Implementing defineAsyncComponent in Vue (TypeScript)

defineAsyncComponent is a crucial Vue 3 feature that allows for lazy-loading components, improving initial load times and overall application performance. This challenge asks you to implement a simplified version of defineAsyncComponent that handles asynchronous component loading and displays a loading component while the actual component is being fetched. This exercise will solidify your understanding of promises, asynchronous operations, and Vue component lifecycle.

Problem Description

You are tasked with creating a function defineAsyncComponent that takes a factory function as an argument. This factory function should return a Promise that resolves with a Vue component. defineAsyncComponent should:

1. Asynchronous Loading: Load the component asynchronously using the provided factory function.
2. Loading Component: Display a provided loadingComponent while the component is being loaded. If no loadingComponent is provided, display a default loading message (e.g., "Loading...").
3. Error Handling: Display an errorComponent if the Promise returned by the factory function rejects. If no errorComponent is provided, display a default error message (e.g., "Error loading component.").
4. Component Caching: Cache the resolved component so that subsequent loads use the cached version, avoiding redundant fetches.
5. Suspense Integration: The function should return a component that can be used within a <Suspense> component.

Key Requirements:

• The function must accept a factory function that returns a Promise resolving to a Vue component.
• The function must accept optional loadingComponent and errorComponent props.
• The function must cache the resolved component.
• The function must handle Promise rejections gracefully.
• The returned component should emit a loaded event when the component is successfully loaded.
• The returned component should emit an error event when an error occurs during loading.

Expected Behavior:

When the component returned by defineAsyncComponent is initially rendered, the loadingComponent (or the default loading message) should be displayed. Once the Promise resolves, the actual component should be rendered, and the loaded event should be emitted. If the Promise rejects, the errorComponent (or the default error message) should be displayed, and the error event should be emitted. Subsequent renders should immediately display the cached component without re-loading.

Edge Cases to Consider:

• Factory function returning a Promise that never resolves or rejects (infinite loading).
• loadingComponent or errorComponent being null or undefined.
• Multiple components using the same factory function – ensure only one load occurs.
• Error handling during the rendering of loadingComponent or errorComponent.

Examples

Example 1:

Input:
const myComponentFactory = () => new Promise(resolve => setTimeout(() => resolve({ render() { return 'My Component'; } } ), 1000));
const loadingComponent = { render() { return 'Loading...'; } };
const errorComponent = { render() { return 'Error!'; } };

const AsyncComponent = defineAsyncComponent(myComponentFactory, { loadingComponent, errorComponent });

Output:
Initially: "Loading..." (rendered by loadingComponent)
After 1 second: "My Component" (rendered by the resolved component)

Explanation: The factory function simulates an asynchronous load. The loadingComponent is displayed for 1 second, then the resolved component ("My Component") is rendered.

Example 2:

Input:
const myComponentFactory = () => new Promise((resolve, reject) => setTimeout(() => reject(new Error('Failed to load')), 500));

const AsyncComponent = defineAsyncComponent(myComponentFactory);

Output:
Initially: "Loading..." (default loading message)
After 0.5 seconds: "Error!" (default error message)

Explanation: The factory function rejects the Promise after 0.5 seconds. The default error message is displayed.

Example 3: (Edge Case - Repeated Load)

Input:
const myComponentFactory = () => new Promise(resolve => setTimeout(() => resolve({ render() { return 'My Component'; } } ), 500));
const AsyncComponent = defineAsyncComponent(myComponentFactory);

Output:
First render: "Loading..." (rendered by default loading message)
After 0.5 seconds: "My Component" (rendered by the resolved component)
Second render (immediately): "My Component" (rendered from cache - no re-loading)

Explanation: The component is rendered twice. The second render uses the cached component, so no re-loading occurs.

Constraints

• The factory function must return a Promise.
• loadingComponent and errorComponent can be Vue components or objects with a render function.
• The implementation should be reasonably performant, avoiding unnecessary re-renders.
• The solution must be written in TypeScript.
• The solution should not rely on external libraries beyond Vue itself.

Notes

• Consider using a simple cache object to store the resolved component.
• Think about how to handle the lifecycle of the loadingComponent and errorComponent. They should be unmounted when the actual component is loaded or an error occurs.
• The defineAsyncComponent function should return a Vue component that can be used within a template.
• Focus on the core functionality of asynchronous loading, error handling, and caching. Advanced features like timeouts or retries are not required.
• The emitted loaded and error events should be standard Vue events.