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4. Separate Slot Getters and Setters

Date: 2022/03/22


Blake Williams




Currently, slots implement a single method for both getting and setting a slot. For example, given a slot named header:

class MyComponent < ViewComponent::Base
  renders_one :header

c =

c.header { "Hello world!" } # sets the slot
c.header # gets the slot

This API was built with the assumption that a slot will always be set by passing an argument and/or passing a block.

This assumption hasn’t remained valid. Specifically, with_content breaks the assumption when passing static content to a slot:

class MyComponent < ViewComponent::Base
  renders_one :header

c =

# c.header returns nil because the getter path is being executed due to having
# no arguments and no block passed:
c.header.with_content("Hello world!") # undefined method `with_content' for nil:NilClass (NoMethodError)

The above example shows off the gap in the slots API via with_content, but it’s likely that as the library continues to grow this gap will appear in other valid use-cases.


Split the slots API into a getter and setter. Keeping the slot name as the getter makes the most sense, but the setter can be renamed to with_#{slot_name}.

For example, the above would become:

class MyComponent < ViewComponent::Base
  renders_one :header

c =

# New API for setting slots
c.with_header { "hello world" }

# Now `with_content` is valid when defining slots
c.with_header.with_content("Hello world!")

Alternatives Considered

We’ve spoken about a few alternatives:

  • Making a special NilSlot class that responds to with_content.
    • Results in extra allocations
    • Can’t treat the NilSlot as falsy. So if header would no longer work even though you would expect it to.
  • Introducing an API for the header.with_content("Hello world!") pattern as explained above (like: c.with_header_content("Hello world!")):
    • The API gap still exists and requires a specific work around for with_content, leaving the gap for future API’s.
    • This API doesn’t allow arguments to be passed 1-to-1 like the current setter API.


The largest consequence of this change is that we’ll need to deprecate the old setter usage (header { "Hello world!"}) in favor of the new setter API (with_header { "Hello world!" }).

I propose that we make at least one release with the new API and no deprecation warning followed by another release that includes the deprecation warning. This will give teams some time to migrate before running into deprecation warnings.