feat: onComplete config option (#4322)

* feat: `onComplete` config option

* tests(e2e): add case for onComplete option

src/core/plugins/on-complete/index.js

@@ -0,0 +1,28 @@
+let engaged = false
+
+export default function() {
+
+  return {
+    statePlugins: {
+      spec: {
+        wrapActions: {
+          updateSpec: (ori) => (...args) => {
+            engaged = true
+            return ori(...args)
+          },
+          updateJsonSpec: (ori, system) => (...args) => {
+            const cb = system.getConfigs().onComplete
+            if(engaged && typeof cb === "function") {
+              // call `onComplete` on next tick, which allows React to
+              // reconcile the DOM before we notify the user
+              setTimeout(cb, 0)
+              engaged = false
+            }
+
+            return ori(...args)
+          }
+        }
+      }
+    }
+  }
+}