Implementing a Vue-like Effect Scheduler

Vue's reactivity system relies on an effect scheduler to manage how and when side effects (like component re-renders or computed property updates) are executed. This challenge is to build a simplified version of such a scheduler in TypeScript. A well-implemented scheduler ensures that effects are run efficiently, preventing redundant computations and managing dependencies correctly.

Problem Description

You need to create a Scheduler class that can:

1. Register Effects: Accept functions (effects) that should be scheduled for execution.
2. Schedule Effects: When an effect is registered, it should be added to a queue of pending effects.
3. Execute Effects: Provide a method to process the queue and execute all pending effects.
4. Debounce Execution: If the schedule method is called multiple times before flush is invoked, the effect should only be scheduled once.
5. Handle Dependencies (Implicitly): While you won't be implementing a full dependency tracking system, your scheduler should be able to handle cases where the same effect might be re-scheduled by different operations.

Key Requirements:

• The Scheduler class should have a schedule(effect: () => void) method.
• The Scheduler class should have a flush() method.
• When schedule(effect) is called, the effect function should be added to a queue.
• If schedule(effect) is called with the same effect function multiple times before flush is called, it should only be enqueued once.
• When flush() is called, all effects currently in the queue should be executed in the order they were scheduled.
• After flush() is called, the queue should be cleared.

Expected Behavior:

• Multiple calls to schedule for different effects will add them to the queue.
• Multiple calls to schedule for the same effect function will not result in duplicates in the queue.
• Calling flush will execute all unique pending effects.
• Subsequent calls to schedule after a flush will add new effects to the queue for the next flush.

Edge Cases:

• Calling flush() when the queue is empty should do nothing.
• Effects themselves might trigger other effects, which should be handled by the scheduler's queuing mechanism.

Examples

Example 1:

// Assume a Scheduler instance is created: const scheduler = new Scheduler();

// effect1 and effect2 are distinct functions
const effect1 = () => console.log('Effect 1 executed');
const effect2 = () => console.log('Effect 2 executed');

scheduler.schedule(effect1);
scheduler.schedule(effect2);
scheduler.schedule(effect1); // Duplicate, should not be added again

console.log('Before flush...');
scheduler.flush();
console.log('After flush.');

Output:

Before flush...
Effect 1 executed
Effect 2 executed
After flush.

Explanation: effect1 and effect2 are scheduled. effect1 is scheduled again, but because it's a duplicate before a flush, it's ignored. flush executes the unique scheduled effects.

Example 2:

// Assume a Scheduler instance is created: const scheduler = new Scheduler();

let counter = 0;
const incrementEffect = () => {
    counter++;
    console.log(`Counter is now: ${counter}`);
    // This effect might re-schedule itself or another effect
    if (counter < 3) {
        scheduler.schedule(incrementEffect);
    }
};

scheduler.schedule(incrementEffect);
console.log('Initial counter:', counter);

console.log('Flushing...');
scheduler.flush();
console.log('Final counter:', counter);

Output:

Initial counter: 0
Flushing...
Counter is now: 1
Counter is now: 2
Counter is now: 3
Final counter: 3

Explanation: The incrementEffect is scheduled. When flush is called, it executes, increments counter to 1, and re-schedules itself because counter < 3. This continues until counter reaches 3. The scheduler correctly handles the self-rescheduling by adding the effect to the queue for the next tick (or in this case, within the same flush cycle, but conceptually it will be processed if the loop continues to run). Note that for a true Vue-like scheduler, this might happen across microtasks, but for this simplified version, sequential execution within flush that re-schedules is acceptable. The key is that incrementEffect is not endlessly duplicated in the queue.

Example 3 (Edge Case):

// Assume a Scheduler instance is created: const scheduler = new Scheduler();

const effectA = () => console.log('Effect A');
const effectB = () => console.log('Effect B');

scheduler.schedule(effectA);
scheduler.schedule(effectA); // Duplicate

console.log('Calling flush when queue has duplicates...');
scheduler.flush(); // Should only run effectA once.

console.log('Calling flush again when queue is empty...');
scheduler.flush(); // Should do nothing.

Output:

Calling flush when queue has duplicates...
Effect A
Calling flush again when queue is empty...

Explanation: The first flush correctly handles the duplicate, running effectA only once. The second flush operates on an empty queue and correctly does nothing.

Constraints

• The Scheduler class must be implemented in TypeScript.
• The effect parameter for schedule will always be a function that takes no arguments and returns void.
• The scheduler should be reasonably efficient for a moderate number of effects (e.g., up to a few thousand effects per flush). The primary concern is avoiding redundant executions and managing the queue correctly.

Notes

• Think about how to keep track of effects that have already been scheduled but not yet flushed to avoid duplicates. A Set is a good candidate for this.
• Consider the order of execution. Effects should run in the order they were first successfully scheduled, not necessarily the order they appear in the queue if duplicates were attempted.
• This challenge focuses on the queuing and de-duplication logic, not on the complex dependency tracking that a real Vue reactivity system would involve. You are responsible for managing the queue of functions to be executed.