perf: more light-weight computed (#452)

* feat: add customRef

* perf: more light-weight computed

src/apis/computed.ts

@@ -1,7 +1,11 @@
 import { getVueConstructor, getCurrentInstance } from '../runtimeContext'
 import { createRef, Ref } from '../reactivity'
-import { defineComponentInstance } from '../utils/helper'
-import { warn } from '../utils'
+import {
+  warn,
+  noopFn,
+  defineComponentInstance,
+  getVueInternalClasses,
+} from '../utils'
 
 interface Option<T> {
   get: () => T
@@ -31,28 +35,61 @@ export function computed<T>(
     set = options.set
   }
 
-  const computedHost = defineComponentInstance(getVueConstructor(), {
-    computed: {
-      $$state: {
-        get,
-        set,
+  let computedSetter
+  let computedGetter
+
+  if (vm) {
+    const { Watcher, Dep } = getVueInternalClasses()
+    let watcher: any
+    computedGetter = () => {
+      if (!watcher) {
+        watcher = new Watcher(vm, get, noopFn, { lazy: true })
+      }
+      if (watcher.dirty) {
+        watcher.evaluate()
+      }
+      if (Dep.target) {
+        watcher.depend()
+      }
+      return watcher.value
+    }
+
+    computedSetter = (v: T) => {
+      if (__DEV__ && !set) {
+        warn('Write operation failed: computed value is readonly.', vm!)
+        return
+      }
+
+      if (set) {
+        set(v)
+      }
+    }
+  } else {
+    // fallback
+    const computedHost = defineComponentInstance(getVueConstructor(), {
+      computed: {
+        $$state: {
+          get,
+          set,
+        },
       },
-    },
-  })
+    })
+
+    computedGetter = () => (computedHost as any).$$state
+    computedSetter = (v: T) => {
+      if (__DEV__ && !set) {
+        warn('Write operation failed: computed value is readonly.', vm!)
+        return
+      }
 
-  vm && vm.$on('hook:destroyed', () => computedHost.$destroy())
+      ;(computedHost as any).$$state = v
+    }
+  }
 
   return createRef<T>(
     {
-      get: () => (computedHost as any).$$state,
-      set: (v: T) => {
-        if (__DEV__ && !set) {
-          warn('Write operation failed: computed value is readonly.', vm!)
-          return
-        }
-
-        ;(computedHost as any).$$state = v
-      },
+      get: computedGetter,
+      set: computedSetter,
     },
     !set
   )

src/utils/helper.ts

@@ -72,3 +72,36 @@ export function resolveSlots(
 
   return res
 }
+
+let vueInternalClasses:
+  | {
+      Watcher: any
+      Dep: any
+    }
+  | undefined
+
+export const getVueInternalClasses = () => {
+  if (!vueInternalClasses) {
+    const vm: any = defineComponentInstance(getVueConstructor(), {
+      computed: {
+        value() {
+          return 0
+        },
+      },
+    })
+
+    // to get Watcher class
+    const Watcher = vm._computedWatchers.value.constructor
+    // to get Dep class
+    const Dep = vm._data.__ob__.dep.constructor
+
+    vueInternalClasses = {
+      Watcher,
+      Dep,
+    }
+
+    vm.$destroy()
+  }
+
+  return vueInternalClasses
+}