feat: add isCommitted/isDiscarded/run to fork

This gives more insight into the state of a fork and also gives you the ability to run another function in the context of the fork. SvelteKit needs this

Author: dummdidumm

.changeset/lemon-walls-judge.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: add `isCommitted/isDiscarded/run` to `fork`

packages/svelte/src/index.d.ts

@@ -366,6 +366,18 @@ export interface Fork {
  * Discard the fork
  */
 discard(): void;
+/**
+ * Whether the fork has been committed
+ */
+isCommitted(): boolean;
+/**
+ * Whether the fork has been discarded
+ */
+isDiscarded(): boolean;
+/**
+ * Run a function within the forked context.
+ */
+run(fn: () => void): Promise<void>;
 }
 
 export * from './index-client.js';

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

@@ -1014,6 +1014,37 @@ export function fork(fn) {
 batches.delete(batch);
 batch.discard();
 }
+},
+isCommitted: () => committed,
+isDiscarded: () => !committed && !batches.has(batch),
+run: async (fn) => {
+if (committed) {
+// TODO error instead?
+await settled;
+fn();
+} else {
+// We want to start with the current state of the world, not the
+// state back when the fork was created. Also important to
+// correctly revert state changes later
+const previous = batch.previous;
+batch.previous = new Map();
+batch.activate();
+
+flushSync(fn);
+
+// revert state changes
+for (var [source, value] of batch.previous) {
+source.v = value;
+}
+
+// merge 'previous' map
+// TODO is this correct?
+for (const [source, value] of previous) {
+if (!batch.previous.has(source)) {
+batch.previous.set(source, value);
+}
+}
+}
 }
 };
 }

packages/svelte/types/index.d.ts

@@ -363,6 +363,18 @@ declare module 'svelte' {
  * Discard the fork
  */
 discard(): void;
+/**
+ * Whether the fork has been committed
+ */
+isCommitted(): boolean;
+/**
+ * Whether the fork has been discarded
+ */
+isDiscarded(): boolean;
+/**
+ * Run a function within the forked context.
+ */
+run(fn: () => void): Promise<void>;
 }
 /**
  * Returns an [`AbortSignal`](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) that aborts when the current [derived](https://svelte.dev/docs/svelte/$derived) or [effect](https://svelte.dev/docs/svelte/$effect) re-runs or is destroyed.