Wire is a code generation tool that automates connecting components using dependency injection. Dependencies between components are represented in Wire as function parameters, encouraging explicit initialization instead of global variables. Because Wire operates without runtime state or reflection, code written to be used with Wire is useful even for hand-written initialization.
Wire has two core concepts: providers and injectors.
The primary mechanism in Wire is the provider: a function that can produce a value. These functions are ordinary Go code.
package foobarbaz
type Foo struct {
X int
}
// ProvideFoo returns a Foo.
func ProvideFoo() Foo {
return Foo{X: 42}
}Provider functions must be exported in order to be used from other packages, just like ordinary functions.
Providers can specify dependencies with parameters:
package foobarbaz
// ...
type Bar struct {
X int
}
// ProvideBar returns a Bar: a negative Foo.
func ProvideBar(foo Foo) Bar {
return Bar{X: -foo.X}
}Providers can also return errors:
package foobarbaz
import (
"context"
"errors"
)
// ...
type Baz struct {
X int
}
// ProvideBaz returns a value if Bar is not zero.
func ProvideBaz(ctx context.Context, bar Bar) (Baz, error) {
if bar == 0 {
return 0, errors.New("cannot provide baz when bar is zero")
}
return Baz{X: bar.X}, nil
}Providers can be grouped into provider sets. This is useful if several
providers will frequently be used together. To add these providers to a new set
called SuperSet, use the wire.NewSet function:
package foobarbaz
import (
// ...
"github.com/google/go-cloud/wire"
)
// ...
var SuperSet = wire.NewSet(ProvideFoo, ProvideBar, ProvideBaz)You can also add other provider sets into a provider set.
package foobarbaz
import (
// ...
"example.com/some/other/pkg"
)
// ...
var MegaSet = wire.NewSet(SuperSet, pkg.OtherSet)An application wires up these providers with an injector: a function that calls providers in dependency order. With Wire, you write the injector's signature, then Wire generates the function's body.
An injector is declared by writing a function declaration whose body is a call
to wire.Build. The return values don't matter as long as they are of the
correct type. The values themselves will be ignored in the generated code. Let's
say that the above providers were defined in a package called
example.com/foobarbaz. The following would declare an injector to obtain a
Baz:
// +build wireinject
// The build tag makes sure the stub is not built in the final build.
package main
import (
"context"
"github.com/google/go-cloud/wire"
"example.com/foobarbaz"
)
func initializeApp(ctx context.Context) (foobarbaz.Baz, error) {
wire.Build(foobarbaz.MegaSet)
return foobarbaz.Baz{}, nil
}Like providers, injectors can be parameterized on inputs (which then get sent to
providers) and can return errors. Arguments to wire.Build are the same as
wire.NewSet: they form a provider set. This is the provider set that gets
used during code generation for that injector.
Any non-injector declarations found in a file with injectors will be copied into the generated file.
You can generate the injector by invoking gowire in the package directory:
gowireWire will produce an implementation of the injector in a file called
wire_gen.go that looks something like this:
// Code generated by gowire. DO NOT EDIT.
//go:generate gowire
//+build !wireinject
package main
import (
"example.com/foobarbaz"
)
func initializeApp(ctx context.Context) (foobarbaz.Baz, error) {
foo := foobarbaz.ProvideFoo()
bar := foobarbaz.ProvideBar(foo)
baz, err := foobarbaz.ProvideBaz(ctx, bar)
if err != nil {
return 0, err
}
return baz, nil
}As you can see, the output is very close to what a developer would write themselves. Further, there is little dependency on Wire at runtime: all of the written code is just normal Go code, and can be used without Wire.
Once wire_gen.go is created, you can regenerate it by running go generate.
The following features all build on top of the concepts of providers and injectors.
Frequently, dependency injection is used to bind a concrete implementation for an interface. Wire matches inputs to outputs via type identity, so the inclination might be to create a provider that returns an interface type. However, this would not be idiomatic, since the Go best practice is to return concrete types. Instead, you can declare an interface binding in a provider set:
type Fooer interface {
Foo() string
}
type Bar string
func (b *Bar) Foo() string {
return string(*b)
}
func ProvideBar() *Bar {
b := new(Bar)
*b = "Hello, World!"
return b
}
var BarFooer = wire.NewSet(
ProvideBar,
wire.Bind(new(Fooer), new(Bar)))The first argument to wire.Bind is a pointer to a value of the desired
interface type and the second argument is a zero value of the concrete type.
Any set that includes an interface binding must also have a provider in the
same set that provides the concrete type.
Structs can also be marked as providers. Instead of calling a function, an
injector will fill in each field using the corresponding provider. For a given
struct type S, this would provide both S and *S. For example, given the
following providers:
type Foo int
type Bar int
func ProvideFoo() Foo {
// ...
}
func ProvideBar() Bar {
// ...
}
type FooBar struct {
Foo Foo
Bar Bar
}
var Set = wire.NewSet(
ProvideFoo,
ProvideBar,
FooBar{})A generated injector for FooBar would look like this:
func injectFooBar() FooBar {
foo := ProvideFoo()
bar := ProvideBar()
fooBar := FooBar{
Foo: foo,
Bar: bar,
}
return fooBar
}And similarly if the injector needed a *FooBar.
Occasionally, it is useful to bind a basic value (usually nil) to a type.
Instead of having injectors depend on a throwaway provider function, you can
add a value expression to a provider set.
type Foo struct {
X int
}
func injectFoo() Foo {
wire.Build(wire.Value(Foo{X: 42}))
return Foo{}
}The generated injector would look like this:
func injectFoo() Foo {
foo := Foo{X: 42}
return foo
}It's important to note that the expression will be copied, so references to
variables will be evaluated during the call to the injector. gowire will emit
an error if the expression calls any functions.
If a provider creates a value that needs to be cleaned up (e.g. closing a file), then it can return a closure to clean up the resource. The injector will use this to either return an aggregated cleanup function to the caller or to clean up the resource if a provider called later in the injector's implementation returns an error.
func provideFile(log Logger, path Path) (*os.File, func(), error) {
f, err := os.Open(string(path))
if err != nil {
return nil, nil, err
}
cleanup := func() {
if err := f.Close(); err != nil {
log.Log(err)
}
}
return f, cleanup, nil
}A cleanup function is guaranteed to be called before the cleanup function of any
of the provider's inputs and must have the signature func().
If you grow weary of writing return foobarbaz.Foo{}, nil at the end of your
injector function declaration, you can instead write it more concisely with a
panic:
func injectFoo() Foo {
panic(wire.Build(/* ... */))
}The following are practices we recommend for using Wire. This list will grow over time.
If you need to inject a common type like string, create a new string type
to avoid conflicts with other providers. For example:
type MySQLConnectionString stringA provider function that includes many dependencies can pair the function with an options struct.
type Options struct {
// Messages is the set of recommended greetings.
Messages []Message
// Writer is the location to send greetings. nil goes to stdout.
Writer io.Writer
}
func NewGreeter(ctx context.Context, opts *Options) (*Greeter, error) {
// ...
}
var GreeterSet = wire.NewSet(Options{}, NewGreeter)