ViewComponent Yield Bug: Double Rendering Issue & Fix

by Sebastian Müller 54 views

Hey guys! There's a tricky bug we've run into with ViewComponent that I wanted to share and discuss. It involves double rendering when using partials with yield, and it's something you might encounter in your projects. Let's dive in!

Discussion Category: ViewComponent

This issue falls squarely into the ViewComponent category, as it directly relates to how components interact with partials and the yield keyword within the ViewComponent framework.

Additional Information: The Backstory

This bug report is actually a re-filing of a previous issue, specifically https://github.com/ViewComponent/view_component/issues/2408#issuecomment-3155202036. The core problem is that components rendering partials that utilize yield are exhibiting a double rendering behavior. This means that certain parts of the view are being rendered twice, leading to unexpected output.

Previously, this issue was resolved in v3.x by enabling config.view_component.capture_compatibility_patch_enabled = true. However, the bug has resurfaced after upgrading to v4, indicating a potential regression or change in behavior.

Why Failing Tests Are Crucial

Before we get into the nitty-gritty, a quick reminder: When reporting bugs, providing a failing test case is incredibly helpful. It gives the maintainers a clear and actionable way to understand and fix the issue. If you can, please consider submitting a pull request with a failing test instead of just a bug report.

Steps to Reproduce: Let's Get Our Hands Dirty

To illustrate this double rendering bug, I've put together a minimal reproducible example. This will allow you guys to see the issue firsthand and hopefully help in figuring out a solution. You can follow these steps to recreate the bug:

  1. Define a Simple Component:

    First, we need a basic ViewComponent. Let's create a component called ExampleComponent:

    # app/components/example_component.rb
class ExampleComponent < ViewComponent::Base
end

  2. Component Template with Partial:

    Next, we'll define the template for our component. This template will render a shared partial and pass a block to it using yield:

    <%# app/components/example_component.html.erb %>
<%= render "shared/partial" do %>
  abc
<% end %>

  3. The Shared Partial:

    Now, let's create the shared partial that uses yield. This is where the magic (or rather, the bug) happens:

    <%# app/views/shared/_partial.html.erb %>
def <%= yield %>

    This partial is designed to simply output the content passed to it via the yield keyword.

Expected Behavior: What We Want to See

So, what should happen when we render this component? Ideally, the output should be something like this (line breaks might vary depending on your setup):

<!-- BEGIN app/components/example_component.html.erb -->

<!-- BEGIN app/views/shared/_partial.html.erb -->def
  abc

<!-- END app/views/shared/_partial.html.erb --><!-- END app/components/example_component.html.erb -->

This output shows that the component template is rendered, then the partial is rendered, and the content from the yield block (abc) is inserted correctly. The HTML comments are added by Rails to indicate the start and end of each rendered template, which is super helpful for debugging.

Actual Behavior: The Double Render

Unfortunately, the actual output is quite different. When we run this code, we get the following:

<!-- BEGIN app/components/example_component.html.erb -->
  abc
<!-- BEGIN app/views/shared/_partial.html.erb -->def
  abc

<!-- END app/views/shared/_partial.html.erb --><!-- END app/components/example_component.html.erb -->

Notice anything weird? The content abc (which is supposed to be the block passed to the partial) is being output twice! This is the double rendering bug in action. It appears that the block content is being rendered both within the component template and within the partial, leading to the duplication.

System Configuration: Under the Hood

To give you guys a complete picture, here's the system configuration I'm using:

  • Rails version: 8.0.2
  • Ruby version: 3.3.5
  • Gem version: 4.0.0 (for ViewComponent)

This information can be crucial for the maintainers to understand if the bug is specific to certain versions or configurations.

Diving Deeper: Why Is This Happening?

Let's try to break down why this double rendering might be occurring. When a ViewComponent renders a partial with a yield, it's essentially capturing the block of code passed to it and then rendering it within the partial's context. The bug suggests that this capture and rendering process is happening twice somehow.

One potential reason could be related to how ViewComponent handles the rendering context and how it interacts with Rails' rendering pipeline. There might be an issue with how the captured content is being stored or passed around, leading to it being rendered multiple times.

Another possibility is that there's a conflict or regression in the way ViewComponent v4 handles yield compared to v3. The fact that the capture_compatibility_patch_enabled setting fixed the issue in v3 suggests that there were already some complexities around this area.

Possible Workarounds (But We Need a Real Fix!)

While we wait for a proper fix, there might be some workarounds we can use in our applications. However, these are just temporary solutions, and it's crucial to address the underlying bug.

  1. Avoid yield in Partials (If Possible):

    The most straightforward workaround is to avoid using yield in partials rendered by ViewComponents. This might involve refactoring your code to pass the content as a variable instead of a block. However, this can be a significant change, especially if you have a lot of existing code that relies on this pattern.

  2. Manual Content Capture:

    Another approach is to manually capture the content of the block and pass it to the partial. This can be done using Rails' capture helper. For example:

    <%# app/components/example_component.html.erb %>
<% content = capture do %>
  abc
<% end %>
<%= render "shared/partial", content: content %>

    <%# app/views/shared/_partial.html.erb %>
def <%= content %>

    This approach gives you more control over the rendering process, but it can also make your code more verbose.

The Path Forward: Let's Collaborate on a Solution

This double rendering bug is a real head-scratcher, and it's something that needs to be addressed to ensure ViewComponent remains a reliable and predictable framework. The next steps involve:

  1. Further Investigation:

    We need to dive deeper into the ViewComponent codebase to understand exactly why this double rendering is happening. This might involve tracing the rendering process, examining the interaction between components and partials, and looking for any potential conflicts or regressions.

  2. Creating a Failing Test Case:

    As mentioned earlier, a failing test case is crucial for fixing this bug. It provides a clear and reproducible way to demonstrate the issue and verify that the fix is working correctly. If you have the time and expertise, please consider contributing a failing test to the ViewComponent repository.

  3. Submitting a Pull Request:

    Once we have a fix, the next step is to submit a pull request to the ViewComponent repository. This will allow the maintainers to review the fix and merge it into the codebase.

Conclusion: Let's Keep the Conversation Going

This double rendering bug in ViewComponent partials using yield is a challenging issue, but it's also an opportunity for us to learn and improve the framework. I encourage you guys to share your thoughts, experiences, and potential solutions in the comments below.

Let's work together to get this bug squashed and make ViewComponent even better! Your insights and contributions are highly valued.