependencies at binding time.

// Bind Config to JsonConfig
err := container.Singleton(func() Config {
    return &JsonConfig{...}
})

// Bind Database to MySQL
err := container.Singleton(func(c Config) Database {
    // `c` will be an instance of `JsonConfig`
    return &MySQL{
        Username: c.Get("DB_USERNAME"),
        Password: c.Get("DB_PASSWORD"),
    }
})

Standalone Instance

By default, the Container keeps your bindings in the global instance. Sometimes you may want to create a standalone instance for a part of your application. If so, create a standalone instance like the example below.

c := container.New()

err := c.Singleton(func() Database {
    return &MySQL{}
})

err := c.Call(func(db Database) {
    db.Query("...")
})

The rest stays the same. The global container is still available.

Tips

Performance

The package Container inevitably uses reflection in binding and resolving processes. If performance is a concern, try to bind and resolve the dependencies out of the processes that run many times (for example, HTTP handlers). Place it where that runs only once when you run your application like main and init functions, instead.

License

GoLobby Container is released under the MIT License.