Fx

# Consuming value groups

To consume a value group of type T, you have to tag a []T dependency with group:"$name" where $name is the name of the value group.

You can do this in one of the following ways:

  • with parameter objects
  • with annotated functions

# With parameter objects

You can use parameter objects to tag a slice parameter of a function as a value group.

Prerequisites

  1. A function that consumes a parameter object.

    type Params struct {
  fx.In

  // ...
}

func New(p Params) (Result, error) {
  // ...

  2. This function is provided to the Fx application.

      fx.Provide(New),

Steps

  1. Add a new exported field to the parameter object with the type []T, where T is the kind of value in the value group. Tag this field with the name of the value group.

    type Params struct {
  fx.In

  // ...
  Watchers []Watcher `group:"watchers"`
}

  2. Consume this slice in the function that takes this parameter object.

    func New(p Params) (Result, error) {
  // ...
  for _, w := range p.Watchers {
    // ...

WARNING

Do not rely on the order of values inside the slice. The order is randomized.

# With annotated functions

You can use annotations to consume a value group from an existing function.

Prerequisites

  1. A function that accepts a slice of the kind of values in the group.

    func NewEmitter(watchers []Watcher) (*Emitter, error) {

  2. The function is provided to the Fx application.

      fx.Provide(
    NewEmitter,
  ),

Steps

  1. Wrap the function passed into fx.Provide with fx.Annotate.

      fx.Provide(
    fx.Annotate(
      NewEmitter,
    ),
  ),

  2. Annotate this function to state that its slice parameter is a value group.

        fx.Annotate(
      NewEmitter,
      fx.ParamTags(`group:"watchers"`),
    ),

  3. Consume this slice in the function.

    func NewEmitter(watchers []Watcher) (*Emitter, error) {
  for _, w := range watchers {
    // ...

Tip: Functions can accept variadic arguments

You can consume a value group from a function that accepts variadic arguments instead of a slice.

func EmitterFrom(watchers ...Watcher) (*Emitter, error) {
  return &Emitter{ws: watchers}, nil
}

Annotate the variadic argument like it was a slice to do this.

    fx.Annotate(
      EmitterFrom,
      fx.ParamTags(`group:"watchers"`),
    ),

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