Replace Decorator with PartBuilder, supporting built-in part class resolution from a namespace

[timriley]

Now, a view controller can be configured with a part_namespace and the default part builder will look up part classes to use from that namespace.

For example:

module MyParts
  class User < Dry::View::Part
  end
end

class MyVC < Dry::View::Controller
  configure do |config|
    config.template = "test"
    config.part_namespace = MyParts
  end

  expose :user
end

my_vc = MyVC.new
my_vc.(user: my_user).locals[:user] # => MyParts::User

This works for decorated attributes, too. If the part classes looked like this:

module MyParts
  class User < Dry::View::Part
    decorate :profile
  end

  class Profile < Dry::View::Part
  end
end

Then locals[:user].profile would be a MyParts::Profile.

Providing aliases for part names continues to work largely as before, e.g. expose :user, as: :admin_user will return the part as a MyParts::AdminUser (if it exists).

The API for providing a the part to use for array values has changed, however. Previously it took the form of as: {:collection_part_name => :item_part_name}. However, this made things feel pretty awkward if you didn't want to supply a custom name for the individual item (as: {:collection_part_name => nil} is pretty bad). Now you can provide one or both name aliases via an array syntax, e.g. as: [:collection_part_name] if you only want to provide an alias for the collection, or as: [:collection_part_name, :item_part_name] if you want to provide both. The array format here also works nicely as a mnemonic device, since it's relating to the wrapping of an array value.

Names are resolved using an inflector (by default a Dry::Inflector instance) which can be configured on view controllers via an inflector setting.

Custom part builders can still be provided to view controllers via a part_builder setting. When the view controller is initialized, the part builder will be prepared with the part namespace and inflector like so:

config.part_builder.new(namespace: config.part_namespace, inflector: config.inflector)

So any custom part builder will need to conform to this interface.

Replacing the decorator name with part_builder not only makes the purpose of the object clearer, but it also paves the way for a related scope_builder to be established as part of an upcoming feature.

Resolves #54, resolves #73

[timriley]

@solnic @jodosha Here's one of the first big steps forward in dry-view functionality I've recently promised. This makes it possible for view parts to have custom classes resolved from a namespace automatically (similar to how rom's struct_namespace works).

I've described most of the behaviour in the PR description. Please have a look and let me know if there's anything you'd like to see adjusted or finessed in this feature (please keep in mind this is one in a series of many steps I'll be taking to improve the gem, there's a bunch of issues tagged 1.0 laying out what will come next).

[solnic]

This is great, it's a very useful feature, we know that already :)