Flutter瀑布流仿写原生的复用机制详解_java

class WaterfallFlowItem extends StatefulWidget{
  Frame? _frame;
  WaterfallFlowItemState? _waterfallFlowItemState;

  WaterfallFlowItem({required Frame frame}){
    _frame = frame;
  }

  Frame getFrame(){
    return _frame!;
  }

  void setFrame({required Frame frame}) {
    _frame = frame;
    _waterfallFlowItemState!.setFrame(frame: frame);
  }

  @override
  State createState() {
    _waterfallFlowItemState = new WaterfallFlowItemState(frame: _frame!);
    return _waterfallFlowItemState!;
  }
}

class WaterfallFlowItemState extends State with AutomaticKeepAliveClientMixin {
  Frame? _frame;

  WaterfallFlowItemState({required Frame frame}){
    _frame = frame;
  }

  void setFrame({required Frame frame}) {
    setState(() {
      _frame = frame;
    });
  }
  @override
  Widget build(BuildContext context) {
    return new Positioned(
        top: _frame!.top,
        left: _frame!.left,
        child: GestureDetector(
          child: new Container(
            color: _frame!.index == 12 ? Colors.red : Color.fromARGB(255, 220, 220, 220),
            width: _frame!.width,
            height: _frame!.heigth,
            child: new Text(_frame!.index.toString()),
          ),
          onTap: (){

          },
        )
    );
  }

  @override
  // TODO: implement wantKeepAlive
  bool get wantKeepAlive => true;
}

@override
Widget build(BuildContext context) {
  return new Container(
    //去掉scrollView顶部空白间隙
    child: MediaQuery.removePadding(
        context: context,
        removeTop: true,
        child: Scrollbar(
          //isAlwaysShown: true,
          //showTrackOnHover: true,
          //scrollView
          child: new SingleChildScrollView(
            controller: _scrollController,
            child: new Container(
              width: MediaQuery.of(context).size.width,
              //最大高度
              height: _maxHeight,
              color: Colors.white,
              child: new Stack(
                //帧布局下的瀑布流单元格item集合
                children: _listWidget,
              ),
            ),
          ),
        )
  ),
  );
}

//瀑布流间隔
double sep = 5;
//瀑布流宽度
double? _width;
//最大高度
double _maxHeight = 0;
//左侧最大高度
double leftHeight = 0;
//右侧最大高度
double rightHeight = 0;
//主界面高度
double _mineCOntentHeight= 0;
//瀑布流item缓存池
List _bufferPoolWidget = [];
//当前显示的瀑布流item
List _listWidget = [];
//当前组渲染frame对象保存
List _fList = [];
//总frame集合
List _frameList = [];
//数据源这里只保存高度
List _list = [
  100,150,45,11,140,89,212,21,434,545,100,150,45,11,140,89,212,21,434,545,
  100,150,45,11,140,89,212,21,434,545,100,150,45,11,140,89,212,21,434,545];
//滑动监听
ScrollController _scrollCOntroller= new ScrollController();
//滑动偏移量
double _scrollOff = 0;

//获取最大高度，并计算出全部的瀑布流位置
void getMaxHeight(){
  List fList = [];
  double width = (_width! - sep * 3) / 2.0;
  double maxHeight = _maxHeight;
  for(int i = _frameList.length;i <_list.length;i++){
    double height = _list[i];
    bool isLeft = (leftHeight <= rightHeight);
    double left = isLeft ? sep : (width + sep * 2);
    maxHeight = isLeft ? leftHeight : rightHeight;
    Frame frame = Frame(leftP: left, topP: maxHeight, widthP: width, heigthP: height,indexP: i);
    if(isLeft == true) {
      leftHeight += (height + sep);
    } else {
      rightHeight += (height + sep);
    }
    fList.add(frame);
  }
  _maxHeight = max(leftHeight, rightHeight);
  _frameList.addAll(fList);
  //刷新
  setState(() {});
}

class Frame{
  double left = 0;//左
  double top = 0;//右
  double width = 0;//宽度
  double heigth = 0;//高度
  int index = 0;//索引
  Frame({required leftP
  ,required topP,
    required widthP,
    required heigthP,
    required indexP}){
    left = leftP * 1.0;
    top = topP * 1.0;
    width = widthP * 1.0;
    heigth = heigthP * 1.0;
    index = indexP;
  }
}

//重用池里生成item
_takeReuseFlowItem(Frame f,dynamic block){
  WaterfallFlowItem? waterfallFlowItem;
  //是否重用，是，直接修改frame；否，重新渲染。
  bool isReUse = false;
  //有，从缓存池里取(缓存中的已在结构树里，可以修改帧布局位置)
  if(_bufferPoolWidget.length > 0){
    waterfallFlowItem = _bufferPoolWidget.last;
    waterfallFlowItem.setFrame(frame: f);
    _bufferPoolWidget.removeLast();
    isReUse = true;
  }
  
  //没有，直接创建(不缓存中的，需要调用setState方法渲染)
  if(waterfallFlowItem == null) {
    waterfallFlowItem = new WaterfallFlowItem(frame: f,);
    isReUse = false;
  }
  block(waterfallFlowItem,isReUse);
}

//渲染瀑布流item
createWaterfallFlow(int index){
  getMaxHeight();
  //这里加点延迟，保证获取最大高度完成（不太严谨，大神有好方法请赐教[抱拳]）
  Future.delayed(Duration(milliseconds: 100),(){
    _mineCOntentHeight= context.size!.height;
    for(var i = 0;i <_frameList.length;i++){
      Frame f = _frameList[i];
      //判断可视化逻辑
      if(f.top <= _mineContentHeight + _scrollOff) {
        _takeReuseFlowItem(f,(WaterfallFlowItem waterfallFlowItem,bool isReuse){
          _listWidget.add(waterfallFlowItem);
        });
      }
    }
    setState(() {
    });
  });
}

//获取上滑状态当前显示的下一个item位置
Frame? _getUpNeedShowFrame(){
  Frame? f;
  WaterfallFlowItem? lastWaterfallFlowItem = _listWidget.last;
  if(lastWaterfallFlowItem.getFrame().index + 1 <_frameList.length) {
    f = _frameList[lastWaterfallFlowItem.getFrame().index + 1];
  }
  return f;
}

//获取下滑状态当前显示的上一个item位置
Frame? _getDownNeedShowFrame(){
  Frame? f;
  WaterfallFlowItem? lastWaterfallFlowItem = _listWidget[0];
  if(lastWaterfallFlowItem.getFrame().index - 1 >= 0) {
    f = _frameList[lastWaterfallFlowItem.getFrame().index - 1];
  }
  return f;
}

//超出界面可视范围的瀑布流加入缓存池
void addFlowItemAddToBufferPool(){

  List list = [];
  for(int i = 0; i <_listWidget.length;i++){
    WaterfallFlowItem? waterfallFlowItem = _listWidget[i];
    Frame? frame = waterfallFlowItem.getFrame();
    if((frame.top + frame.heigth) <_scrollOff || frame.top > _mineContentHeight + _scrollOff) {
      _bufferPoolWidget.add(waterfallFlowItem);
      list.add(waterfallFlowItem);
    }
  }
  if(list.length != 0) {
    for(int i= 0;i  _scrollOff && downNextFrame.top + downNextFrame.heigth <_mineContentHeight + _scrollOff) {
      debugPrint('我在下滑重置第${downNextFrame.index}个frame');
      _takeReuseFlowItem(downNextFrame,(WaterfallFlowItem waterfallFlowItem,bool isReuse){
        _listWidget.insert(0, waterfallFlowItem);
        if(!isReuse){
          debugPrint('我不是复用');
          setState(() {});
        } else {
          debugPrint('我是复用');
          waterfallFlowItem.setFrame(frame: downNextFrame);
        }
      });
    }
  }
}

_scrollController.addListener(() {
  _scrollOff = _scrollController.offset;
  //加入缓存池,并进行复用
  addFlowItemAddToBufferPool();
  debugPrint('总共：${_listWidget.length + _bufferPoolWidget.length} 个');
});