Added new withSwitch observe utility function. (#69)

* bump json

* explicit unsubscribe to null just in case

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: krisnye

packages/data/src/observe/public.ts

@@ -25,6 +25,7 @@ export * from "./with-async-map.js";
 export * from "./with-map.js";
 export * from "./with-map-data.js";
 export * from "./with-optional.js";
+export * from "./with-switch.js";
 export * from "./with-unwrap.js";
 export * from "./with-lazy.js";
 export * from "./with-batch.js";

packages/data/src/observe/with-switch.test.ts

@@ -0,0 +1,179 @@
+// © 2026 Adobe. MIT License. See /LICENSE for details.
+
+import { describe, test, expect, assertType } from "vitest";
+import { withSwitch } from "./with-switch.js";
+import { fromConstant } from "./from-constant.js";
+import { createState } from "./create-state.js";
+import type { Observe } from "./index.js";
+
+describe("withSwitch", () => {
+  test("should switch and observe the selected observable", () => {
+    const record = {
+      a: fromConstant(1),
+      b: fromConstant(2),
+      c: fromConstant(3),
+    };
+    const key = fromConstant("b" as const);
+    const picked = withSwitch(record, key);
+
+    let result: number | undefined;
+    picked((value) => {
+      result = value;
+    })();
+
+    expect(result).toBe(2);
+  });
+
+  test("should switch observables when key changes", () => {
+    const record = {
+      a: fromConstant(10),
+      b: fromConstant(20),
+      c: fromConstant(30),
+    };
+    const [key, setKey] = createState<"a" | "b" | "c">("a");
+    const picked = withSwitch(record, key);
+
+    const values: number[] = [];
+    const unsubscribe = picked((value) => {
+      values.push(value);
+    });
+
+    setKey("b");
+    setKey("c");
+
+    unsubscribe();
+
+    expect(values).toEqual([10, 20, 30]);
+  });
+
+  test("should unsubscribe from previous observable when key changes", () => {
+    const [observableA, setA] = createState(100);
+    const [observableB, setB] = createState(200);
+    const record = { a: observableA, b: observableB };
+    const [key, setKey] = createState<"a" | "b">("a");
+    const picked = withSwitch(record, key);
+
+    const values: number[] = [];
+    const unsubscribe = picked((value) => {
+      values.push(value);
+    });
+
+    setA(101); // Should be observed
+    setKey("b"); // Switch to b
+    setA(102); // Should NOT be observed (unsubscribed from a)
+    setB(201); // Should be observed
+
+    unsubscribe();
+
+    expect(values).toEqual([100, 101, 200, 201]);
+  });
+
+  test("should clean up all subscriptions on unobserve", () => {
+    const [observableA, setA] = createState(1);
+    const [observableB, setB] = createState(2);
+    const record = { a: observableA, b: observableB };
+    const [key, setKey] = createState<"a" | "b">("a");
+    const picked = withSwitch(record, key);
+
+    const values: number[] = [];
+    const unsubscribe = picked((value) => {
+      values.push(value);
+    });
+
+    setA(10);
+    unsubscribe();
+
+    // After unsubscribe, no further notifications
+    setA(20);
+    setB(30);
+    setKey("b");
+
+    expect(values).toEqual([1, 10]);
+  });
+
+  test("should handle rapid key changes", () => {
+    const record = {
+      x: fromConstant("first"),
+      y: fromConstant("second"),
+      z: fromConstant("third"),
+    };
+    const [key, setKey] = createState<"x" | "y" | "z">("x");
+    const picked = withSwitch(record, key);
+
+    const values: string[] = [];
+    const unsubscribe = picked((value) => {
+      values.push(value);
+    });
+
+    setKey("y");
+    setKey("z");
+    setKey("x");
+    setKey("z");
+
+    unsubscribe();
+
+    expect(values).toEqual(["first", "second", "third", "first", "third"]);
+  });
+
+  test("should throw error when key is not in record", () => {
+    const record = {
+      a: fromConstant(1),
+      b: fromConstant(2),
+    };
+    const [key, setKey] = createState<string>("a");
+    const picked = withSwitch(record, key);
+
+    const unsubscribe = picked(() => {});
+
+    expect(() => {
+      setKey("invalid");
+    }).toThrow('Key "invalid" not found in observable record');
+
+    unsubscribe();
+  });
+
+  test("type inference: should infer union type from subset of keys", () => {
+    // Compile-time type test
+    const record = {
+      a: fromConstant(true),
+      b: fromConstant("hello"),
+      c: fromConstant(42),
+    };
+    
+    const key = fromConstant("a" as "a" | "b");
+    const result = withSwitch(record, key);
+    
+    // Type should be Observe<boolean | string>, not Observe<boolean | string | number>
+    assertType<Observe<boolean | string>>(result);
+  });
+
+  test("type inference: should work with all keys", () => {
+    // Compile-time type test
+    const record = {
+      a: fromConstant(true),
+      b: fromConstant("hello"),
+      c: fromConstant(42),
+    };
+    
+    const key = fromConstant("a" as "a" | "b" | "c");
+    const result = withSwitch(record, key);
+    
+    // Type should be Observe<boolean | string | number>
+    assertType<Observe<boolean | string | number>>(result);
+  });
+
+  test("type inference: should work with single key", () => {
+    // Compile-time type test
+    const record = {
+      a: fromConstant(true),
+      b: fromConstant("hello"),
+      c: fromConstant(42),
+    };
+    
+    const key = fromConstant("b" as const);
+    const result = withSwitch(record, key);
+    
+    // Type should be Observe<string>
+    assertType<Observe<string>>(result);
+  });
+});

packages/data/src/observe/with-switch.ts

@@ -0,0 +1,56 @@
+// © 2026 Adobe. MIT License. See /LICENSE for details.
+import { Observe, Unobserve } from "./index.js";
+
+/**
+ * Dynamically switches between observables in a record based on a key observable.
+ * When the key changes, automatically unsubscribes from the previous observable and subscribes to the new one.
+ *
+ * @example
+ * ```typescript
+ * const data = {
+ *   home: homeData$,
+ *   profile: profileData$,
+ *   settings: settingsData$
+ * };
+ * const currentTab$ = createState('home');
+ * const currentData$ = withSwitch(data, currentTab$);
+ * // When currentTab$ changes, automatically switches to observing the corresponding data observable
+ * ```
+ */
+export function withSwitch<K extends string, T extends Record<K, Observe<any>>>(
+  record: T,
+  key: Observe<K>
+): Observe<T[K] extends Observe<infer U> ? U : never> {
+  return (observer) => {
+    let currentUnsubscribe: Unobserve | null = null;
+
+    const keyUnsubscribe = key((selectedKey) => {
+      // Unsubscribe from the previous observable before subscribing to the new one
+      currentUnsubscribe?.();
+      if (currentUnsubscribe) {
+        currentUnsubscribe();
+        currentUnsubscribe = null;
+      }
+
+      // Validate that the key exists in the record
+      if (!(selectedKey in record)) {
+        throw new Error(
+          `Key "${selectedKey}" not found in observable record. Available keys: ${Object.keys(record).join(", ")}`
+        );
+      }
+
+      // Subscribe to the newly selected observable
+      const selectedObservable = record[selectedKey];
+      currentUnsubscribe = selectedObservable(observer);
+    });
+
+    // Return a new unsubscribe function that unsubscribes from both the key observable and current selected observable
+    return () => {
+      keyUnsubscribe();
+      if (currentUnsubscribe) {
+        currentUnsubscribe();
+        currentUnsubscribe = null;
+      }
+    };
+  };
+}