Managing state in React is difficult. While many suggest that lifting state up
to the highest level of the app, and frequently even outside of the component
tree, most apps aren't that simple.
One case where this becomes complicated is when app authors need to interact
with complex UI components within their application. I internally wrestle with
this concept frequently as I build these UI components everyday at work.
One of the most recent cases where I am running into this is our new carousel
component which I started work on about 6 months ago as of the time of writing
this blog post. This component is perhaps the most complicated UI component we
have at Wayfair.
It manages quite a bit of internal state, ranging from actual React state to
many references to the rendered dom nodes as well. The problem here is that we
also want end users (app developers) the ability to control the carousel from
their business logic.
We started with the concept that render props and prop getters would be
sufficient to allow end users to control the carousel, however over the months
of development, we realized that many (maybe 90%) of frontend developers at
Wayfair (and really anywhere) don't want to manage a lot of state and helpers
that comes with a render prop.
So we mostly scrapped the render prop pattern on the carousel
[1]Jump to footnote. Since then we have continued to iterate on the carousel
but we were still blocked by this battle of how to properly let end developers
control the carousel while also not needing to manage a ton of internal logic
(and possibly duplicating that across the codebase).
Skip forward ~ 2 months
On Memorial day weekend of 2018, I decided to spend some time in Codesandbox to
work on this problem. One of the rough patterns that I still don't fully grasp
from the Downshift component published by Paypal, is the idea of passing back up
functions in event handlers to help compute the next state
[2]Jump to footnote.
I started with a simple example, a component that can be both controlled as well
as uncontrolled (a counter component). I broke the problem down into two pieces:
Local state management
External state management
The common logic between these two pieces is the actual methods behind how to
update this state. Locally we call setState with either an object or a function.
Using an object for setState generally means that we have all the information we
need to determine the next state. Whereas using a function means that we need to
know the current state of the world to derive the next state.
As with a counting component, a carousel only really needs to know the current
slide, the action the user is taking (i.e. clicking next), and the number of
slides to scroll by. With this information (which can be derived by the event as
well as the current state and props) we can derive the next active index.
So the solution was a concept I will be calling State Updaters, these are
functions which can be (and often are) curried methods that lastly return a
function that takes in state and props and returns some new state. They are
curried because they need additional information, such as selectors and
transformers to return the correct state.
Lets break down this step by step in some code.
class Counter extends React.Component { state = { count: this.props.defaultCount, propCount: this.props.count, }; static getDerivedStateFromProps(nextProps, prevState) { /* we'll get to this later */ } // This is used for local control flow // We determine if we call setState or not isControlled = () => typeof
So we start off with some local state, a class method for handling the click
event from the button, and an isControlled method for determining if an
implementer is controlling the component or not.
The next step is to handle the click logic:
// ...handleClick = event => { // If we aren't controlled (i.e. we manage our own state) // call setState with the return value of the state updater if (!this.isControlled()) { this.setState(stateUpdater()); } // Always call the prop handleClick handler // first passing the event, and second passing an object // with a stateUpdater argument this.props.handleClick(event, { // defined in the module // could also be exported as well stateUpdater,
Once we have the local handler resolved, now we can dive into the state updater
method.
// return a function that accepts an object with three key fields// 1. transformState: this determines if the returned state should be nested// 2. selectState: this selects the correct UI component state within the parent state// 3. fieldName: this is a string that is the key inside state that we updateexport const stateUpdater = ({ transformState = state => state, selectState = state => state, fieldName = 'count',} = {}) =>// then return another function that accepts state and props(state, props) => { return
Now that we know what the stateUpdater looks like, we can dive back into the
event handler above. Inside the !this.isControlled() check, we setState using
the returned function after calling stateUpdater with undefined (results in
our default argument for transformState, selectState and fieldName).
There is one other thing we need to cover to fully make the UI component
controllable:
class Counter extends React.Component { // ... state = { count: this.props.defaultCount, // this needs to be an exact reference to the value provided by props propCount: this.props.count, }; static getDerivedStateFromProps(nextProps, prevState) { // if the exact comparison between propCount in local state // and the count provided by the props is false // then its time to update the component if (
Now we can put all this together into a working demo, which you can find here on
Codesandbox.
Footnotes:
👆Back to reference Most of the render prop code still lives in the carousel but it
will most likely be removed before we open source.
👆Back to reference Side note, I haven't even validated that this is what Downshift
does, I have only some second hand knowledge of their helpers that they pass
back up.
I recently launched a rewrite and redesign of this personal website, I figured I'd talk a bit about the changes and new features that I added along the way!