perf(reactivity): ref should not trigger if value did not change

Note: shallowRef will always trigger on assignment because it does not
account for deep mutations

close #1012

packages/reactivity/__tests__/ref.spec.ts

@@ -22,11 +22,19 @@ describe('reactivity/ref', () => {
   it('should be reactive', () => {
     const a = ref(1)
     let dummy
+    let calls = 0
     effect(() => {
+      calls++
       dummy = a.value
     })
+    expect(calls).toBe(1)
     expect(dummy).toBe(1)
     a.value = 2
+    expect(calls).toBe(2)
+    expect(dummy).toBe(2)
+    // same value should not trigger
+    a.value = 2
+    expect(calls).toBe(2)
     expect(dummy).toBe(2)
   })
 
@@ -174,6 +182,22 @@ describe('reactivity/ref', () => {
     expect(dummy).toBe(2)
   })
 
+  test('shallowRef force trigger', () => {
+    const sref = shallowRef({ a: 1 })
+    let dummy
+    effect(() => {
+      dummy = sref.value.a
+    })
+    expect(dummy).toBe(1)
+
+    sref.value.a = 2
+    expect(dummy).toBe(1) // should not trigger yet
+
+    // force trigger
+    sref.value = sref.value
+    expect(dummy).toBe(2)
+  })
+
   test('isRef', () => {
     expect(isRef(ref(1))).toBe(true)
     expect(isRef(computed(() => 1))).toBe(true)