fix: prevent computed from being called before accessing it

Resolves: #79

Author: liximomo

src/reactivity/ref.ts

@@ -95,7 +95,10 @@ class RefImpl<T> implements Ref<T> {
 }
 
 export function createRef<T>(options: RefOption<T>) {
-  return new RefImpl<T>(options);
+  // seal the ref, this could prevent ref from being observed
+  // It's safe to seal the ref, since we really shoulnd't extend it.
+  // related issues: #79
+  return Object.seal(new RefImpl<T>(options));
 }
 
 type RefValue<T> = T extends Ref<infer V> ? V : UnwrapRef<T>;

test/apis/state.spec.js

@@ -1,5 +1,5 @@
 const Vue = require('vue/dist/vue.common.js');
-const { reactive, ref, watch, set, toRefs } = require('../../src');
+const { reactive, ref, watch, set, toRefs, computed } = require('../../src');
 
 describe('api/ref', () => {
   it('should work with array', () => {
@@ -350,4 +350,19 @@ describe('unwrapping', () => {
     });
     expect(dummy).toBe(a);
   });
+
+  it('should not call the computed property until accessing it', () => {
+    const spy = jest.fn();
+    const state = reactive({
+      count: 1,
+      double: computed(() => {
+        spy();
+        return state.count * 2;
+      }),
+    });
+
+    expect(spy).not.toHaveBeenCalled();
+    expect(state.double).toBe(2);
+    expect(spy).toHaveBeenCalled();
+  });
 });