add loadmore widget

lib/widgets/LoadMore.dart

@@ -0,0 +1,81 @@
+
+import 'package:flutter/widgets.dart';
+
+import 'package:farmer_client/models/entities/PagedResult.dart';
+
+class LoadMore extends StatefulWidget {
+  LoadMore({
+    Key? key,
+    required this.builder,
+    required this.fetch,
+    this.onError,
+  }): super(key: key);
+
+  Future Function(Map param) fetch;
+  Function? onError;
+
+  Widget Function({
+    BuildContext context,
+    ScrollController scrollController,
+    PagedResult data,
+    bool haveMore,
+  }) builder;
+
+  @override
+  State<StatefulWidget> createState () => _LoadMoreState();
+}
+
+class _LoadMoreState extends State<LoadMore> {
+  final _controller = ScrollController();
+  // 是否还有更多
+  bool haveMore = true;
+
+  PagedResult result = PagedResult(total: 0, size: 0, pages: 0, current: 0);
+  List<dynamic> listData = [];
+
+  @override
+  initState() {
+    super.initState();
+
+    // 添加滚动监听
+    _controller.addListener(() {
+      //当滚动到最底部的时候，加载新的数据
+      if (_controller.position.pixels == _controller.position.maxScrollExtent) {
+        //当还有更多数据的时候才会进行加载新数据
+        if (haveMore) {
+          fetchData();
+        }
+      }
+    });
+
+    fetchData();
+  }
+
+  Future fetchData() {
+    final pageNum = result.current + 1;
+    final pageSize = result.size == 0 ? 10 : result.size;
+    Map params = { "pageNum": pageNum, "pageSize": pageSize };
+    return Future.value(widget.fetch(params).then((res) {
+      setState(() {
+        result = PagedResult.fromJson(res);
+        // 如果当前页小于总页数
+        haveMore = result.current < result.pages;
+      });
+    }).catchError((err) {
+      print(err);
+      if (null != widget.onError) {
+        widget.onError!(err);
+      }
+    }));
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return widget.builder(
+      context: context,
+      scrollController: _controller,
+      data: result,
+      haveMore: haveMore,
+    );
+  }
+}