廢話開篇：iOS與android在實現列表介面的時候是有重用機制的，目的就是減少記憶體開銷，用時間換空間。個人感覺flutter並沒有特別強調複用，關於listView.builder 的“複用”個人感覺應該是銷燬跟重建的過程，所以這裡用flutter實現了簡單的複用機制。程式碼拙劣，大神勿噴，共同進步
先看複用效果
複用狀態列印
右側是簡單實現瀑布流介面，裡面顯示的是一共有39個Widget。左側是控制檯列印一共建立的12個Widget，所以這裡就簡單的實現了Widget複用。
問題一、實現思路是什麼？
這裡先簡單的說一下實現思路。
- 在渲染介面前，通過計算得出全部的Widget的位置座標。
- 首次渲染建立一屏可視瀑布流Widget.
- 監聽滑動，判斷當前頁面滾動方向展示的瀑布流Widget,先去快取池裡拿，如果沒有就直接建立，新增到元件中進行渲染。如果快取池裡有，修改Widget的相對佈局位置。
問題二、UI佈局程式碼分析。
tip: WaterfallFlow.dart 瀑布流主頁面；WaterfallFlowItem.dart 瀑布流單元item
效果展示：
WaterfallFlowItem.dart 瀑布流item檔案
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<StatefulWidget> createState() {
_waterfallFlowItemState = new WaterfallFlowItemState(frame: _frame!);
return _waterfallFlowItemState!;
}
}
class WaterfallFlowItemState extends State<WaterfallFlowItem> 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;
}
複製程式碼WaterfallFlow.dart 主介面檔案
builder 實現
@override
Widget build(BuildContext context) {
return new Container(
//去掉scrollView頂部空白間隙
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(
//幀佈局下的瀑布流單元格item集合
children: _listWidget,
),
),
),
)
),
);
}
複製程式碼宣告的屬性
//瀑布流間隔
double sep = 5;
//瀑布流寬度
double? _width;
//最大高度
double _maxHeight = 0;
//左側最大高度
double leftHeight = 0;
//右側最大高度
double rightHeight = 0;
//主介面高度
double _mineContentHeight = 0;
//瀑布流item快取池
List<WaterfallFlowItem> _bufferPoolWidget = [];
//當前顯示的瀑布流item
List<WaterfallFlowItem> _listWidget = [];
//當前組渲染frame物件儲存
List<Frame> _fList = [];
//總frame集合
List<Frame> _frameList = [];
//資料來源這裡只儲存高度
List<double> _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];
//滑動監聽
ScrollController _scrollController = new ScrollController();
//滑動偏移量
double _scrollOff = 0;
複製程式碼計算主視窗scrollView 高度
//獲取最大高度，並計算出全部的瀑布流位置
void getMaxHeight(){
List<Frame> 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);
//重新整理
setState(() {});
}
複製程式碼Frame 位置資訊類
class Frame{
double left = 0;//左
double top = 0;//右
double width = 0;//寬度
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;
}
}
複製程式碼生成瀑布流Widget單元item
//重用池裡生成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，從快取池裡取(快取中的已在結構樹裡，可以修改幀佈局位置)
if(_bufferPoolWidget.length > 0){
waterfallFlowItem = _bufferPoolWidget.last;
waterfallFlowItem.setFrame(frame: f);
_bufferPoolWidget.removeLast();
isReUse = true;
}
//沒有，直接建立(不快取中的，需要呼叫setState方法渲染)
if(waterfallFlowItem == null) {
waterfallFlowItem = new WaterfallFlowItem(frame: f,);
isReUse = false;
}
block(waterfallFlowItem,isReUse);
}
複製程式碼建立首屏全部可視瀑布流Widget單元元件
//渲染瀑布流item
createWaterfallFlow(int index){
getMaxHeight();
//這裡加點延遲，保證獲取最大高度完成（不太嚴謹，大神有好方法請賜教[抱拳]）
Future.delayed(Duration(milliseconds: 100),(){
_mineContentHeight = context.size!.height;
for(var i = 0;i < _frameList.length;i++){
Frame f = _frameList[i];
//判斷視覺化邏輯
if(f.top <= _mineContentHeight + _scrollOff) {
_takeReuseFlowItem(f,(WaterfallFlowItem waterfallFlowItem,bool isReuse){
_listWidget.add(waterfallFlowItem);
});
}
}
setState(() {
});
});
}
複製程式碼滑動過程中進行重用渲染
//獲取上滑狀態當前顯示的下一個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;
}
//獲取下滑狀態當前顯示的上一個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;
}
//超出介面可視範圍的瀑布流加入快取池
void addFlowItemAddToBufferPool(){
List<WaterfallFlowItem> 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 < list.length;i++){
WaterfallFlowItem? waterfallFlowItem = list[i];
if(_listWidget.contains(waterfallFlowItem)){
_listWidget.remove(waterfallFlowItem);
}
}
}
//從快取池裡獲取item
//上滑狀態
Frame? upNextFrame = _getUpNeedShowFrame();
if(upNextFrame != null) {
//debugPrint('我是在複用 ${upNextFrame.index} ,${upNextFrame.top},${_mineContentHeight + _scrollOff}');
if(upNextFrame.top <= _mineContentHeight + _scrollOff) {
debugPrint('我在上滑重置第${upNextFrame.index}個frame');
_takeReuseFlowItem(upNextFrame,(WaterfallFlowItem waterfallFlowItem,bool isReuse){
_listWidget.add(waterfallFlowItem);
if(!isReuse){
debugPrint('我不是複用');
setState(() {});
} else {
debugPrint('我是複用');
waterfallFlowItem.setFrame(frame: upNextFrame);
}
});
}
}
//下滑狀態
Frame? downNextFrame = _getDownNeedShowFrame();
if(downNextFrame != null) {
//debugPrint('我是在複用 ${downNextFrame.index} ,${downNextFrame.top},${_mineContentHeight + _scrollOff}');
if(downNextFrame.top + downNextFrame.heigth > _scrollOff && downNextFrame.top + downNextFrame.heigth < _mineContentHeight + _scrollOff) {
debugPrint('我在下滑重置第${downNextFrame.index}個frame');
_takeReuseFlowItem(downNextFrame,(WaterfallFlowItem waterfallFlowItem,bool isReuse){
_listWidget.insert(0, waterfallFlowItem);
if(!isReuse){
debugPrint('我不是複用');
setState(() {});
} else {
debugPrint('我是複用');
waterfallFlowItem.setFrame(frame: downNextFrame);
}
});
}
}
}
複製程式碼滾動監聽
_scrollController.addListener(() {
_scrollOff = _scrollController.offset;
//加入快取池,並進行復用
addFlowItemAddToBufferPool();
debugPrint('總共：${_listWidget.length + _bufferPoolWidget.length} 個');
});
複製程式碼基本上flutter的瀑布流複用邏輯就完成了，程式碼拙劣，裡面有些地方需要優化，比如：快速滑動防護，item的內容渲染。flutter對於介面渲染已經很極致了，重寫複用有點倒退的趕腳。大神勿噴，互相學習。[抱拳][抱拳][抱拳]