Idea

This is almost react transition group, but for state management...

Why people react transition group? Because it 1) takes a pause between steps letting classNames be applied and 2) keeps children after you remove them, to perform a fade animation.

We are doing the same, but using state.

It's all about tracking what the value would be, what is right now, and what it was. We call this ContinuousState

ContinuousState

• future - the next value. The value you just set.
• present - to be synchronized with the future "later"
• past - to be synchronized with the present "later"

and

• defined - an indication that any of future, past or present are truthy.

API

useContinuousState(value, options)

• value - a value to assign to the future
• options
  • delayPresent - time in ms between future becoming present
  • delayPast - time in ms between present becoming past. For transitions, it usually equals to exist animation duration
  • initialValue - a value to be set as initial to the past and present. future is always equal to the value given as a first arg

useScatteredContinuousState(value, options)

Call signature is equal to useContinuousState, returns an object with extra property DefinePresent. See example below.

Usage

Problem statement

Let's imagine you have a switch. Which controls visibility of something, but you also want to add some animation.

Let's handle these cases separately:

const App = () => {
    const [on, setOn] = useState(false);

    return (
        <div>
            <button onClick={() => setOn(on => !on)}>Toggle</button>
            // would be instanly hidden and shown
            {on && <Content/>}
            // would be animated, but would be ALWAYS rendered
            <ContentWithAnimation visible={on}/>}
        </div>
    );
};

Now let's imagine you want to not render Content when it's not visible and not required.

Ok, "when is this when"?

• render ContentWithAnimation when it is about to be displayed
• render ContentWithAnimation when it is displayed
• render ContentWithAnimation when it is no longer visible, but still animating toward hidden state

import { ContinuousContainer, useContinuousState } from '@theuiteam/continuous-container';

const App = () => {
  const [on, setOn] = useState(false);
  const continuousState = useContinuousState(on);

  return (
    <div>
      <button onClick={() => setOn((on) => !on)}>Toggle</button>
      {/*render if any of past/preset/future is set to true*/}
      {continuousState.defined && (
        <ContentWithAnimation visible={continuousState.present} />
        // wire the "present" state
      )}
      {/* or */}
      <ContinuousContainer value={on} timeout={300}>
        {
          (past, present, future) => (past || present || future) && <ContentWithAnimation visible={present} />
          // ^^ use the "present" value
        }
      </ContinuousContainer>
    </div>
  );
};

Scattered

There are more sophisticated situations, when setting up something to display does not mean "display". Lazy loading is a good case

const App = () => {
  const continuousState = useContinuousState(on);
  return continuousState.defined && <LazyLoadedContentWithAnimation visible={continuousState.present} />;
};

In such case ContinuousState will update from future to present before LazyLoadedContentWithAnimation component is loaded, breaking a connection between states.

In order to handle this problem one might need to tap into rendering process using useScatteredContinuousState

const continuousState = useScatteredContinuousState(on);
return (
  continuousState.defined && (
    <Suspense>
      <LazyLoadedContentWithAnimation visible={continuousState.present}>
        <continuousState.DefinePresent />
        {/*this component will advance ContinuousState once rendered*/}
      </LazyLoadedContentWithAnimation>
    </Suspense>
  )
);

For readability purposes we recommend putting DefinePresent to a separate slot different from children.

<LazyLoadedContentWithAnimation visible={continuousState.present} effector={<continuousState.DefinePresent />} />

⚠️⚠️⚠️⚠️⚠️⚠️⚠️

The following code will NOT work as DefinePresent will be rendered instantly, even if suspense will be in fallback

<Suspense>
  // will not be rendred until ready
  <LazyLoadedContentWithAnimation visible={continuousState.present} />
  // will be rendered too early
  <continuousState.DefinePresent />
</Suspense>

See also

• Phased Container from a recondition library

License

MIT