Singleton Provider

[ehsannm]

Is there anyway to make a provider function as singleton ?

There are cases we have one dependency object which must be initialized only once but is used by many other objects.

How to handle those cases in uber/dig ?

[abhinav]

Hello @ehsannm.
All functions provided to Dig/Fx are treated as singletons and re-used in every other function.

For example, if you have,

func NewFoo() *Foo {
	fmt.Println("building Foo")
	return // ...
}

And you use that with two constructors NewBar(*Foo) *Bar and NewBaz(*Foo) *Bar,
both Dig and Fx will instantiate *Foo only once (calling that print statement only once)
and share it between Bar and Baz.

Sample code with Dig (run it at https://play.golang.org/p/bHol91PaDj3):

package main

import (
	"fmt"

	"go.uber.org/dig"
)

type Foo struct{}

func NewFoo() *Foo {
	fmt.Println("building Foo")
	return new(Foo)
}

type Bar struct{}

func NewBar(*Foo) *Bar {
	fmt.Println("using Foo in Bar")
	return new(Bar)
}

type Baz struct{}

func NewBaz(*Foo) *Baz {
	fmt.Println("using Foo in Baz")
	return new(Baz)
}

func main() {
	c := dig.New()

	if err := c.Provide(NewFoo); err != nil {
		panic(err)
	}

	if err := c.Provide(NewBar); err != nil {
		panic(err)
	}

	if err := c.Provide(NewBaz); err != nil {
		panic(err)
	}

	if err := c.Invoke(run); err != nil {
		panic(err)
	}
}

func run(bar *Bar, baz *Baz) {
	fmt.Println("running application")
}

Closing because answered.
If I misunderstood the question, feel free to open a new issue with more details.
Thanks!

[dalianzhu]

@abhinav hi, i have a new question, if i want to create new instance each time, could you tell me how can do that?