Component State: Overcoming Tunnel Vision

Redux can eliminate the need for component state by storing all data in one immutable object. However Redux introduces new complexity to manage global state and actions. For domain resources (usually fetched over HTTP) a global store enables sophisticated caching strategies. But for application state, it’s often more trouble than it’s worth.

React lets us organize applications by the features in our domain (e.g. ProjectPage, Search, ValidatedField) rather than by technical patterns (e.g. views, containers, actions, reducers). Effective product teams work in terms of features, not abstract technical domains.

Deciding to work with component state puts pressure on our designs. Developers must decide which components should own state and then connect parent and child components to that state with data props and callbacks. Because components can make API calls, manage internal state, and render HTML they often grow with every new feature.

Let’s focus on one design pressure we often feel when using component state: tunneling data though layers of components.

Tunneling Application State

Most front-end developers know that they could forego global state if they were willing to pass every bit of state through every component. This practice is sometimes called “tunneling”.

Tunneling properties through components starts small. But every time a developer extracts an intermediate component, they must tunnel all the properties through that new component. It’s like they’re being punished every time they improve the design.

Let’s look at an example of a search component that follows a naive design path.

Already there’s a little boilerplate in our render method. We’re mapping names (e.g. onFilterClear to this.handleFilterClear) without providing any new information. It’s redundant but it’s a reasonable way to expose a public API to SearchLayout.

Now let’s check out the SearchLayout component:

SearchLayout has some presentation responsibilities (notice the heading and the horizontal rule), but it also has to divvy up props from its owner to its children. This time we’re mapping props that our component doesn’t care about to other props that our component doesn’t care about. This is where we start to feel the “tunneling” pain.

Search, meet SearchField

SearchLayout shouldn’t know how to connect the prop names between Search and SearchField. It should only care about presentation things: headings, horizontal rules, and other stylistic elements.

So let’s pull the configuration of SearchField and SearchResult up the stack, right where that information lives: inside Search.

So what does SearchLayout look like now?

SearchLayout decides where to put the field prop and results prop, but no longer needs to worry about the props exchanged between SearchField, SearchResults, and Search.

Is this actually better?

Are we really improving our design or just moving stuff around?

I argue this design is better because we’ve eliminated the repetition of tunneling our properties through the SearchLayout. We now map the Search properties to owned components once rather than twice. We’ve also identified an implicit responsibility (configuration) that was shared between two components and is now confined to Search.

However, if we look at the Search component before and after this refactoring we notice that it’s more complicated. Although SearchLayout is now dead simple, Search knows about three components (SearchLayout, SearchField, and SearchResults) rather than one (SearchLayout). Because Search is the top-level component in our Search domain, it’s easy for responsibilities to collect here.

Search handles both the concern of querying and the concern of coordinating communication between our search components. Let’s extract a new component to handle our lower-level, “querying” concern.

Introducing the SearchQuery Component

An alternative, and probably a more popular option, is to create a higher order component that accepts Search as an argument and renders it with a query prop. I prefer the render callback because it declaratively shows how all of our search components work together inside of Search.

Notice that I cheated a little. Not only did I extract a component to handle querying, I also bundling the props and callbacks into a domain object that I’ll call query inside of Search.

Search is looking pretty good at this point.

We set out to remove some prop tunneling and at the same time dramatically increased the flexibility of our design. It would be trivial to create another Search component that uses a different SearchQuery component to accommodate a different backend. We could inject a different presentation for our search results into our SearchLayout component. The flexibility comes with a cost: separating concerns creates more abstract pieces to learn about. But in this case, the level of abstraction in each component is more consistent, making our components easier to understand.

Goodbye Tunneling

If you squint a little you can see query as a little Redux bundle with one immutable state object and 3 actions (setFilter, clearFilter, setTerm).

By creating a domain object and a component dedicated to composition, we removed the state tunneling pain. As a happy side-effect we created a design that is more flexible, has constant levels of abstraction, and is easier to test.

Published on Aug 20, 2017