Result Objects

A result object is an object with the sole purpose of carrying results for a specific function or method.

As with parameter objects, the object is defined exclusively for a single function, and not shared with other functions.

In Fx, result objects contain exported fields exclusively, and are always tagged with fx.Out.

Related

• Parameter objects are the parameter analog of result objects.

Using result objects

To use result objects in Fx, take the following steps:

1. Define a new struct type named after your constructor with a Result suffix. If the constructor is named NewClient, name the struct ClientResult. If the constructor is named New, name the struct Result. This naming isn't strictly necessary, but it's a good convention to follow.

   type ClientResult struct {
   }
   

2. Embed fx.Out into this struct.

   type ClientResult struct {
     fx.Out
   

3. Use this new type as the return value of your constructor by value.

   func NewClient() (ClientResult, error) {
   

4. Add values produced by your constructor as exported fields on this struct.

   type ClientResult struct {
     fx.Out
   
     Client *Client
   }
   

5. Set these fields and return an instance of this struct from your constructor.

   func NewClient() (ClientResult, error) {
     client := &Client{
       // ...
     }
     return ClientResult{Client: client}, nil
   }
   

Once you have a result object on a function, you can use it to access other advanced features of Fx:

• Feeding value groups with result objects

Adding new results

You can add new values to an existing result object in a completely backwards compatible manner.

1. Take an existing result object.

   type Result struct {
     fx.Out
   
     Client *Client
   }
   
   func New() (Result, error) {
     client := &Client{
       // ...
     }
     return Result{
       Client: client,
     }, nil
   

2. Add a new field to it for your new result.

   type Result struct {
     fx.Out
   
     Client *Client
     Inspector *Inspector
   }
   

3. In your constructor, set this field.

     return Result{
       Client: client,
       Inspector: &Inspector{
         // ...
       },
     }, nil