Go Modules

A dependency injection library using struct tags.

Overview

This library simplifies the wiring of an application by injecting dependencies between modules.

A module is a go struct containing fields tagged with 'inject' or 'provide' keys. When a set of modules are bound, fields tagged 'inject' are set with values from corresponding fields tagged 'provide', respecting type and name. Provided fields may be set normally prior to binding, during binding from a module's Provide method, or from Injectors triggered by additional tag keys.

How to Use

Modules

A module is any tagged struct. The 'inject' and 'provide' tag keys are treated specially. Other tag keys may be registered with a Binder to trigger Injectors. Unexported fields, and fields without recognized tags will be ignored during binding.

type simpleModule struct {
  // Requires a string value named 'injectMe' to be injected.
  FieldA string 'inject:"injectMe"'
  // Provides a string value named 'provideMe'.
  FieldB int 'provide:"provideMe"'
  // These fields are ignored by the Binder.
  FieldC bool
  fieldD string
}

When simpleModule is bound, it provides the string value named 'provideMe' via FieldB to the Binder, and expects the string dependency named 'injectMe' to be provided by another module and injected into FieldA. FieldC and fieldD will be ignored by the Binder.

Providers

There are a few different ways for a module to provide values.

Fields may be set normally prior to binding.

module := struct {
  FieldA string 'provide:"provideMe"'
} {
  FieldA: "value"
}
// or
module.FieldA = "value"

Modules implementing the Provider interface may set fields from the Provide method.

type module struct {
  Field string 'inject:"injectMe"'
  Connection func() Connection 'provide:"provideMe"'
}
// Implements modules.Provider
func (m *Module) Provide() error {
  m.ConnectionProvider = func() Connection {
    return NewConnection(m.Field)
  }
  return nil
}

The Provide method is called during binding. Injected fields have not yet necessarily been set when Provide is called, so they may not be accessed directly, but they may be closed over.

Additionally, a Binder may be configured to recognize certain tag keys and call an Injector to set a value. The 'literal' tag key is built-in, and parses string tag values into standard supported types.

type module struct {
  FieldA string 'provide:"stringField" literal:"someString"'
  FieldB int    'provide:"intField" literal:"10"'
  FieldC complex128 'provide:"complexField" literal:"-1,1"'
}

Other built-in tag keys include:

• 'env' for environment variables
• 'file' for os.File handles, and decoding of txt, json, xml, and gob
• 'flag' for command line arguments

Binders

Modules are bound using a Binder. Binders are created with the NewBinder function, which optionally accepts functional option arguments.

binder := modules.NewBinder(modules.LogWriter(os.Stdout))

This binder logs information to stdout.

The Bind method binds a set of modules. All binding and injection occurs during this call. Modules implementing Provider will have their Provide method called. Exported fields are inspected for 'provide', 'inject' or other registered tag keys.

_ := binder.Bind(appModule, dataModule, serviceModule)

This call binds 3 modules. Each module's provided fields are available for injection into any module.

Tags and Injectors

The functional option Injectors can be used to map tag keys (anything besides "provide" and "inject") to custom or third party Injectors.

injectors := modules.Injectors(map[string]Injector{
  "customTag": customInjector,
})
binder := modules.NewBinder(injectors)
module := struct{
  FieldA CustomType 'provide:"someField" customTag:"tagValueArgument"'
}
_ := binder.Bind(module)

When this module is bound, customInjector may set the value of FieldA based on the tag value "tagValueArgument".

The Injector interface is defined in the inject package.

// An Injector sets a value based on a string.
type Injector interface {
	// May set a value based on string.
	// Returns (true, nil) when a value has been set, or (false, nil) when a value has not been set (e.g. environment
	// variable not set, file not found, etc.).
	Inject(reflect.Value, string) (bool, error)
}

If a field is tagged with multiple keys, Inject will be called for each Injector until one sets the value.

module := struct{
  Field string `provide:"setting" flag:"setting" env:"SETTING" literal:"defaultValue"`
}

This module provides a string value named 'setting', which may be set via a command-line flag or environment variable, and which falls back to the default literal 'defaultValue'.

See the GoDoc for more api documentation, and a working example.