Vue Cached Computed Property

Vue's computed properties are a powerful way to derive reactive data. However, by default, they re-evaluate on every render if their dependencies change. This challenge asks you to implement a "cached computed" functionality that only re-computes when necessary, significantly improving performance for expensive computations.

This is useful for scenarios where a computed property involves complex calculations, data fetching, or filtering that should not be performed on every single re-render, but only when the underlying data that influences its result has actually changed.

Problem Description

You need to create a composable function in Vue 3 (using TypeScript) that mimics Vue's computed but adds caching. This means the computation should only run if:

1. The computed property is accessed for the first time.
2. Any of its reactive dependencies have changed since the last access.

If the computed property is accessed and its dependencies haven't changed, the previously computed value should be returned immediately.

Key Requirements:

• Create a composable function, let's call it cachedComputed.
• cachedComputed should accept a getter function as its argument. This getter function should be reactive and can depend on other Vue refs or computeds.
• The composable should return a Vue ComputedRef that holds the cached value.
• The getter function should only be executed when necessary (as described above).
• The composable should work seamlessly within a Vue 3 Composition API setup.

Expected Behavior:

• When cachedComputed is first created and accessed, the getter function will execute.
• Subsequent accesses to the returned ComputedRef (without dependency changes) will return the cached value without executing the getter.
• When any reactive dependency used within the getter function changes, the getter will execute again on the next access, and the new result will be cached.

Edge Cases to Consider:

• What happens if the getter function throws an error? (For this challenge, assume the getter is well-behaved and doesn't throw).
• The composable should handle dependencies correctly, even if they are nested or complex.

Examples

Example 1:

Let's imagine a scenario where we have a large list of items and we need to filter them.

import { ref, cachedComputed } from 'vue'; // Assume cachedComputed is imported

const items = ref([
  { id: 1, name: 'Apple' },
  { id: 2, name: 'Banana' },
  { id: 3, name: 'Orange' },
  { id: 4, name: 'Mango' },
]);

const filterText = ref('');

// Expensive filtering operation (simulated)
const filteredItems = cachedComputed(() => {
  console.log('Filtering items...'); // This should only log when filterText changes or initially
  if (!filterText.value) {
    return items.value;
  }
  return items.value.filter(item =>
    item.name.toLowerCase().includes(filterText.value.toLowerCase())
  );
});

// --- Usage in a component ---

// Initial access
console.log('Initial filtered items:', filteredItems.value); // Logs: Filtering items..., then the full list

// Access again without changing filterText
console.log('Filtered items again:', filteredItems.value); // Should return cached value, "Filtering items..." should NOT log

// Change filterText
filterText.value = 'a';

// Access after dependency change
console.log('Filtered items after change:', filteredItems.value); // Logs: Filtering items..., then the filtered list

// Access again without changing filterText
console.log('Filtered items again after change:', filteredItems.value); // Should return cached value, "Filtering items..." should NOT log

Output of Example 1 (console logs):

Filtering items...
Initial filtered items: [ { id: 1, name: 'Apple' }, { id: 2, name: 'Banana' }, { id: 3, name: 'Orange' }, { id: 4, name: 'Mango' } ]
Filtered items again: [ { id: 1, name: 'Apple' }, { id: 2, name: 'Banana' }, { id: 3, name: 'Orange' }, { id: 4, name: 'Mango' } ]
Filtering items...
Filtered items after change: [ { id: 1, name: 'Apple' }, { id: 4, name: 'Mango' } ]
Filtered items again after change: [ { id: 1, name: 'Apple' }, { id: 4, name: 'Mango' } ]

Explanation:

The console.log('Filtering items...'); statement is used to track when the getter function is actually executed. Initially, it runs. When accessed again without filterText changing, it returns the cached value, and the log doesn't appear. When filterText changes to 'a', the getter is executed again on the next access, and the result is cached.

Example 2:

A computed property that performs a slow calculation.

import { ref, cachedComputed } from 'vue';

const multiplier = ref(2);
const baseValue = ref(10);

// Simulates a computationally expensive operation
const slowCalculation = cachedComputed(() => {
  console.log('Performing slow calculation...'); // This should only log when multiplier or baseValue changes
  let result = 0;
  for (let i = 0; i < 1000000; i++) {
    result += multiplier.value * baseValue.value;
  }
  return result;
});

// --- Usage in a component ---

// Initial access
console.log('Initial result:', slowCalculation.value); // Logs: Performing slow calculation..., then the calculated result

// Access again without changing dependencies
console.log('Result again:', slowCalculation.value); // Should return cached value, "Performing slow calculation..." should NOT log

// Change a dependency
baseValue.value = 20;

// Access after dependency change
console.log('Result after change:', slowCalculation.value); // Logs: Performing slow calculation..., then the new calculated result

Output of Example 2 (console logs):

Performing slow calculation...
Initial result: 2000000
Result again: 2000000
Performing slow calculation...
Result after change: 4000000

Explanation:

Similar to Example 1, the log statement shows that the expensive calculation is only performed when the reactive dependencies (multiplier or baseValue) change.

Constraints

• The cachedComputed composable must be implemented in TypeScript.
• It should leverage Vue 3's Composition API.
• The underlying getter function can use any reactive primitives (ref, computed) available in Vue.
• Performance: For scenarios with many re-renders but few dependency changes, the cached computed should be significantly faster than a standard computed property.

Notes

• Think about how Vue's computed internally tracks dependencies. You'll likely need a similar mechanism.
• Consider storing the previous value and a flag indicating whether it's valid.
• When dependencies change, you need to invalidate the cache.
• This is a foundational exercise for understanding reactivity and performance optimization in Vue.