02_Flutter自定义Sliver组件实现分组列表吸顶效果

一.先上效果图

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二.列表布局实现

比较简单，直接上代码，主要使用CustomScrollView和SliverToBoxAdapter实现

_buildSection(String title) {return SliverToBoxAdapter(child: RepaintBoundary(child: Container(height: 50,color: Colors.brown,alignment: Alignment.center,child: Text(title),),));
}_buildItem(String title) {return SliverToBoxAdapter(child: RepaintBoundary(child: Container(padding: const EdgeInsets.symmetric(horizontal: 15),height: 70,color: Colors.cyanAccent,alignment: Alignment.centerLeft,child: Text(title),),));
}CustomScrollView(slivers: [_buildSection("蜀汉五虎将"),_buildItem("关羽"),_buildItem("张飞"),_buildItem("赵云"),_buildItem("马超"),_buildItem("黄忠"),_buildSection("虎贲双雄"),_buildItem("许褚"),_buildItem("典韦"),_buildSection("五子良将"),_buildItem("张辽"),_buildItem("乐进"),_buildItem("于禁"),_buildItem("张郃"),_buildItem("徐晃"),_buildSection("八虎骑"),_buildItem("夏侯惇"),_buildItem("夏侯渊"),_buildItem("曹仁"),_buildItem("曹纯"),_buildItem("曹洪"),_buildItem("曹休"),_buildItem("夏侯尚"),_buildItem("曹真")],
)

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三.SliverToBoxAdapter和SliverPersistentHeader

可以使用Flutter提供的SliverPersistentHeader组件实现，在使用SliverPersistentHeader时要求我们明确指定子控件的高度，不支持吸顶上推效果，使用起来不够灵活，所以我们参考并结合SliverToBoxAdapter和SliverPersistentHeader源码，自己实现一个自适应高度的吸顶Sliver组件，并在此基础上一步步实现吸顶上推效果。

编写StickySliverToBoxAdapter类，继承自SingleChildRenderObjectWidget

class StickySliverToBoxAdapter extends SingleChildRenderObjectWidget {const StickySliverToBoxAdapter({super.key,super.child});@overrideRenderObject createRenderObject(BuildContext context) => _StickyRenderSliverToBoxAdapter();}

SingleChildRenderObjectWidget类要求我们自己实现createRenderObject方法，返回一个RenderObject对象，而对于一个S liver组件而言，这个RenderObject必须是RenderSilver的子类。

编写_StickyRenderSliverToBoxAdapter，继承RenderSliverSingleBoxAdapter

class _StickyRenderSliverToBoxAdapter extends RenderSliverSingleBoxAdapter {@overridevoid performLayout() {// TODO: implement performLayout}}

RenderSliverSingleBoxAdapter要求子类实现performLayout方法，performLayout会对widegt的布局和绘制做控制，实现吸顶效果的关键就在于performLayout方法的实现。先依次看下SliverToBoxAdapter和SliverPersistentHeader对应RenderObject的performLayout相关方法的实现。

RenderSliverToBoxAdapter#performLayout

@override
void performLayout() {if (child == null) {geometry = SliverGeometry.zero;return;}final SliverConstraints constraints = this.constraints;//摆放子View，并把constraints传递给子Viewchild!.layout(constraints.asBoxConstraints(), parentUsesSize: true);//获取子View在滑动主轴方向的尺寸final double childExtent;switch (constraints.axis) {case Axis.horizontal:childExtent = child!.size.width;case Axis.vertical:childExtent = child!.size.height;}final double paintedChildSize = calculatePaintOffset(constraints, from: 0.0, to: childExtent);final double cacheExtent = calculateCacheOffset(constraints, from: 0.0, to: childExtent);assert(paintedChildSize.isFinite);assert(paintedChildSize >= 0.0);//更新SliverGeometrygeometry = SliverGeometry(scrollExtent: childExtent,paintExtent: paintedChildSize,cacheExtent: cacheExtent,maxPaintExtent: childExtent,hitTestExtent: paintedChildSize,hasVisualOverflow: childExtent > constraints.remainingPaintExtent || constraints.scrollOffset > 0.0,);//更新paintOffset，由滑动偏移量constraints.scrollOffset决定setChildParentData(child!, constraints, geometry!);
}

RenderSliverFloatingPersistentHeader#performLayout

SliverPersistentHeader的performLayout方法中调用了updateGeometry方法去更新geometry，而吸顶的关键就在updateGeometry方法中，也就是paintOrigin的值。constraints.overlap的值代表前一个Sliver和当前Sliver被覆盖部分的高度。

@override
double updateGeometry() {final double minExtent = this.minExtent;final double minAllowedExtent = constraints.remainingPaintExtent > minExtent ?minExtent :constraints.remainingPaintExtent;final double maxExtent = this.maxExtent;final double paintExtent = maxExtent - _effectiveScrollOffset!;final double clampedPaintExtent = clampDouble(paintExtent,minAllowedExtent,constraints.remainingPaintExtent,);final double layoutExtent = maxExtent - constraints.scrollOffset;final double stretchOffset = stretchConfiguration != null ?constraints.overlap.abs() :0.0;geometry = SliverGeometry(scrollExtent: maxExtent,paintOrigin: math.min(constraints.overlap, 0.0),paintExtent: clampedPaintExtent,layoutExtent: clampDouble(layoutExtent, 0.0, clampedPaintExtent),maxPaintExtent: maxExtent + stretchOffset,maxScrollObstructionExtent: minExtent,hasVisualOverflow: true, // Conservatively say we do have overflow to avoid complexity.);return 0.0;
}

四.吸顶效果实现

直接把上面updateGeometry中设置SliverGeometry的代码拷贝到_StickyRenderSliverToBoxAdapter#performLayout实现中，maxExtent和minExtent这两个值是由SliverPersistentHeader传入的SliverPersistentHeaderDelegate对象提供的。这里可以自己去看SliverPersistentHeaderDelegate的源码，就不多废话了。我们只需要把maxExtent和minExtent这两个值都改为子控件在主轴方向的尺寸大小即可。

 _buildSection(String title) {return StickySliverToBoxAdapter(child: RepaintBoundary(child: Container(height: 50,color: Colors.brown,alignment: Alignment.center,child: Text(title),),));}class _StickyRenderSliverToBoxAdapter extends RenderSliverSingleBoxAdapter {@overridevoid performLayout() {if (child == null) {geometry = SliverGeometry.zero;return;}final SliverConstraints constraints = this.constraints;//摆放子View，并把constraints传递给子Viewchild!.layout(constraints.asBoxConstraints(), parentUsesSize: true);//获取子View在滑动主轴方向的尺寸final double childExtent;switch (constraints.axis) {case Axis.horizontal:childExtent = child!.size.width;case Axis.vertical:childExtent = child!.size.height;}final double minExtent = childExtent;final double minAllowedExtent = constraints.remainingPaintExtent > minExtent ?minExtent : constraints.remainingPaintExtent;final double maxExtent = childExtent;final double paintExtent = maxExtent;final double clampedPaintExtent = clampDouble(paintExtent,minAllowedExtent,constraints.remainingPaintExtent,);final double layoutExtent = maxExtent - constraints.scrollOffset;geometry = SliverGeometry(scrollExtent: maxExtent,paintOrigin: min(constraints.overlap, 0.0),paintExtent: clampedPaintExtent,layoutExtent: clampDouble(layoutExtent, 0.0, clampedPaintExtent),maxPaintExtent: maxExtent,maxScrollObstructionExtent: minExtent,hasVisualOverflow: true, // Conservatively say we do have overflow to avoid complexity.);}
}

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仔细看上面的效果，貌似只有第一个Sliver吸顶了，我们把分组item的背景改成透明的，再来看看效果，就知道怎么回事了😄。

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可以看到，所有的分组section都已经吸顶了，只不过吸顶位置都是0，并且前一个section把后一个section覆盖了，我们下一步实现上推功能后，这个问题自热而然的就解决了。

五.实现上推效果

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如图，当前section与前一个section重合了多少，前一个section就往上移动多少，也就是移动constraints.overlap即可，往下滑动也是同样的道理。

//查找前一个吸顶的section
RenderSliver? _prev() {if(parent is RenderViewportBase) {RenderSliver? current = this;while(current != null) {current = (parent as RenderViewportBase).childBefore(current);if(current is _StickyRenderSliverToBoxAdapter && current.geometry != null) {return current;}}}return null;
}@override
void performLayout() {if (child == null) {geometry = SliverGeometry.zero;return;}final SliverConstraints constraints = this.constraints;//摆放子View，并把constraints传递给子Viewchild!.layout(constraints.asBoxConstraints(), parentUsesSize: true);//获取子View在滑动主轴方向的尺寸final double childExtent;switch (constraints.axis) {case Axis.horizontal:childExtent = child!.size.width;case Axis.vertical:childExtent = child!.size.height;}final double minExtent = childExtent;final double minAllowedExtent = constraints.remainingPaintExtent > minExtent ?minExtent : constraints.remainingPaintExtent;final double maxExtent = childExtent;final double paintExtent = maxExtent;final double clampedPaintExtent = clampDouble(paintExtent,minAllowedExtent,constraints.remainingPaintExtent,);final double layoutExtent = maxExtent - constraints.scrollOffset;geometry = SliverGeometry(scrollExtent: maxExtent,paintOrigin: min(constraints.overlap, 0.0),paintExtent: clampedPaintExtent,layoutExtent: clampDouble(layoutExtent, 0.0, clampedPaintExtent),maxPaintExtent: maxExtent,maxScrollObstructionExtent: minExtent,hasVisualOverflow: true, // Conservatively say we do have overflow to avoid complexity.);//上推关键代码: 当前吸顶的Sliver被覆盖了多少，前一个吸顶的Sliver就移动多少RenderSliver? prev = _prev();if(prev != null && constraints.overlap > 0) {setChildParentData(_prev()!, constraints.copyWith(scrollOffset: constraints.overlap), _prev()!.geometry!);}
}

搞定，可以洗洗睡了，嘿嘿。

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六.Fixed: 吸顶section点击事件失效

重写childMainAxisPosition方法返回0即可

class _StickyRenderSliverToBoxAdapter extends RenderSliverSingleBoxAdapter {...// 必须重写，否则点击事件失效。@overridedouble childMainAxisPosition(covariant RenderBox child) => 0.0;}