sarulabs/di

package di

import (
    "context"
    "net/http"
)

// ContainerKey is a type that can be used to store a container
// in the context.Context of an http.Request.
// By default, it is used in the C function and the HTTPMiddleware.
type ContainerKey string

// HTTPMiddleware adds a container in the request context.
//
// The container injected in each request, is a new sub-container
// of the app container given as parameter.
//
// It can panic, so it should be used with another middleware
// to recover from the panic, and to log the error.
//
// It uses logFunc, a function that can log an error.
// logFunc is used to log the errors during the container deletion.
func HTTPMiddleware(h http.HandlerFunc, app Container, logFunc func(msg string)) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // create a request container from tha app container
        ctn, err := app.SubContainer()
        if err != nil {
            panic(err)
        }
        defer func() {
            if err := ctn.Delete(); err != nil && logFunc != nil {
                logFunc(err.Error())
            }
        }()

        // call the handler with a new request
        // containing the container in its context
        h(w, r.WithContext(
            context.WithValue(r.Context(), ContainerKey("di"), ctn),
        ))
    }
}

// C retrieves a Container from an interface.
// The function panics if the Container can not be retrieved.
//
// The interface can be :
// - a Container
// - an *http.Request containing a Container in its context.Context
//   for the ContainerKey("di") key.
//
// The function can be changed to match the needs of your application.
var C = func(i interface{}) Container {
    if c, ok := i.(Container); ok {
        return c
    }

    r, ok := i.(*http.Request)
    if !ok {
        panic("could not get the container with C()")
    }

    c, ok := r.Context().Value(ContainerKey("di")).(Container)
    if !ok {
        panic("could not get the container from the given *http.Request")
    }

    return c
}

// Get is a shortcut for C(i).Get(name).
func Get(i interface{}, name string) interface{} {
    return C(i).Get(name)
}