Function preload

packages/melonjs/src/loader/loader.js:420–459

(assets, onloadcb, switchToLoadState = true)

 * @category Assets
 */
export function preload(assets, onloadcb, switchToLoadState = true) {
	// set the onload callback if defined
	if (typeof onloadcb !== "undefined") {
		onload = onloadcb;
	}

	// parse the resources and collect a promise for each asset. `load()` now
	// returns a Promise in its no-callback form, so there's no need to wrap a
	// callback in `new Promise` here — just layer the progress / error reporting
	// on top of it. Each asset counts as one toward the progress total (parsers
	// load 0 or 1 resource; a cached asset resolves immediately and still
	// reports progress, keeping `loadCount`/`resourceCount` balanced).
	const promises = assets.map((asset) => {
		resourceCount += 1;
		return load(asset)
			.then(() => {
				onResourceLoaded(asset);
			})
			.catch((err) => {
				// onLoadingError emits LOADER_ERROR and throws by design, so this
				// re-rejects → Promise.all (and the returned promise) reject and
				// `await loader.preload(...)` surfaces the failure. The trailing
				// throw keeps the rejection meaningful should it ever stop throwing.
				onLoadingError.call(this, asset);
				throw err;
			});
	});

	if (switchToLoadState === true) {
		// switch to the loading screen
		state.change(state.LOADING);
	}

	// Resolve once every asset has loaded; call the completion callback on
	// success (back-compat). Returned so callers can `await loader.preload(...)`
	// instead of (or as well as) passing a callback.
	return Promise.all(promises).then(() => {
		completeLoading(onload);
	});
}

/**
 * retry loading assets after a loading failure