Flutter开发之Widget自定义总结

Z时代
2024-01-10
分类：IT

前言

在Flutter实际开发中，大家可能会遇到flutter框架中提供的widget达不到我们想要的效果，这时就需要我们去自定义widget，从Flutter构建、布局、绘制三部曲中我们了解到，实际的测量、布局、绘制操作都在RenderObject中，我们是可以进行继承相关的RenderObject来实现自定义的。但是其实flutter框架在设计之初就给我们预留出了自定义的入口，方便我们进行自定义。

CustomPaint自定义绘制

例：圆形进度条

思路：使用CustomPaint绘制需要的效果


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中写真正的绘画逻辑
 );
 }
}
class CircleProgressPainter extends CustomPainter {
 ...省略
 @override
 void paint(Canvas canvas, Size size) {
 ...绘制的具体逻辑，size是画布的大小
 }
}

CustomSingleChildLayout对单一child进行布局

例：实现对child约束成正方形

思路：使用CustomSingleChildLayout对child进行布局，并约束为正方形


class RectLayout extends StatelessWidget {
 final Widget child;
 RectLayout({@required this.child});
 @override
 Widget build(BuildContext context) {
 return CustomSingleChildLayout(
 delegate: RectLayoutDelegate(),//进行布局的代理
 child: child,
 );
 }
}
class RectLayoutDelegate extends SingleChildLayoutDelegate {
 //确定layout的size，constraints是parent传过来的约束
 @override
 Size getSize(BoxConstraints constraints) => super.getSize(constraints);
 ///是否需要relayout
 @override
 bool shouldRelayout(SingleChildLayoutDelegate oldDelegate) => false;
 ///确定child的位置，返回一个相对于parent的偏移值，size是layout的大小，由getsize确定，childSize由getConstraintsForChild得出的Constraints对child进行约束，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);
 }
 ///确定child的约束，用于确定child的大小
 @override
 BoxConstraints getConstraintsForChild(BoxConstraints constraints) {//
 double maxEdge = min(constraints.maxWidth, constraints.maxHeight);
 return BoxConstraints(maxWidth: maxEdge, maxHeight: maxEdge);
 }
}

CustomSingleChildLayout对多个child进行布局

例：实现网格布局

思路：使用CustomSingleChildLayout对child进行布局、定位，使其成为网格的布局


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(//真正的布局实现
 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) {
	//对每个child进行逐一布局
 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++;
 }
	//对每一个child逐一进行定位
 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;
 }
 //确定layout的size，constraints是parent传过来的约束
 @override
 Size getSize(BoxConstraints constraints) => super.getSize(constraints);
}

组合自定义

一般情况，组合自定义应该是我们最经常用的方式，通过继承自StatelessWidget或StatefulWidget,把多个Widget组合起来，从而达到我们需要的效果。

例：下拉刷新，上拉加载

实现一：通过自带的RefreshIndictor和ScrollController组合实现

思路：通过对滚动进行监听来触发加载更多


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

实现二：通过NotificationListener监听scroll的整体状态，让后结合平移、动画来实现

思路：通过监听用户overscroll的距离来平移内容区域，从而达到下拉刷新，上拉加载的效果


@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,
  ),
  ),
 ),
 )
 ],
 );
}

例：上下左右滑动的layout

实现：通过GestureDetector监听手势滑动，然后通过平移来达到效果

思路：主要处理滑动边界，以及开关的零界点


@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,
 ),
 ],
 ),
 );
}