src/player/transitions/use-transition-progress.ts

import React from "react";
import { EnteringContext, ExitingContext } from "./context";

export type TransitionState = {
  entering: number;
  exiting: number;
  isInTransitionSeries: boolean;
};

/**
 * Gets the progress and direction of a transition with a context() presentation.
 */
export const useTransitionProgress = (): TransitionState => {
  const entering = React.useContext(EnteringContext);
  const exiting = React.useContext(ExitingContext);

  if (!entering && !exiting) {
    return {
      isInTransitionSeries: false,
      entering: 1,
      exiting: 0
    };
  }

  return {
    isInTransitionSeries: true,
    entering: entering?.enteringProgress ?? 1,
    exiting: exiting?.exitingProgress ?? 0
  };
};