class CircleProgress extends StatelessWidget {
 final Size size;
 final double progress;

 CircleProgress({@required this.size, @required this.progress});

 @override
 Widget build(BuildContext context) {
 return CustomPaint(
 size: size,
 painter: CircleProgressPainter(endDegree: progress * 360),//在Painter中寫真正的繪畫邏輯
 );
 }
}

class CircleProgressPainter extends CustomPainter {
 ...省略

 @override
 void paint(Canvas canvas, Size size) {
 ...繪制的具體邏輯，size是畫布的大小
 }
}

class RectLayout extends StatelessWidget {
 final Widget child;

 RectLayout({@required this.child});

 @override
 Widget build(BuildContext context) {
 return CustomSingleChildLayout(
 delegate: RectLayoutDelegate(),//進(jìn)行布局的代理
 child: child,
 );
 }
}

class RectLayoutDelegate extends SingleChildLayoutDelegate {
 //確定layout的size，constraints是parent傳過來的約束
 @override
 Size getSize(BoxConstraints constraints) => super.getSize(constraints);

 ///是否需要relayout
 @override
 bool shouldRelayout(SingleChildLayoutDelegate oldDelegate) => false;

 ///確定child的位置，返回一個(gè)相對于parent的偏移值，size是layout的大小，由getsize確定，childSize由getConstraintsForChild得出的Constraints對child進(jìn)行約束，得到child自身的size
 @override
 Offset getPositionForChild(Size size, Size childSize) {
 double dx = (size.width - childSize.width) / 2;
 double dy = (size.height - childSize.height) / 2;
 return Offset(dx, dy);
 }

 ///確定child的約束，用于確定child的大小
 @override
 BoxConstraints getConstraintsForChild(BoxConstraints constraints) {//
 double maxEdge = min(constraints.maxWidth, constraints.maxHeight);
 return BoxConstraints(maxWidth: maxEdge, maxHeight: maxEdge);
 }
}

class GridLayout extends StatelessWidget {
 final List<Widget> children;
 final double horizontalSpace;
 final double verticalSpace;

 GridLayout(
 {@required this.children,
 @required this.horizontalSpace,
 @required this.verticalSpace});

 @override
 Widget build(BuildContext context) {
 List<Widget> layoutChildren = new List();
 for (int index = 0; index < children.length; index++) {
 layoutChildren.add(LayoutId(id: index, child: children[index]));
 }
 return CustomMultiChildLayout(
 delegate: GridLayoutDelegate(//真正的布局實(shí)現(xiàn)
 horizontalSpace: horizontalSpace,
 verticalSpace: verticalSpace,
 ),
 children: layoutChildren,
 );
 }
}

class GridLayoutDelegate extends MultiChildLayoutDelegate {
 final double horizontalSpace;
 final double verticalSpace;
 List<Size> _itemSizes = List();

 GridLayoutDelegate(
 {@required this.horizontalSpace, @required this.verticalSpace});

 @override
 void performLayout(Size size) {
	//對每個(gè)child進(jìn)行逐一布局
 int index = 0;
 double width = (size.width - horizontalSpace) / 2;
 var itemConstraints = BoxConstraints(
 minWidth: width, maxWidth: width, maxHeight: size.height);
 while (hasChild(index)) {
 _itemSizes.add(layoutChild(index, itemConstraints));
 index++;
 }
	//對每一個(gè)child逐一進(jìn)行定位
 index = 0;
 double dx = 0;
 double dy = 0;
 while (hasChild(index)) {
 positionChild(index, Offset(dx, dy));
 dx = index % 2 == 0 ? width + horizontalSpace : 0;
 if (index % 2 == 1) {
 double maxHeight =
  max(_itemSizes[index].height, _itemSizes[index - 1].height);
 dy += maxHeight + verticalSpace;
 }
 index++;
 }
 }

 @override
 bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) {
 return oldDelegate != this;
 }

 //確定layout的size，constraints是parent傳過來的約束
 @override
 Size getSize(BoxConstraints constraints) => super.getSize(constraints);
}

_scrollController.addListener(() {
 var maxScroll = _scrollController.position.maxScrollExtent;
 if (_scrollController.offset >= maxScroll) {
 if (widget.loadMoreStatus != LoadMoreStatus.noData) {
 widget.onLoadMore();
 }
 }
});

@override
Widget build(BuildContext context) {
 double topHeight =
 _pullDirection == PullDirection.DOWN ? _overScrollOffset.dy.abs() : 0;
 double bottomHeight =
 _pullDirection == PullDirection.UP ? _overScrollOffset.dy.abs() : 0;
 return Stack(
 children: <Widget>[
 widget.headerBuilder.buildTip(_state, topHeight),
 Align(
 alignment: Alignment.bottomCenter,
 child: widget.footerBuilder.buildTip(_state, bottomHeight),
 ),
 Transform.translate(
 offset: _overScrollOffset,
 child: NotificationListener<ScrollNotification>(
  onNotification: handleScrollNotification,
  child: DecoratedBox(
  decoration: BoxDecoration(color: Colors.grey[100]),
  child: ListView.builder(
  itemBuilder: buildItem,
  itemCount: 30,
  ),
  ),
 ),
 )
 ],
 );
}

@override
Widget build(BuildContext context) {
 //debugPrint('_slideOffset:${_slideOffset.toString()}');
 return GestureDetector(
 onPanUpdate: handlePanUpdate,
 onPanEnd: handlePanEnd,
 child: Stack(
 children: <Widget>[
 widget.background,
 Transform.translate(
  child: widget.foreground,
  offset: _slideOffset,
 ),
 ],
 ),
 );
}