Implement React's startTransition API

React's startTransition API allows you to defer non-urgent state updates, preventing them from blocking more important updates like user input. This challenge asks you to build a simplified version of this API to understand its core concepts and implementation.

Problem Description

Your task is to create a custom hook, useCustomStartTransition, that mimics the behavior of React's startTransition. This hook should accept a callback function that contains a state update. The hook should then execute this callback in a way that allows other urgent updates (simulated in this challenge) to be prioritized.

Key Requirements:

1. useCustomStartTransition Hook: Create a TypeScript hook named useCustomStartTransition.
2. Callback Execution: The hook should accept a single argument: a callback function (stateUpdater: () => void) => void. This callback will be responsible for performing the state update.
3. Prioritization: Updates initiated via useCustomStartTransition should be considered "non-urgent" and should yield to "urgent" updates. In this simulation, urgent updates will be represented by immediate state changes.
4. Return Value: The hook should return a function that, when called, triggers the execution of the provided callback.

Expected Behavior:

When the function returned by useCustomStartTransition is invoked:

• If there are any "urgent" updates pending (simulated by other immediate state setters), those urgent updates should be processed first.
• The non-urgent update provided in the callback should be deferred.
• Once urgent updates are cleared, the non-urgent update should be executed.

Edge Cases:

• Multiple startTransition calls: How does your implementation handle multiple non-urgent updates being scheduled? (For simplicity, we'll focus on one at a time for this challenge).
• No urgent updates: If there are no urgent updates, the non-urgent update should execute relatively quickly after being scheduled.

Examples

Example 1: Basic Usage

import React, { useState, useEffect } from 'react';
import { useCustomStartTransition } from './useCustomStartTransition'; // Assuming your hook is in this file

function App() {
  const [urgentValue, setUrgentValue] = useState('');
  const [nonUrgentValue, setNonUrgentValue] = useState('');

  const startUpdatingNonUrgent = useCustomStartTransition();

  const handleInputChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    // This is an urgent update
    setUrgentValue(e.target.value);
  };

  const handleNonUrgentButtonClick = () => {
    startUpdatingNonUrgent(() => {
      // Simulate a more complex or time-consuming update
      setNonUrgentValue('Updated after transition');
    });
  };

  return (
    <div>
      <input type="text" value={urgentValue} onChange={handleInputChange} placeholder="Type here (urgent)" />
      <p>Urgent Value: {urgentValue}</p>
      <button onClick={handleNonUrgentButtonClick}>Update Non-Urgent</button>
      <p>Non-Urgent Value: {nonUrgentValue}</p>
    </div>
  );
}

Input: User types "a", then clicks "Update Non-Urgent".

Output: The input field (Urgent Value) updates immediately as the user types. The Non-Urgent Value updates shortly after, but not instantaneously with the typing. If the user types very rapidly, the "Updated after transition" text will appear after they stop typing.

Explanation: Typing into the input field causes an immediate setUrgentValue call (urgent update). Clicking the button schedules setNonUrgentValue via startUpdatingNonUrgent (non-urgent update). The typing updates should take precedence, and the non-urgent value update is deferred until the urgent updates from typing have settled.

Example 2: Demonstrating Deferral

import React, { useState, useEffect } from 'react';
import { useCustomStartTransition } from './useCustomStartTransition';

function App() {
  const [count, setCount] = useState(0);
  const startUpdatingCount = useCustomStartTransition();

  const incrementUrgent = () => {
    setCount(c => c + 1); // Urgent
  };

  const incrementNonUrgent = () => {
    startUpdatingCount(() => {
      setCount(c => c + 1); // Non-urgent
    });
  };

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={incrementUrgent}>Increment Urgent</button>
      <button onClick={incrementNonUrgent}>Increment Non-Urgent</button>
    </div>
  );
}

Input: User clicks "Increment Urgent" twice rapidly, then clicks "Increment Non-Urgent".

Output: The count will increment by 2 immediately. Then, after a slight delay, the count will increment by an additional 1. The final count will be 3.

Explanation: The two "Increment Urgent" clicks result in immediate state updates. The "Increment Non-Urgent" click schedules a state update. Because it's scheduled via startUpdatingCount, it's treated as a lower priority and executed after the urgent updates from the first two button clicks have finished.

Constraints

• The simulation of "urgent" updates will be done using standard useState setters directly within event handlers or effects.
• Your useCustomStartTransition hook should not rely on any actual browser scheduling mechanisms like requestIdleCallback or internal React scheduler APIs. You need to simulate the prioritization logic yourself.
• The hook should be implemented in TypeScript.

Notes

Consider how you might manage a queue of non-urgent updates. How can you signal when urgent updates have finished so that your non-urgent updates can proceed? Think about using useEffect to observe state changes or other mechanisms to detect when the main thread is less busy. You are essentially building a simplified scheduler.