listView新增下拉刷新，上拉加载，bug修复

netrain_flutter_app/lib/xuehao/list_page.dart

@@ -40,11 +40,11 @@ class _ListPageState extends State<List_Page> {
               itemBuilder: _getDatas,
             ),
             onRefresh: () async {
-              setState(() {
-                refreshData();
-              });
+              refreshData();
+            },
+            onLoad: () async {
+              loadData();
             },
-            onLoad: () async {},
           ),
         ));
   }
@@ -76,15 +76,32 @@ class _ListPageState extends State<List_Page> {
       },
     ));
   }
-}
 
-Future<void> refreshData() async {
-  await Future.delayed(Duration(seconds: 3), () {
-    for (int i = 0; i < datas.length; i++) {
-      datas[i].name = "刷新后的数据$i";
-    }
-    _controller.finishRefresh(success: true);
-  });
+  Future<void> refreshData() async {
+    await Future.delayed(Duration(seconds: 3), () {
+      setState(() {
+        for (int i = 0; i < datas.length; i++) {
+          datas[i].name = "刷新后的数据$i";
+        }
+        _controller.finishRefresh(success: true);
+      });
+    });
+  }
+
+  ///加载数据
+  Future<void> loadData() async {
+    await Future.delayed(Duration(seconds: 3), () {
+      setState(() {
+        for (int i = 0; i < 10; i++) {
+          datas.add(Car(
+              name: "新增数据$i",
+              imageUrl:
+                  "https://img2.baidu.com/it/u=2010980676,2375641500&fm=11&fmt=auto&gp=0.jpg"));
+        }
+        _controller.finishLoad(success: true);
+      });
+    });
+  }
 }
 
 //定义一个全局的变量：模型数组