wire/docs/guide.md

# Wire User Guide

## Basics

Wire has two core concepts: providers and injectors.

### Defining Providers

The primary mechanism in Wire is the **provider**: a function that can produce a
value. These functions are ordinary Go code.

```go
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:

```go
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:

```go
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.X == 0 {
        return Baz{}, 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:

```go
package foobarbaz

import (
    // ...
    "github.com/google/wire"
)

// ...

var SuperSet = wire.NewSet(ProvideFoo, ProvideBar, ProvideBaz)
```

You can also add other provider sets into a provider set.

```go
package foobarbaz

import (
    // ...
    "example.com/some/other/pkg"
)

// ...

var MegaSet = wire.NewSet(SuperSet, pkg.OtherSet)
```

### Injectors

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`:

```go
// +build wireinject
// The build tag makes sure the stub is not built in the final build.

package main

import (
    "context"

    "github.com/google/wire"
    "example.com/foobarbaz"
)

func initializeBaz(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 Wire in the package directory:

```shell
wire
```

Wire will produce an implementation of the injector in a file called
`wire_gen.go` that looks something like this:

```go
// Code generated by Wire. DO NOT EDIT.

//go:generate wire
//+build !wireinject

package main

import (
    "example.com/foobarbaz"
)

func initializeBaz(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`].

[`go generate`]: https://blog.golang.org/generate

## Advanced Features

The following features all build on top of the concepts of providers and
injectors.

### Binding Interfaces

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:

```go
type Fooer interface {
    Foo() string
}

type MyFooer string

func (b *MyFooer) Foo() string {
    return string(*b)
}

func provideMyFooer() *MyFooer {
    b := new(MyFooer)
    *b = "Hello, World!"
    return b
}

type Bar string

func provideBar(f Fooer) string {
    // f will be a *MyFooer.
    return f.Foo()
}

var Set = wire.NewSet(
    provideMyFooer,
    wire.Bind(new(Fooer), new(*MyFooer)),
    provideBar)
```

The first argument to `wire.Bind` is a pointer to a value of the desired
interface type and the second argument is a pointer to a value of the type that
implements the interface. Any set that includes an interface binding must also
have a provider in the same set that provides the concrete type.

[type identity]: https://golang.org/ref/spec#Type_identity
[return concrete types]: https://github.com/golang/go/wiki/CodeReviewComments#interfaces

### Struct Providers

Structs can also be marked as providers. Use the `wire.Struct` function to
inject a struct type and tell the injector which field(s) should be injected.
The injector will fill in each field using the provider for the field's type.
For a given struct type `S`, this would provide both `S` and `*S`. For example,
given the following providers:

```go
type Foo int
type Bar int

func ProvideFoo() Foo {/* ... */}

func ProvideBar() Bar {/* ... */}

type FooBar struct {
    MyFoo Foo
    MyBar Bar
}

var Set = wire.NewSet(
    ProvideFoo,
    ProvideBar,
    wire.Struct(new(FooBar), "MyFoo", "MyBar"))
```

A generated injector for `FooBar` would look like this:

```go
func injectFooBar() FooBar {
    foo := ProvideFoo()
    bar := ProvideBar()
    fooBar := FooBar{
        MyFoo: foo,
        MyBar: bar,
    }
    return fooBar
}
```

The first argument to `wire.Struct` is a pointer to the desired struct type and
the subsequent arguments are the names of fields to be injected. A special
string `"*"` can be used as a shortcut to tell the injector to inject all
fields. So `wire.Struct(new(FooBar), "*")` produces the same result as above.

For the above example, you can specify only injecting `"MyFoo"` by changing the
`Set` to:

```go
var Set = wire.NewSet(
    ProvideFoo,
    wire.Struct(new(FooBar), "MyFoo"))
```

Then the generated injector for `FooBar` would look like this:

```go
func injectFooBar() FooBar {
    foo := ProvideFoo()
    fooBar := FooBar{
        MyFoo: foo,
    }
    return fooBar
}
```

And similarly if the injector needed a `*FooBar`.

### Binding Values

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.

```go
type Foo struct {
    X int
}

func injectFoo() Foo {
    wire.Build(wire.Value(Foo{X: 42}))
    return Foo{}
}
```

The generated injector would look like this:

```go
func injectFoo() Foo {
    foo := _wireFooValue
    return foo
}

var (
    _wireFooValue = Foo{X: 42}
)
```

It's important to note that the expression will be copied to the injector's
package; references to variables will be evaluated during the injector package's
initialization. Wire will emit an error if the expression calls any functions or
receives from any channels.

For interface values, use `InterfaceValue`:

```go
func injectReader() io.Reader {
    wire.Build(wire.InterfaceValue(new(io.Reader), os.Stdin))
    return nil
}
```

### Cleanup 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.

```go
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()`.

### Alternate Injector Syntax

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`:

```go
func injectFoo() Foo {
    panic(wire.Build(/* ... */))
}
```