refactor(fitView): use same fitView function for viewport helper and initial prop

src/hooks/useOnInitHandler.ts

@@ -0,0 +1,18 @@
+import { useEffect, useRef } from 'react';
+
+import useReactFlow from './useReactFlow';
+import { OnInit } from '../types';
+
+function useOnInitHandler(onInit: OnInit<any> | undefined) {
+  const ReactFlowInstance = useReactFlow();
+  const isInitialized = useRef<boolean>(false);
+
+  useEffect(() => {
+    if (!isInitialized.current && ReactFlowInstance.viewportInitialized && onInit) {
+      setTimeout(() => onInit(ReactFlowInstance), 1);
+      isInitialized.current = true;
+    }
+  }, [onInit, ReactFlowInstance.viewportInitialized]);
+}
+
+export default useOnInitHandler;