fix: Issue with @signal's cache: computed instances sharing the same cache

src/index.ts

@@ -111,15 +111,23 @@ function computedDecorator<Value = any>(
     throw new Error(`Can only use @cached on getters.`);
   }
 
-  let caches = new WeakMap<typeof target, Signal.Computed<Value>>();
+  const caches = new WeakMap<
+    typeof target,
+    WeakMap<object, Signal.Computed<Value>>
+  >();
 
   return function (this: unknown) {
     let cache = caches.get(target);
     if (!cache) {
-      cache = new Signal.Computed(() => target.call(this));
+      cache = new WeakMap();
       caches.set(target, cache);
     }
+    let effect = cache.get(this as object);
+    if (!effect) {
+      effect = new Signal.Computed(() => target.call(this));
+      cache.set(this as object, effect);
+    }
 
-    return cache.get();
+    return effect.get();
   };
 }

tests/@signal.test.ts

@@ -83,4 +83,25 @@ describe("@signal (getter)", () => {
       new State();
     }, /@signal can only be used on accessors or getters/);
   });
+
+  it("not shared between instances", () => {
+    class State {
+      @signal accessor #value = 3;
+
+      @signal
+      get doubled() {
+        return this.#value * 2;
+      }
+
+      constructor(value: number) {
+        this.#value = value;
+      }
+    }
+
+    let state1 = new State(1);
+    let state2 = new State(2);
+
+    assert.strictEqual(state1.doubled, 2);
+    assert.strictEqual(state2.doubled, 4);
+  });
 });