feat: skip constant trees and memoize fn

packages/runtime-core/src/optional/memoize.ts

@@ -0,0 +1,53 @@
+// Used for memoizing trees inside render functions.
+//
+// Example (equivalent of v-once):
+//
+//   render() {
+//     return memoize(h('div', this.msg), this, 0)
+//   }
+//
+// Memoize baesd on keys:
+//
+//   render() {
+//     return memoize(h('div', this.msg + this.count), this, 0, [this.msg])
+//   }
+
+import { Component } from '../component'
+import { warn } from '../warning'
+
+const memoizeMap = new WeakMap()
+
+export function memoize(
+  getter: () => any,
+  instance: Component,
+  id: number,
+  keys?: any[]
+): any {
+  if (__DEV__ && !Array.isArray(keys)) {
+    warn(
+      `keys passed to v-memo or memoize must be an array. Got ${String(keys)}`
+    )
+  }
+  let storage = memoizeMap.get(instance)
+  if (!storage) {
+    storage = []
+    memoizeMap.set(instance, storage)
+  }
+  const record = storage[id]
+  if (!record) {
+    const value = getter()
+    storage[id] = [value, keys]
+    return value
+  } else {
+    const [prevValue, prevKeys] = record
+    record[1] = keys
+    if (keys) {
+      for (let i = 0; i < keys.length; i++) {
+        if (keys[i] !== prevKeys[i]) {
+          return (record[0] = getter())
+        }
+      }
+    }
+    return prevValue
+  }
+}