better API

packages/svelte/src/internal/client/reactivity/batch.js

@@ -850,17 +850,18 @@ export function clear() {
 
 /**
  * @param {() => void} fn
- * @returns {Promise<{ commit: () => void, discard: () => void }>}
+ * @returns {{ commit: () => void, discard: () => void }}
  */
 export function fork(fn) {
-	/** @type {Promise<{ commit: () => void, discard: () => void }>} */
-	const promise = new Promise((fulfil) => {
-		// TODO does qmt guarantee this will run outside a batch?
-		// because it needs to
-		queue_micro_task(async () => {
+	if (current_batch !== null) {
+		throw new Error('cannot fork here'); // TODO better error
+	}
+
 	const batch = Batch.ensure();
 	batch.is_fork = true;
 
+	const promise = batch.fork_settled();
+
 	flushSync(fn);
 	const deferred_inspect_effects = inspect_effects;
 
@@ -869,10 +870,8 @@ export function fork(fn) {
 		source.v = value;
 	}
 
-			await batch.fork_settled();
-
-			fulfil({
-				commit: () => {
+	return {
+		commit: async () => {
 			if (!batches.has(batch)) {
 				throw new Error('Cannot commit this batch'); // TODO better error
 			}
@@ -884,6 +883,8 @@ export function fork(fn) {
 				}
 			}
 
+			await promise;
+
 			for (const [source, value] of batch.current) {
 				source.v = value;
 			}
@@ -906,9 +907,5 @@ export function fork(fn) {
 		discard: () => {
 			batches.delete(batch);
 		}
-			});
-		});
-	});
-
-	return promise;
+	};
 }