Reading a computed makes it live

src/computed.ts

@@ -16,6 +16,7 @@ import {
   ReactiveNode,
   SIGNAL,
 } from './graph.js';
+import {beginEnsureLive} from './liveConsumer.js';
 
 /**
  * A computation, which derives a value from a declarative reactive expression.
@@ -48,11 +49,16 @@ export type ComputedGetter<T> = (() => T) & {
 };
 
 export function computedGet<T>(node: ComputedNode<T>) {
-  // Check if the value needs updating before returning it.
-  producerUpdateValueVersion(node);
-
-  // Record that someone looked at this signal.
-  producerAccessed(node);
+  const endEnsureLive = beginEnsureLive();
+  try {
+    // Check if the value needs updating before returning it.
+    producerUpdateValueVersion(node);
+
+    // Record that someone looked at this signal.
+    producerAccessed(node);
+  } finally {
+    endEnsureLive();
+  }
 
   if (node.value === ERRORED) {
     throw node.error;

src/liveConsumer.ts

@@ -0,0 +1,25 @@
+import {
+  consumerBeforeComputation,
+  consumerDestroy,
+  getActiveConsumer,
+  REACTIVE_NODE,
+  setActiveConsumer,
+} from './graph';
+
+const noop = () => {};
+export const beginEnsureLive = () => {
+  const activeConsumer = getActiveConsumer();
+  if (activeConsumer) {
+    return noop;
+  }
+
+  const liveConsumer = Object.create(REACTIVE_NODE);
+  liveConsumer.consumerIsAlwaysLive = true;
+  liveConsumer.consumerAllowSignalWrites = false;
+
+  setActiveConsumer(liveConsumer);
+  return () => {
+    setActiveConsumer(null);
+    consumerDestroy(liveConsumer);
+  };
+};

tests/behaviors/liveness.test.ts

@@ -10,9 +10,11 @@ describe('liveness', () => {
       [Signal.subtle.unwatched]: unwatchedSpy,
     });
     const computed = new Signal.Computed(() => state.get());
-    computed.get();
-    expect(watchedSpy).not.toBeCalled();
-    expect(unwatchedSpy).not.toBeCalled();
+    computed.get(); // reading a computed is considered watching and unwatching it
+    expect(watchedSpy).toHaveBeenCalledTimes(1);
+    expect(unwatchedSpy).toHaveBeenCalledTimes(1);
+    watchedSpy.mockClear();
+    unwatchedSpy.mockClear();
 
     const w = new Signal.subtle.Watcher(() => {});
     const w2 = new Signal.subtle.Watcher(() => {});
@@ -43,9 +45,11 @@ describe('liveness', () => {
       [Signal.subtle.unwatched]: unwatchedSpy,
     });
 
-    c.get();
-    expect(watchedSpy).not.toBeCalled();
-    expect(unwatchedSpy).not.toBeCalled();
+    c.get(); // reading a computed is considered watching and unwatching it
+    expect(watchedSpy).toHaveBeenCalledTimes(1);
+    expect(unwatchedSpy).toHaveBeenCalledTimes(1);
+    watchedSpy.mockClear();
+    unwatchedSpy.mockClear();
 
     const w = new Signal.subtle.Watcher(() => {});
     w.watch(c);