refactor(examples): switch examples from cra to nextjs

examples/nextjs/pages/DragNDrop/index.tsx

@@ -0,0 +1,88 @@
+import React, { useState, DragEvent } from 'react';
+import ReactFlow, {
+  ReactFlowProvider,
+  addEdge,
+  Controls,
+  ReactFlowInstance,
+  Connection,
+  Edge,
+  Node,
+  useNodesState,
+  useEdgesState,
+} from '@react-flow/core';
+
+import Sidebar from './Sidebar';
+
+import styles from './dnd.module.css';
+
+const initialNodes: Node[] = [
+  {
+    id: '1',
+    type: 'input',
+    data: { label: 'input node' },
+    position: { x: 250, y: 5 },
+  },
+];
+
+const onDragOver = (event: DragEvent) => {
+  event.preventDefault();
+  event.dataTransfer.dropEffect = 'move';
+};
+
+let id = 0;
+const getId = () => `dndnode_${id++}`;
+
+const DnDFlow = () => {
+  const [reactFlowInstance, setReactFlowInstance] =
+    useState<ReactFlowInstance>();
+  const [nodes, setNodes, onNodesChange] = useNodesState(initialNodes);
+  const [edges, setEdges, onEdgesChange] = useEdgesState([]);
+
+  const onConnect = (params: Connection | Edge) =>
+    setEdges((eds) => addEdge(params, eds));
+  const onInit = (rfi: ReactFlowInstance) => setReactFlowInstance(rfi);
+
+  const onDrop = (event: DragEvent) => {
+    event.preventDefault();
+
+    if (reactFlowInstance) {
+      const type = event.dataTransfer.getData('application/reactflow');
+      const position = reactFlowInstance.project({
+        x: event.clientX,
+        y: event.clientY - 40,
+      });
+      const newNode: Node = {
+        id: getId(),
+        type,
+        position,
+        data: { label: `${type} node` },
+      };
+
+      setNodes((nds) => nds.concat(newNode));
+    }
+  };
+
+  return (
+    <div className={styles.dndflow}>
+      <ReactFlowProvider>
+        <div className={styles.wrapper}>
+          <ReactFlow
+            nodes={nodes}
+            edges={edges}
+            onEdgesChange={onEdgesChange}
+            onNodesChange={onNodesChange}
+            onConnect={onConnect}
+            onInit={onInit}
+            onDrop={onDrop}
+            onDragOver={onDragOver}
+          >
+            <Controls />
+          </ReactFlow>
+        </div>
+        <Sidebar />
+      </ReactFlowProvider>
+    </div>
+  );
+};
+
+export default DnDFlow;