[Rendering] Custom renderer support

[anuj-pal27]

Summary

Implements the config.renderer(:name) { ... } DSL proposed in the issue.
Registers custom renderer blocks and generates render_<name> methods on
RageController::API at boot time.

Key behavior

• config.renderer(:csv) { |obj, **opts| ... } registers a renderer block
• After __finalize, every controller automatically gets render_csv, render_phlex, etc.
• Generated methods support status: as both Symbol (:created) and Integer (201)
• Unknown status symbols raise ArgumentError instead of setting nil status
• Renderer blocks run in controller context — headers, request, params all work
• Double-render is prevented whether render happens via return value or internal render call
• RageController::API.__custom_renderers registry makes finalize idempotent across re-runs

Tests

9 specs covering:

• Basic registration and method generation
• Status handling (symbol, integer, default 200)
• Duplicate renderer registration guard
• Method name conflict detection
• Controller context access (headers, params)
• nil return value → empty string body
• Renderer block calling render internally
• Double-render protection

Notes

Renderer blocks are converted to named methods via define_dynamic_method rather
than using instance_exec per request — this keeps the hot path allocation-free
and consistent with how Rage handles action methods internally.

Fixes: #234

Screenshots

CSV renderer:

Phlex renderer:

[rsamoilov]

Hi @anuj-pal27

That's a great start! I've left some comments. Please let me know if you have any questions.

[anuj-pal27]

hii @rsamoilov I have one question that I updated render plain: to use @__headers["content-type"] ||= "text/plain; charset=utf-8" so custom renderers can set their own content-type (like text/csv) without it being overwritten when delegating to render. But there's a problem — RageController::API#initialize already sets the default content-type to application/json, so ||= ends up keeping JSON instead of switching to text/plain, which breaks existing specs. Should render plain: always force text/plain, or should it keep the existing content-type and I update the specs?

[rsamoilov]

But there's a problem — RageController::API#initialize already sets the default content-type to application/json, so ||= ends up keeping JSON instead of switching to text/plain

Exactly! Which is why you would need to assign the default application/json content type to a constant. And then you would update the condition in the plain branch to set the content type only if it's the default one.

[anuj-pal27]

@rsamoilov , Done — changed __define_custom_renderers to iterate with @renderers.each since _finalize already guards if @Renderers.
Also removed the explicit double-render raise from the generated render method and rely on render for raising.
For the content-type behavior: introduced DEFAULT_CONTENT_TYPE (JSON) and now render plain: only switches to text/plain when the current content-type is nil or equals the default JSON; custom types like text/csv are preserved.

Thank you for the clear pointers/review — they helped a lot.

[rsamoilov]

We don't need this value as a constant.

Suggested change

DEFAULT_PLAIN_CONTENT_TYPE = "text/plain; charset=utf-8"

[anuj-pal27]

Sorry about that — you’re right. I’ll remove DEFAULT_PLAIN_CONTENT_TYPE and keep only the default JSON content-type as a constant, and update the plain branch to override only when it’s still the default.

[rsamoilov]

Suggested change

	@__headers["content-type"] = DEFAULT_PLAIN_CONTENT_TYPE if ct.nil? || ct == DEFAULT_CONTENT_TYPE
	@__headers["content-type"] = "text/plain; charset=utf-8" if ct.nil? || ct == DEFAULT_CONTENT_TYPE

[rsamoilov]

I'm really sorry, I didn't realise that return if @__rendered will prevent the code from going into render and raising the exception. We actually need it, could you please add it back? Sorry again.

Suggested change

	def render_#{name}(*args, status: nil, **kwargs)
	def render_#{name}(*args, status: nil, **kwargs)
	raise "Render was called multiple times in this action." if @__rendered

[anuj-pal27]

No problem — I’ll add the raise Render was called multiple times in this action.if @__rendered back at the start of render_#{name} (and keep return if @__rendered after the block).

[rsamoilov]

This looks great!

Thinking about responsibilities, there's one more thing I'd love to improve. Currently, the configuration class defines the custom renderer methods on RageController::API. Ideally though, we want the configuration class to simply hold the data and know as little about the outside world as possible.

• Let's update the renderer method to not call Rage::Internal.define_dynamic_method - all it should do is just store the block in a RendererEntry
• In __define_custom_renderers, let's move everything inside the RageController::API.method_defined? condition into a dedicated method in RageController::API, e.g. __register_rederer

This way, Configuration won't know the internal details of RageController::API, and the loop inside __define_custom_renderers will simply iterate over the renderers, call __register_rederer and flip the applied flag.

[anuj-pal27]

Sounds good — I’ll refactor so Configuration#renderer only stores the block in RendererEntry, and move the method-definition/conflict-check logic into RageController::API.__register_renderer. Then __define_custom_renderers will just iterate, call __register_renderer, and mark the entry as applied.