refactor(examples): switch examples from cra to nextjs

examples/nextjs/pages/DragNDrop/Sidebar.tsx

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+import React, { DragEvent } from 'react';
+import styles from './dnd.module.css';
+
+const onDragStart = (event: DragEvent, nodeType: string) => {
+  event.dataTransfer.setData('application/reactflow', nodeType);
+  event.dataTransfer.effectAllowed = 'move';
+};
+
+const Sidebar = () => {
+  return (
+    <aside className={styles.aside}>
+      <div className={styles.description}>
+        You can drag these nodes to the pane on the left.
+      </div>
+      <div
+        className='react-flow__node-input'
+        onDragStart={(event: DragEvent) => onDragStart(event, 'input')}
+        draggable
+      >
+        Input Node
+      </div>
+      <div
+        className='react-flow__node-default'
+        onDragStart={(event: DragEvent) => onDragStart(event, 'default')}
+        draggable
+      >
+        Default Node
+      </div>
+      <div
+        className='react-flow__node-output'
+        onDragStart={(event: DragEvent) => onDragStart(event, 'output')}
+        draggable
+      >
+        Output Node
+      </div>
+    </aside>
+  );
+};
+
+export default Sidebar;

examples/nextjs/pages/DragNDrop/dnd.module.css

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+.dndflow {
+  flex-direction: column;
+  display: flex;
+  height: 100%;
+}
+
+.aside {
+  border-right: 1px solid #eee;
+  padding: 15px 10px;
+  font-size: 12px;
+  background: #fcfcfc;
+}
+
+.aside > * {
+  margin-bottom: 10px;
+  cursor: grab;
+}
+
+.description {
+  margin-bottom: 10px;
+}
+
+.wrapper {
+  flex-grow: 1;
+  height: 100%;
+}
+
+@media screen and (min-width: 768px) {
+  .aside {
+    width: 20%;
+    max-width: 180px;
+  }
+
+  .dndflow {
+    flex-direction: row;
+  }
+}

examples/nextjs/pages/DragNDrop/index.tsx

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+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;