React Slots: a New Way of Composition

Hey, it's my great honor to be here to share with you some of my thoughts on the design system. My name is Gongwu Nie, you can call me Neo. Today, my topic is React Snots, a new way of composition.

Before we dive into the topic, I'd like to tell the backstory first. When working on the design system, I'm always thinking, how to build flexible and future-proof components? For me, the hardest thing is not creating new components or fixing weird bugs. It's the component API design.

Every time when I found a new design system, the first thing I would do is to see how they implement the text input component, because it's one of the most mysterious components. Let me show you. Text input is simple and straightforward. It comes with a label, an input, and has a test. Let's add accessibility support first. When building a shareable component, please don't forget to forward ref. We need the ref to the input to focus it in form validation. But what about the wrapper? We also need the ref to the wrapper to attach propover or tootip to it in some cases. Well, we can add a wrapper for it.

But wait, have you noticed a potential problem with the current approach? What will happen if we are trying to add some margins to the component from the other component? It will increase the margin between label, input and description instead, because we spread all the rest prompts to the inner input element. The fix is simple. We can pass the class name and style to the component wrapper instead. Looks good? The problem is that the component is getting confusing for the consumers. Which prompt will be passed to the wrapper? Which prompt will be passed to the input? The component becomes a black box. In reality, the product team will keep asking for more options to access the internal elements, like attaching point events or adding data attributes to the label or description. Similar to WrapRef, we can introduce wrapper prompts, label prompts, and description prompts. It works. But it doesn't end here. What if we need to add surroundings to the input, more prompts. Now you can see how API gets bloated with this approach. Here is the API of text input from man time. Don't get me wrong. Man time is a great components library. I love it a lot.

They choose the configuration way to design the component API. In the configuration way, we have to support flexibility with API bloating, and the components become a completely black box for consumers.

Let's take one step back and think, how do we build our web app in React? We compose components. Each component just do its own job and that's it. Simple and flexible.

Then, what if we use the same approach for shareable components? We gain the same clearance, simplicity, and flexibility. A lot of popular components libraries are using the CompositionWay to implement accessible components like Chakra UI, Redux primitives, Hedonist UI, and more.

To support accessibility, we have to use React Context to communicate between parent and children to coordinate the accessibility attributes. Configuration is simple to use. You really want to write your module in the left style rather than the right style. But the CompositionWay is not the final answer.

Take the Chakra UI for example. What if we swap the position of inputLeftAddon and inputRightAddon? We get inputLeftAddon shown on the right side and inputRightAddon shown on the left side. It breaks the semantic with a broken style. I have a hobby when I found a new design system is testing the resonance of components. By adding extra nodes to the items, we see how it breaks. For a dedicated design system, we deliver well-designed components. But you just can't imagine how alternative developers will break them in various ways, and unfortunately, we can't prevent them from doing that.

You would say, it doesn't matter if we follow the design order correctly. OK, let's see another more realistic problem. Building an Accessible List, the parents need to know the children's information to make it accessibility compliant for Azure Active Descendant is easy to achieve using the configuration way because the parent knows all the items from the ItemPrompt. But it's a bit cumbersome for the composition way. The menu can't see its internal items. The component is blind. To solve the problem, we can use react-children with react-clone-element. Then we can take control of the rendering of items in parent component. But in terms of extension, it will break again. Even we can iterate the children, the parent is still blind as it can't see the deep details. The problem is well described by root UI, if you are interested, here I have a homework for you figuring out how those libraries are trying to solve this problem.

Let's recap, the configuration way provides the best consistency, as the component is single source of truth, but it's hard to extend, and it becomes a black box when multiple items are used. On the contrary, the composition way provides the best flexibility, you can customize your sub-components freely, but it cause another problem, consistency, it leads to uncontrolled use cases and wrong patterns. Then, can we have a way to build components with flexibility while keeping the consistency? Yes, the answer is no. Slots is the standard for web-components. In web-components, slots are identified by their name attribute and allow you to define placeholders in your templates that can be filled with any markup fragment you want when the element is used in the markup. It's easier to understand the concept in code. First, we define a template with some named placeholders and define the web-component. Then we fill the named placeholders with named slots. Finally, we get the result with injected elements. Here you can see how the consistency is guaranteed. The layout is defined in the template, not described by the composition order. Some popular frameworks like Vue and Svelte also implemented the slot composition pattern from framework level in a similar behavior.

But as you know, React still doesn't support web components very well, not saying slots. There are a lot of approaches from the community trying to bring slots to React world. The most common way is using react-children with react-clone-element. But it doesn't scale very well as component is blind. React Spectrum use context and use grid order to solve the disorder problem, but the functionality is limited. For example, with grid order, we change the type index in an unexpected way. GitHub Primer uses double rendering to approach slots. It renders the subcomponents into a browser with empty content and gathers the slot information first, and then uses the slot in the second render. The problem is obvious. It doesn't support server-side rendering. There was an experimental package from the React core team called React-core-return, which provides a way to interpolate the children during render phase. It could be the best candidate to bring slots to React, but unfortunately, it was removed shortly. So I created React Slots RFC to outline the problems with the proposal. Let me show you the code directly.

Again, the typical text input component implemented in React Slots RFC. In the RFC, we introduced two new APIs called createHost and createSlot, which is very similar to React call return but with a much simpler mental model. The createHost creates the template and the createSlot creates placeholders, and then compose the Slot in the host component. It's quite similar to Slot for web components, but way much powerful, as we don't simply inject the elements, we interplay them.

It opens the third eye of the component, now the parent can see the deep details of his children, no matter how it was extended. Now, demo time. I will show you how the previous problems will be perfectly solved by React Slots RFC. Here, I implemented the React Slots RFC with react-core-return in 10 lines. And then, the text field, first, we create the slots for label, input, and tag. We pass the children to createHost, then it gives us the slots. Here you can see how we can interpolate the slots. That means we can do everything we want in a single place. Here you can see how I support accessibility for the text input. We can read the input ID directly for the label, and also we can iterate all the text.

Now, I will show you some magic. No matter how I compose the slots, it doesn't change the result because the layout is defined in the template by createHost. Meanwhile, we are still free to extend the slots. For example, we extend the tag, we extend the input, and we can change the order of the tag. It will change the result because it's a list. And the other problem component resilience. Let me try to wrap the tag with an extra node. It will work because it's not a node. That's very important for a dedicated design system, because we have prepared a very... We have prepared the components. We should not change the layout easily. Let's change it back. Another use case is that whatever we use, it's not outside of the host. Nothing will be rendered. That's also important for a consistent design system. The nested slots are also supported.

Have you noticed that how we create the text inputs, we don't need to create a styled text field label or text field input. Instead, we style it in the template. The slot itself renders nothing but only data carrier. That means a lot. For example, we can create a tree builder. Here is the example. We create a tree, but it renders nothing to the browser. But you can see in the console log. Here is the tree, the nested menu, but here is how we compose them. But it's not rendered to the browser at all. But still, we can get data, because the slot is just a data carrier. It's not needed to be rendered to the browser at all. And you can see how it's reactive. Or remove the nested menu. So we can still benefit from all React provides to us. The life cycles, stays, context, all the other things.