feat(hooks): add useMountedState hook

src/__tests__/useMountedState.test.ts

@@ -0,0 +1,22 @@
+import Vue from 'vue';
+import { onMounted } from 'vue-function-api';
+import useMountedState from '../useMountedState';
+import renderHook from '../util/renderHook';
+
+describe('useMountedState', () => {
+  it('should be defined', () => {
+    expect(useMountedState).toBeDefined();
+  });
+
+  it('should return true on mounted', () => {
+    renderHook(() => {
+      const isMounted = useMountedState();
+      expect(isMounted.value).toBe(false);
+
+      onMounted(async () => {
+        await Vue.nextTick();
+        expect(isMounted.value).toBe(true);
+      });
+    });
+  });
+});

src/useMountedState.ts

@@ -0,0 +1,13 @@
+import Vue from 'vue';
+import { value, onMounted } from 'vue-function-api';
+
+export default function useMountedState() {
+  const isMounted = value(false);
+
+  onMounted(async () => {
+    await Vue.nextTick();
+    isMounted.value = true;
+  });
+
+  return isMounted;
+}