example_test.go

package inject_test

import (
	"fmt"
	"net/http"
	"os"

	"github.com/facebookgo/inject"
)

// Our Awesome Application renders a message using two APIs in our fake
// world.
type HomePlanetRenderApp struct {
	// The tags below indicate to the inject library that these fields are
	// eligible for injection. They do not specify any options, and will
	// result in a singleton instance created for each of the APIs.

	NameAPI   *NameAPI   `inject:""`
	PlanetAPI *PlanetAPI `inject:""`
}

func (a *HomePlanetRenderApp) Render(id uint64) string {
	return fmt.Sprintf(
		"%s is from the planet %s.",
		a.NameAPI.Name(id),
		a.PlanetAPI.Planet(id),
	)
}

// Our fake Name API.
type NameAPI struct {
	// Here and below in PlanetAPI we add the tag to an interface value.
	// This value cannot automatically be created (by definition) and
	// hence must be explicitly provided to the graph.

	HTTPTransport http.RoundTripper `inject:""`
}

func (n *NameAPI) Name(id uint64) string {
	// in the real world we would use f.HTTPTransport and fetch the name
	return "Spock"
}

// Our fake Planet API.
type PlanetAPI struct {
	HTTPTransport http.RoundTripper `inject:""`
}

func (p *PlanetAPI) Planet(id uint64) string {
	// in the real world we would use f.HTTPTransport and fetch the planet
	return "Vulcan"
}

func Example() {
	// Typically an application will have exactly one object graph, and
	// you will create it and use it within a main function:
	var g inject.Graph

	// We provide our graph two "seed" objects, one our empty
	// HomePlanetRenderApp instance which we're hoping to get filled out,
	// and second our DefaultTransport to satisfy our HTTPTransport
	// dependency. We have to provide the DefaultTransport because the
	// dependency is defined in terms of the http.RoundTripper interface,
	// and since it is an interface the library cannot create an instance
	// for it. Instead it will use the given DefaultTransport to satisfy
	// the dependency since it implements the interface:
	var a HomePlanetRenderApp
	err := g.Provide(
		&inject.Object{Value: &a},
		&inject.Object{Value: http.DefaultTransport},
	)
	if err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(1)
	}

	// Here the Populate call is creating instances of NameAPI &
	// PlanetAPI, and setting the HTTPTransport on both to the
	// http.DefaultTransport provided above:
	if err := g.Populate(); err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(1)
	}

	// There is a shorthand API for the simple case which combines the
	// three calls above is available as inject.Populate:
	//
	//   inject.Populate(&a, http.DefaultTransport)
	//
	// The above API shows the underlying API which also allows the use of
	// named instances for more complex scenarios.

	fmt.Println(a.Render(42))

	// Output: Spock is from the planet Vulcan.
}