Add a ...$updating property for each computed property.

src/index.js

@@ -41,6 +41,7 @@ const AsyncComputed = {
             } else {
               data[key] = null
             }
+            data[key + '$updating'] = false
           }
           return data
         }
@@ -58,18 +59,22 @@ const AsyncComputed = {
 
         for (const key in this.$options.asyncComputed || {}) {
           let promiseId = 0
+          const updatingKey = key + '$updating'
           this.$watch(prefix + key, newPromise => {
             const thisPromise = ++promiseId
+            this[updatingKey] = true
 
             if (!newPromise || !newPromise.then) {
               newPromise = Promise.resolve(newPromise)
             }
 
             newPromise.then(value => {
               if (thisPromise !== promiseId) return
+              this[updatingKey] = false
               this[key] = value
             }).catch(err => {
               if (thisPromise !== promiseId) return
+              this[updatingKey] = false
 
               if (pluginOptions.errorHandler === false) return
 

test/index.js

@@ -502,3 +502,42 @@ test("Async computed values can be calculated lazily with a default", t => {
     })
   })
 })
+
+test("Updating flag is set while porperty is re-computed", t => {
+  t.plan(10)
+  const vm = new Vue({
+    asyncComputed: {
+      a () {
+        return new Promise(resolve => {
+          setTimeout(() => resolve('done'), 10)
+        })
+      },
+      b () {
+        const data = this.c
+        return new Promise(resolve => {
+          setTimeout(() => resolve(data), 20)
+        })
+      }
+    },
+    data: () => ({
+      c: false
+    })
+  })
+  t.equal(vm.a$updating, true)
+  t.equal(vm.b$updating, true)
+  vm.$watch('a', function (val) {
+    t.equal(val, 'done')
+    t.equal(vm.a$updating, false)
+    t.equal(vm.b$updating, true)
+  })
+  vm.$watch('b', function (val) {
+    t.equal(val, vm.c)
+    t.equal(vm.b$updating, false)
+    if (!vm.c) {
+      vm.c = true
+      Vue.nextTick(() => {
+        t.equal(vm.b$updating, true)
+      })
+    }
+  })
+})