Define a baseline loading Moose only if not present

Moose is a big project and loading it each time can take a long time.

Multiple people came to me asking if there was a way in the baseline of a project to say “Load Moose only if needed”.

Since it is a common question, here is a guide on how to do that!

One way to do it is to use custom project attributes from Metacello. The principle is simple; depending on the state of your image, you can define project attributes and use them in your baseline.

To define the attributes you need to implement a method #customProjectAttributes that will return an array with the custom attributes.

If I want to ensure Famix is in the image I can define it like this:

customProjectAttributes

  'BaselineOfFamix' asPackageIfAbsent: [ ^ #( #NeedsFamix ) ].
  ^ #(  )

Note

You need to adapt this to the project you depend on. Maybe you’ll need to check the baseline of Moose instead of Famix, or a specifi package not present in the default famix.

And now I can use those attributes in a baseline via the #for:do: message.

Full example

customProjectAttributes

  'BaselineOfFamix' asPackageIfAbsent: [ ^ #( #NeedsFamix ) ].
  ^ #(  )

baseline: spec
    <baseline>
    spec
        for: #common
        do: [ spec package: 'MyProject' ].

    spec
        for: #(#'NeedsFamix') do: [
            spec baseline: 'Famix' with: [ spec repository: 'github://moosetechnology/Famix/src' ]
            spec package: 'MyProject' with: [ spec requires: #('Famix') ] ]