【flutter】ListView.builder封装各个页面通

flutter当中提供了无限加载的组件，再此基础上又做了一层封装。
这个可复用的无限加载列表包括以下特点：

• 上滑加载
• 数据不满足加载条件，提示到达底部
• 根据具体参数的变化，搜索并重新加载数据（可以轻松跟filter和search结合）
• 不同的列表只需要传入任意定义的RowItem即可
• 根据RESTFUL当中limit,offset分页机制加载

组件封装当中比较复杂一点的就是dart如何进行泛型实例化。

• RowItem父类

import 'package:flutter/widgets.dart';

abstract  class ListItemWidget extends StatelessWidget {
    const ListItemWidget(
      {Key key, this.item}
    ):super(key:key);

  final Map item;
  call(Map item)=>this;
}

• 无线加载列表组件

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:flutter_easyrefresh/easy_refresh.dart';
import 'package:flutter_easyrefresh/material_header.dart';
import 'package:flutter_lim/common/http.dart';
import 'package:flutter_lim/widgets/listItem.dart';

typedef RequestCallback = Stream<dynamic> Function({Map paramObj});
typedef S ItemCreator<S extends ListItemWidget>(dynamic item); // new T()泛型实例化实现

// 无限加载列表
//使用InheritedWidget，每个列表管理自己的搜索参数
class ShareParams extends InheritedWidget {
  ShareParams(
      {@required this.specialParams, this.data, this.count, Widget child})
      : super(child: child);

  final Map<String, dynamic> specialParams;
  final List<dynamic> data;
  final int count;

  static ShareParams of(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType<ShareParams>();
  }

  @override
  bool updateShouldNotify(ShareParams oldWidget) {
    Map<String, dynamic> oldParams = oldWidget.specialParams;
    bool isUpdate = false;
    if (this.specialParams.keys.length != oldParams.length) {
      isUpdate = true;
    } else {
      for (String key in this.specialParams.keys) {
        if (oldParams[key] == null ||
            oldParams[key] != this.specialParams[key]) {
          isUpdate = true;
          break;
        }
      }
    }

    return isUpdate;
  }
}

@immutable
class InfiniteListViewWidget<T extends ListItemWidget> extends StatefulWidget {
  final RequestCallback request; //数据请求路径
  final List<dynamic> data; //初始化数据
  final int count; //列表总个数
  final ItemCreator<T> creator; //列表Item泛型实例化
  final Map<String, dynamic> specialParam; //特殊请求参数,若无须共享参数,传值
  final Function refresh;
  InfiniteListViewWidget(this.request, this.data, this.count, this.creator,
      {this.specialParam, this.refresh});

  @override
  _InfiniteListViewWidgetState<T> createState() =>
      new _InfiniteListViewWidgetState<T>(
          this.request, this.data, this.count, this.creator,
          specialParam: this.specialParam, refresh: this.refresh);
}

@override
class _InfiniteListViewWidgetState<T extends ListItemWidget>
    extends State<InfiniteListViewWidget> {
  static const loadingTag = {"position": "bottom"}; //表尾标记
  Http http = new Http();
  InfiniteListParamObj infiParams;
  RequestCallback request; //数据请求
  int count; //列表数据总数量
  List<dynamic> data;
  ItemCreator<T> creator;
  @optionalTypeArgs
  Map<String, dynamic> specialParam; //特殊请求参数
  Function refresh;

  EasyRefreshController _refreshController = new EasyRefreshController();

  _InfiniteListViewWidgetState(
      this.request, this.data, this.count, this.creator,
      {this.specialParam, this.refresh});

  @override
  void initState() {
    super.initState();
  }

  void didChangeDependencies() {
    super.didChangeDependencies();
    if (ShareParams.of(context) != null) {
      print(
          'LISTEN PARAMS didChange=>${ShareParams.of(context).specialParams}');
      print('LISTEN PARAMS didChange=>${ShareParams.of(context).data}');
      this.specialParam = ShareParams.of(context).specialParams;
      this.data = ShareParams.of(context).data;
      this.count = ShareParams.of(context).count;
    }
    if (this.specialParam != null) {
      this._setInfiniteParams(this.specialParam);
    }
  }

  @override
  Widget build(BuildContext context) {
//EasyRefresh是下拉刷新组件，不需要的话可以直接去掉
    return EasyRefresh(
      header: MaterialHeader(),
      child: ListView.builder(
        itemCount: this.data.length,
        itemBuilder: (BuildContext context, int index) {
          var word = this.data[index];
          var nextWord;
          if (index == this.data.length - 1) {
            nextWord = null;
          } else {
            nextWord = this.data[index + 1];
          }
          if (nextWord == null && this.data.length == index + 1) {
            // 根据返回的count
            final curCount = this.data.length;
            if (curCount < this.count) {
              //offset + limit  >= count 的时候不再下拉
              this.infiParams.offset = this.data.length;
              //获取数据
              _retrieveData(this.resolveListData);
              //加载时显示loading
              return Column(
                children: <Widget>[
                  creator(word),
                  Container(
                    padding: const EdgeInsets.all(16.0),
                    alignment: Alignment.center,
                    child: SizedBox(
                        width: 24.0,
                        height: 24.0,
                        child: CircularProgressIndicator(strokeWidth: 2.0)),
                  )
                ],
              );
            } else {
              //加载数据总数已经超过count了不再加载
              return Column(
                children: <Widget>[
                  creator(word),
                  Container(
                      alignment: Alignment.center,
                      padding: EdgeInsets.all(16.0),
                      child: Text(
                        "没有更多了",
                        style: TextStyle(color: Colors.grey),
                      ))
                ],
              );
            }
          }
//由于build方法中返回的Widget必须实例化，所以要实现泛型实例化
          T item = creator(word);
          return item;
        },
      ),
      controller: _refreshController,
      onRefresh: () async {
        this.refresh == null
            ? _refreshController.finishRefresh(success: true)
            : this.refresh();
      },
    );
  }

  void _setInfiniteParams(Map<String, dynamic> specialParam,
      {int limit, int offset, String ordering}) {
    if (this.infiParams == null) {
      limit = limit != null ? limit : 10;
      offset = offset != null ? offset : 0;
      ordering = ordering != null ? ordering : '-create_time';
      this.infiParams =
          new InfiniteListParamObj(limit, offset, ordering, specialParam);
    } else {
      this.infiParams.paramsObj = specialParam;
    }
  }

  void _retrieveData(Function resovle) {
    Map<String, dynamic> paramObj = this.infiParams.getParams();
    request(paramObj: paramObj).listen(resovle);
  }

  void resolveListData(dynamic data) {
    this.count = data['count'];
    setListData(data['results']);
  }

  void setListData(dynamic list) {
    this.data.addAll(list);
    setState(() {});
  }
}

• 列表默认参数，可以根据具体的设计修改

@override
class InfiniteListParamObj {
  int limit = 10;
  int offset = 0;
  String ordering = '';
  Map<String, dynamic> paramsObj = {};
  InfiniteListParamObj(this.limit, this.offset, this.ordering, this.paramsObj);
  Map<String, dynamic> getParams() {
    Map<String, dynamic> basic = {
      "limit": this.limit,
      "offset": this.offset,
      "ordering": this.ordering
    };
    basic.addAll(this.paramsObj);
    return basic;
  }

  void resetBasicParams() {
    this.limit = 10;
    this.offset = 0;
  }
}

• 如何使用

 Expanded(
          child: ShareParams(
        specialParams: this.paramsObj,
        data: this.dataList,//数据列表
        count: this.count,//数据总count
        child: new InfiniteListViewWidget<ProductItem>(//ProductItem具体集成自ListItemWidget的列表项目
          getUniqueProdList,//数据加载url
          this.dataList,
          this.count,
          (item) => new ProductItem(item),
          specialParam: this.paramsObj,//列表搜索参数
          refresh: this.onRefresh,//自定义列表刷新的方法
        ),
      ));