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前言
入坑Flutter一年了，接触到Flutter也只是冰山一角，很多东西可能知道是怎么用的，但是不是很明白其中的原理，俗话说唯有深入，方能浅出。本系列将对Sliver相关源码一一进行分析，希望能够举一反三，不再惧怕Sliver。
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看完Flutter Sliver一生之敌 你将不会害怕使用Sliver，Sliver将成为你的一生之爱。欢迎加入Flutter Candies <a target="_blank" href="https://jq.qq.com/?_wv=1027&k=5bcc0gy"><img border="0" src="https://user-gold-cdn.xitu.io/2019/10/27/16e0ca3f1a736f0e?w=90&h=22&f=png&s=1827" alt="flutter-candies" title="flutter-candies"></a> QQ群: 181398081。
下面是全部滚动的组件，以及他们的关系
| Widget | Build | Viewport |
|---|---|---|
| SingleChildScrollView | Scrollable | _SingleChildViewport |
| ScrollView | Scrollable | ShrinkWrappingViewport/Viewport |
Sliver系列继承于ScrollView
| Widget | Extends |
|---|---|
| CustomScrollView | ScrollView |
| NestedScrollView | CustomScrollView |
| ListView/GridView | BoxScrollView => ScrollView |
简单讲滚动组件由Scrollable获取用户手势反馈，将滚动反馈和Slivers传递给Viewport计算出Sliver的位置。注意Sliver可以是单孩子(SliverPadding/SliverPersistentHeader/SliverToBoxAdapter等等)也可以是多孩子(SliverList/SliverGrid)。下面我们通过分析源码，探究其中奥秘。
ScrollView
下面为build方法中的关键代码，这里是我们上面说的Scrollable，主要负责用户手势监听反馈。
final Scrollable scrollable = Scrollable(
dragStartBehavior: dragStartBehavior,
axisDirection: axisDirection,
controller: scrollController,
physics: physics,
semanticChildCount: semanticChildCount,
viewportBuilder: (BuildContext context, ViewportOffset offset) {
return buildViewport(context, offset, axisDirection, slivers);
},
);
我们再看看buildViewport方法
@protected
Widget buildViewport(
BuildContext context,
ViewportOffset offset,
AxisDirection axisDirection,
List<Widget> slivers,
) {
if (shrinkWrap) {
return ShrinkWrappingViewport(
axisDirection: axisDirection,
offset: offset,
slivers: slivers,
);
}
return Viewport(
axisDirection: axisDirection,
offset: offset,
slivers: slivers,
cacheExtent: cacheExtent,
center: center,
anchor: anchor,
);
}
根据shrinkWrap的不同，分成了2种Viewport
Scrollable
用于监听各种用户手势并实现滚动，下面为build方法中的关键代码。
//InheritedWidget组件，为了共享position数据
Widget result = _ScrollableScope(
scrollable: this,
position: position,
// TODO(ianh): Having all these global keys is sad.
child: Listener(
onPointerSignal: _receivedPointerSignal,
child: RawGestureDetector(
key: _gestureDetectorKey,
gestures: _gestureRecognizers,
behavior: HitTestBehavior.opaque,
excludeFromSemantics: widget.excludeFromSemantics,
child: Semantics(
explicitChildNodes: !widget.excludeFromSemantics,
child: IgnorePointer(
key: _ignorePointerKey,
ignoring: _shouldIgnorePointer,
ignoringSemantics: false,
//通过Listener监听手势，将滚动position通过viewportBuilder回调。
child: widget.viewportBuilder(context, position),
),
),
),
),
);
//这里可以看到为什么安卓和ios上面对于滚动越界(overscrolls)时候的操作不一样
return _configuration.buildViewportChrome(context, result, widget.axisDirection);
安卓和fuchsia上面使用GlowingOverscrollIndicator来显示滚动不了之后的水波纹效果。
/// Wraps the given widget, which scrolls in the given [AxisDirection].
///
/// For example, on Android, this method wraps the given widget with a
/// [GlowingOverscrollIndicator] to provide visual feedback when the user
/// overscrolls.
Widget buildViewportChrome(BuildContext context, Widget child, AxisDirection axisDirection) {
// When modifying this function, consider modifying the implementation in
// _MaterialScrollBehavior as well.
switch (getPlatform(context)) {
case TargetPlatform.iOS:
return child;
case TargetPlatform.android:
case TargetPlatform.fuchsia:
return GlowingOverscrollIndicator(
child: child,
axisDirection: axisDirection,
color: _kDefaultGlowColor,
);
}
return null;
}
Viewport
通过只显示(计算绘制)滚动视图中的一部分内容来实现滚动可视化设计，大大降低内存消耗。比如ListView可视区域为666像素，但其列表元素的总高度远远超过666像素，但实际上我们只是关心这个666像素中的元素(当然如果设置了CacheExtent，还要算上这个距离)
在Scrollview中将Scrollable滚动反馈以及Slivers传递给了Viewport。Viewport 是一个MultiChildRenderObjectWidget，lei了lei了，这是一个自绘多孩子的组件。直接找到createRenderObject方法，看到返回一个RenderViewport
RenderViewport
重头戏来了，我们看看构造参数有哪些。
RenderViewport({
//主轴方向，默认向下
AxisDirection axisDirection = AxisDirection.down,
//纵轴方向，跟主轴方向以及有关系
@required AxisDirection crossAxisDirection,
//Scrollable中回调的用户反馈
@required ViewportOffset offset,
//当scrollOffset = 0，第一个child在viewport的位置（0 <= anchor <= 1.0），0.0在leading，1.0在trailing，0.5在中间
double anchor = 0.0,
//sliver孩子们
List<RenderSliver> children,
//The first child in the [GrowthDirection.forward] growth direction.
//计算时候的基准，默认为第一个娃，这个参数估计极少有人使用
RenderSliver center,
//缓存区域大小
double cacheExtent,
//决定cacheExtent是实际大小还是根据viewport的百分比
CacheExtentStyle cacheExtentStyle = CacheExtentStyle.pixel,
})... {
addAll(children);
if (center == null && firstChild != null)
_center = firstChild;
}
可以看到构造中把全部孩子都加进入了，而且如果外部不传递center，center默认为第一个孩子。
划重点代码分析
sizedByParent
在Viewport中这个值永远返回true，
@override
bool get sizedByParent => true;
来看看这个属性的解释。即如果这个值为true，那么组件的大小只跟它的parent告诉它的大小constraints有关系，与它的 child 都无关.
就是说RenderViewport的大小约束是由它的parent告诉它的，跟里面的Slivers没有关系。说到这个我们看一个新手经常错误的代码。
Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text(
'测试',
),
ListView.builder(itemBuilder: (context,index){})
],
),
我们前面知道ListView最终是一个ScrollView,其中的Viewport在Column当中是无法知道自己的有效大小的，该代码的会导致Viewport的高度为无限大，将会报错(当然你这里可以把shrinkWrap设置为true，但是这样会导致ListView的全部元素都被计算，列表将失去滚动，这个我们后面会讲)
继续看代码中看到，当sizedByParent为true的时候调用performResize方法，指定Size只根据constraints。
if (sizedByParent) {
assert(() {
_debugDoingThisResize = true;
return true;
}());
try {
performResize();
assert(() {
debugAssertDoesMeetConstraints();
return true;
}());
} catch (e, stack) {
_debugReportException('performResize', e, stack);
}
assert(() {
_debugDoingThisResize = false;
return true;
}());
}
performResize
看看RenderViewport的performResize中做了什么。有一大堆assert，就一句话，我不能无限大。最后将自己的size设置为constraints.biggest。
(size是自己的大小，constraints是parent给的限制)
@override
void performResize() {
assert(() {
if (!constraints.hasBoundedHeight || !constraints.hasBoundedWidth) {
switch (axis) {
case Axis.vertical:
if (!constraints.hasBoundedHeight) {
throw FlutterError.fromParts(<DiagnosticsNode>[
ErrorSummary('Vertical viewport was given unbounded height.'),
ErrorDescription(
'Viewports expand in the scrolling direction to fill their container. '
'In this case, a vertical viewport was given an unlimited amount of '
'vertical space in which to expand. This situation typically happens '
'when a scrollable widget is nested inside another scrollable widget.'
),
ErrorHint(
'If this widget is always nested in a scrollable widget there '
'is no need to use a viewport because there will always be enough '
'vertical space for the children. In this case, consider using a '
'Column instead. Otherwise, consider using the "shrinkWrap" property '
'(or a ShrinkWrappingViewport) to size the height of the viewport '
'to the sum of the heights of its children.'
)
]);
}
if (!constraints.hasBoundedWidth) {
throw FlutterError(
'Vertical viewport was given unbounded width.\n'
'Viewports expand in the cross axis to fill their container and '
'constrain their children to match their extent in the cross axis. '
'In this case, a vertical viewport was given an unlimited amount of '
'horizontal space in which to expand.'
);
}
break;
case Axis.horizontal:
if (!constraints.hasBoundedWidth) {
throw FlutterError.fromParts(<DiagnosticsNode>[
ErrorSummary('Horizontal viewport was given unbounded width.'),
ErrorDescription(
'Viewports expand in the scrolling direction to fill their container.'
'In this case, a horizontal viewport was given an unlimited amount of '
'horizontal space in which to expand. This situation typically happens '
'when a scrollable widget is nested inside another scrollable widget.'
),
ErrorHint(
'If this widget is always nested in a scrollable widget there '
'is no need to use a viewport because there will always be enough '
'horizontal space for the children. In this case, consider using a '
'Row instead. Otherwise, consider using the "shrinkWrap" property '
'(or a ShrinkWrappingViewport) to size the width of the viewport '
'to the sum of the widths of its children.'
)
]);
}
if (!constraints.hasBoundedHeight) {
throw FlutterError(
'Horizontal viewport was given unbounded height.\n'
'Viewports expand in the cross axis to fill their container and '
'constrain their children to match their extent in the cross axis. '
'In this case, a horizontal viewport was given an unlimited amount of '
'vertical space in which to expand.'
);
}
break;
}
}
return true;
}());
size = constraints.biggest;
// We ignore the return value of applyViewportDimension below because we are
// going to go through performLayout next regardless.
switch (axis) {
case Axis.vertical:
offset.applyViewportDimension(size.height);
break;
case Axis.horizontal:
offset.applyViewportDimension(size.width);
break;
}
}
performLayout
负责布局RenderViewport的Children
//从size中得到主轴和纵轴的大小
double mainAxisExtent;
double crossAxisExtent;
switch (axis) {
case Axis.vertical:
mainAxisExtent = size.height;
crossAxisExtent = size.width;
break;
case Axis.horizontal:
mainAxisExtent = size.width;
crossAxisExtent = size.height;
break;
}
//如果单Sliver孩子的viewport高度为100，anchor为0.5，centerOffsetAdjustment设置为50.0的话，当scroll offset is 0.0的时候，center会刚好在viewport中间。
final double centerOffsetAdjustment = center.centerOffsetAdjustment;
double correction;
int count = 0;
do {
assert(offset.pixels != null);
correction = _attemptLayout(mainAxisExtent, crossAxisExtent, offset.pixels + centerOffsetAdjustment);
///如果不为0.0的话，是因为child中有需要修正(这个我们将在后面系列中讲到，这里我们就简单认为在layout child过程中出现了问题)，我们需要改变scroll offset之后重新layout chilren。
if (correction != 0.0) {
offset.correctBy(correction);
} else {
///告诉Scrollable 最小滚动距离和最大滚动距离
if (offset.applyContentDimensions(
math.min(0.0, _minScrollExtent + mainAxisExtent * anchor),
math.max(0.0, _maxScrollExtent - mainAxisExtent * (1.0 - anchor)),
))
break;
}
count += 1;
} while (count < _maxLayoutCycles);
如果超过最大次数，children还是layout还是有问题的话，将警告提示。
下面我们看看_attemptLayout方法中做了什么。
double _attemptLayout(double mainAxisExtent, double crossAxisExtent, double correctedOffset) {
assert(!mainAxisExtent.isNaN);
assert(mainAxisExtent >= 0.0);
assert(crossAxisExtent.isFinite);
assert(crossAxisExtent >= 0.0);
assert(correctedOffset.isFinite);
_minScrollExtent = 0.0;
_maxScrollExtent = 0.0;
_hasVisualOverflow = false;
//centerOffset的数值将使用anchor和offset.pixels + centerOffsetAdjustment进行修正。前面有讲
final double centerOffset = mainAxisExtent * anchor - correctedOffset;
//反向RemainingPaintExtent，就是center之前还有多少距离可以拿来绘制
final double reverseDirectionRemainingPaintExtent = centerOffset.clamp(0.0, mainAxisExtent);
//正向RemainingPaintExtent，就是center之后还有多少距离可以拿来绘制
final double forwardDirectionRemainingPaintExtent = (mainAxisExtent - centerOffset).clamp(0.0, mainAxisExtent);
switch (cacheExtentStyle) {
case CacheExtentStyle.pixel:
_calculatedCacheExtent = cacheExtent;
break;
case CacheExtentStyle.viewport:
_calculatedCacheExtent = mainAxisExtent * cacheExtent;
break;
}
///总的计算区域包含前后2个cacheExtent
final double fullCacheExtent = mainAxisExtent + 2 * _calculatedCacheExtent;
///加上cacheExtent的center位置，跟前面的比就是多了cache
final double centerCacheOffset = centerOffset + _calculatedCacheExtent;
//反向RemainingPaintExtent，就是center之前还有多少距离可以拿来绘制，跟前面的比就是多了cache
final double reverseDirectionRemainingCacheExtent = centerCacheOffset.clamp(0.0, fullCacheExtent);
//正向RemainingPaintExtent，就是center之后还有多少距离可以拿来绘制，跟前面的比就是多了cache
final double forwardDirectionRemainingCacheExtent = (fullCacheExtent - centerCacheOffset).clamp(0.0, fullCacheExtent);
final RenderSliver leadingNegativeChild = childBefore(center);
///如果在center之前还有child，将向前layout child，计算前面布局前面的child
if (leadingNegativeChild != null) {
// negative scroll offsets
final double result = layoutChildSequence(
child: leadingNegativeChild,
scrollOffset: math.max(mainAxisExtent, centerOffset) - mainAxisExtent,
overlap: 0.0,
layoutOffset: forwardDirectionRemainingPaintExtent,
remainingPaintExtent: reverseDirectionRemainingPaintExtent,
mainAxisExtent: mainAxisExtent,
crossAxisExtent: crossAxisExtent,
growthDirection: GrowthDirection.reverse,
advance: childBefore,
remainingCacheExtent: reverseDirectionRemainingCacheExtent,
cacheOrigin: (mainAxisExtent - centerOffset).clamp(-_calculatedCacheExtent, 0.0),
);
if (result != 0.0)
return -result;
}
///布局center后面的child
// positive scroll offsets
return layoutChildSequence(
child: center,
scrollOffset: math.max(0.0, -centerOffset),
overlap: leadingNegativeChild == null ? math.min(0.0, -centerOffset) : 0.0,
layoutOffset: centerOffset >= mainAxisExtent ? centerOffset: reverseDirectionRemainingPaintExtent,
remainingPaintExtent: forwardDirectionRemainingPaintExtent,
mainAxisExtent: mainAxisExtent,
crossAxisExtent: crossAxisExtent,
growthDirection: GrowthDirection.forward,
advance: childAfter,
remainingCacheExtent: forwardDirectionRemainingCacheExtent,
cacheOrigin: centerOffset.clamp(-_calculatedCacheExtent, 0.0),
);
}
注意scrollOffset ，在向前和向后layout的时候不一样，
一个是 math.max(mainAxisExtent, centerOffset) - mainAxisExtent
一个是 math.max(0.0, -centerOffset)
我们有说过center其实是scrolloffset为0的基准，viewport里面如果有多个slivers，我们可以指定其中一个为center(默认第一个为center)，那么想前滚centerOffset会变大，想后滚centerOffset会变成负数。感觉还是有点抽象，下面给一个栗子，我给第2个sliver增加了key，并且把CustomScrollView的center赋值为这个key。小声逼逼，Center这个参数我估计百分之99的人没有用过，用过的请留言，我看看有多少人知道这个。
CustomScrollView(
center: key,
slivers: <Widget>[
SliverList(),
SliverGrid(key:key),
运行起来初始centerOffset为0的时候SliverGrid在初始位置。
image
向前滚动，可以看到我们得到了逆向的SliverList，从我们的参数中也可以验证到。而offset.pixels（ScollView的滚动位置）当然也为0.(而不是你们想的SliverList的高度)
image
再看下layoutChildSequence方法，注意到advance方法，向前其实调用的是childBefore，向后是调用的childAfter
double layoutChildSequence({
@required RenderSliver child,
@required double scrollOffset,
@required double overlap,
@required double layoutOffset,
@required double remainingPaintExtent,
@required double mainAxisExtent,
@required double crossAxisExtent,
@required GrowthDirection growthDirection,
@required RenderSliver advance(RenderSliver child),
@required double remainingCacheExtent,
@required double cacheOrigin,
}) {
assert(scrollOffset.isFinite);
assert(scrollOffset >= 0.0);
final double initialLayoutOffset = layoutOffset;
final ScrollDirection adjustedUserScrollDirection =
applyGrowthDirectionToScrollDirection(offset.userScrollDirection, growthDirection);
assert(adjustedUserScrollDirection != null);
double maxPaintOffset = layoutOffset + overlap;
double precedingScrollExtent = 0.0;
while (child != null) {
final double sliverScrollOffset = scrollOffset <= 0.0 ? 0.0 : scrollOffset;
// If the scrollOffset is too small we adjust the paddedOrigin because it
// doesn't make sense to ask a sliver for content before its scroll
// offset.
final double correctedCacheOrigin = math.max(cacheOrigin, -sliverScrollOffset);
final double cacheExtentCorrection = cacheOrigin - correctedCacheOrigin;
assert(sliverScrollOffset >= correctedCacheOrigin.abs());
assert(correctedCacheOrigin <= 0.0);
assert(sliverScrollOffset >= 0.0);
assert(cacheExtentCorrection <= 0.0);
//输入
child.layout(SliverConstraints(
axisDirection: axisDirection,
growthDirection: growthDirection,
userScrollDirection: adjustedUserScrollDirection,
scrollOffset: sliverScrollOffset,
precedingScrollExtent: precedingScrollExtent,
overlap: maxPaintOffset - layoutOffset,
remainingPaintExtent: math.max(0.0, remainingPaintExtent - layoutOffset + initialLayoutOffset),
crossAxisExtent: crossAxisExtent,
crossAxisDirection: crossAxisDirection,
viewportMainAxisExtent: mainAxisExtent,
remainingCacheExtent: math.max(0.0, remainingCacheExtent + cacheExtentCorrection),
cacheOrigin: correctedCacheOrigin,
), parentUsesSize: true);
//输出
final SliverGeometry childLayoutGeometry = child.geometry;
assert(childLayoutGeometry.debugAssertIsValid());
// If there is a correction to apply, we'll have to start over.
if (childLayoutGeometry.scrollOffsetCorrection != null)
return childLayoutGeometry.scrollOffsetCorrection;
// We use the child's paint origin in our coordinate system as the
// layoutOffset we store in the child's parent data.
final double effectiveLayoutOffset = layoutOffset + childLayoutGeometry.paintOrigin;
// `effectiveLayoutOffset` becomes meaningless once we moved past the trailing edge
// because `childLayoutGeometry.layoutExtent` is zero. Using the still increasing
// 'scrollOffset` to roughly position these invisible slivers in the right order.
if (childLayoutGeometry.visible || scrollOffset > 0) {
updateChildLayoutOffset(child, effectiveLayoutOffset, growthDirection);
} else {
updateChildLayoutOffset(child, -scrollOffset + initialLayoutOffset, growthDirection);
}
//更新最大绘制位置
maxPaintOffset = math.max(effectiveLayoutOffset + childLayoutGeometry.paintExtent, maxPaintOffset);
scrollOffset -= childLayoutGeometry.scrollExtent;
//前一个child的滚动距离
precedingScrollExtent += childLayoutGeometry.scrollExtent;
layoutOffset += childLayoutGeometry.layoutExtent;
if (childLayoutGeometry.cacheExtent != 0.0) {
remainingCacheExtent -= childLayoutGeometry.cacheExtent - cacheExtentCorrection;
cacheOrigin = math.min(correctedCacheOrigin + childLayoutGeometry.cacheExtent, 0.0);
}
// 更新_maxScrollExtent和_minScrollExtent
// https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/rendering/viewport.dart#L1449
updateOutOfBandData(growthDirection, childLayoutGeometry);
// move on to the next child
// layout下一个child
child = advance(child);
}
// we made it without a correction, whee!
//完美，全部的children都没有错误
return 0.0;
}
SliverConstraints为layout child的输入，SliverGeometry为layout child之后的输出，layout之后viewport将更新_maxScrollExtent和_minScrollExtent，然后layout下一个sliver。至于child.layout方法里面内容，我们将会在下一个章当中讲到。
RenderShrinkWrappingViewport
当我们把shrinkWrap设置为true的时候，最终的Viewport使用的是RenderShrinkWrappingViewport。那么我们看看其中的区别是什么。
先看看官方对shrinkWrap参数的解释。设置shrinkWrap为true，viewport的大小将不是由它的父亲而决定，而是由它自己决定。我们经常碰到由人使用ListView嵌套ListView的情况， 外面的ListView在layout child的时候需要知道里面ListView的大小，而我们前面知道ListView中的Viewport的大小是由它parent告诉它的。
parent：hi, child,你有多大，我给你一个无限纵轴大小的限制。
child: hi, parent，我也不知道啊，你不告诉我，我的viewport有多大。那么我只能将我的全部child都layout出来才知道我总的大小了。那我得换一个viewport了，RenderShrinkWrappingViewport才能知道计算出我的总高度。
由于ListView的parent无法告诉它的child ListView的可丈量大小，所以我们必须设置shrinkWrap为true，内部使用RenderShrinkWrappingViewport计算。
由于RenderShrinkWrappingViewport的大小不再只由parent决定，所以不再调用performResize方法。那么我们来关注下performLayout方法。
performLayout
@override
void performLayout() {
if (firstChild == null) {
switch (axis) {
case Axis.vertical:
//如果是竖直，你起码要告诉我水平最大限制吧？
assert(constraints.hasBoundedWidth);
size = Size(constraints.maxWidth, constraints.minHeight);
break;
//如果是水平，你起码要告诉我垂直最大限制吧？
case Axis.horizontal:
assert(constraints.hasBoundedHeight);
size = Size(constraints.minWidth, constraints.maxHeight);
break;
}
offset.applyViewportDimension(0.0);
_maxScrollExtent = 0.0;
_shrinkWrapExtent = 0.0;
_hasVisualOverflow = false;
offset.applyContentDimensions(0.0, 0.0);
return;
}
double mainAxisExtent;
double crossAxisExtent;
switch (axis) {
case Axis.vertical:
//如果是竖直，你起码要告诉我水平最大限制吧？说到这个我想起来了Flutter中为啥没有支持水平和垂直都能滚动的容器了。
assert(constraints.hasBoundedWidth);
mainAxisExtent = constraints.maxHeight;
crossAxisExtent = constraints.maxWidth;
break;
case Axis.horizontal:
assert(constraints.hasBoundedHeight);
//如果是水平，你起码要告诉我垂直最大限制吧？
mainAxisExtent = constraints.maxWidth;
crossAxisExtent = constraints.maxHeight;
break;
}
double correction;
double effectiveExtent;
do {
assert(offset.pixels != null);
correction = _attemptLayout(mainAxisExtent, crossAxisExtent, offset.pixels);
if (correction != 0.0) {
offset.correctBy(correction);
} else {
switch (axis) {
case Axis.vertical:
effectiveExtent = constraints.constrainHeight(_shrinkWrapExtent);
break;
case Axis.horizontal:
effectiveExtent = constraints.constrainWidth(_shrinkWrapExtent);
break;
}
final bool didAcceptViewportDimension = offset.applyViewportDimension(effectiveExtent);
final bool didAcceptContentDimension = offset.applyContentDimensions(0.0, math.max(0.0, _maxScrollExtent - effectiveExtent));
if (didAcceptViewportDimension && didAcceptContentDimension)
break;
}
} while (true);
switch (axis) {
case Axis.vertical:
size = constraints.constrainDimensions(crossAxisExtent, effectiveExtent);
break;
case Axis.horizontal:
size = constraints.constrainDimensions(effectiveExtent, crossAxisExtent);
break;
}
}
_maxScrollExtent和
_shrinkWrapExtent都是关键先生。当mainAxisExtent不为double.Infinity(无限大)的时候，其实效果跟Viewport里面计算(除掉Center相关)是一样; 当mainAxisExtent为double.Infinity(无限大)，我们将会将全部的child都layout出来获得总的大小
@override
void updateOutOfBandData(GrowthDirection growthDirection, SliverGeometry childLayoutGeometry) {
assert(growthDirection == GrowthDirection.forward);
_maxScrollExtent += childLayoutGeometry.scrollExtent;
if (childLayoutGeometry.hasVisualOverflow)
_hasVisualOverflow = true;
_shrinkWrapExtent += childLayoutGeometry.maxPaintExtent;
}
这里也就是为啥我们之前说Column里面或者ListView放ListView(子)，ListView(子)会全部元素都build，并且失去滚动的原因。
剧透
这一章看起来有些枯燥，都是源码分析。下一章(Flutter Sliver一生之敌 (ExtendedList))，我们将顺着ListView/GridView=> SliverList/SliverGrid => RenderSliverList/RenderSliverGrid的感情线，了解最终Sliver是怎么将children绘制出来的。下一章将不只是枯燥的源码分析，我们将举一反N，告诉你如何处理图片列表内存爆炸闪退，将告诉你列表元素特殊的layout方式等等。
结语
ExtendedList WaterfallFlow 和 LoadingMoreList 都是可以食用的状态。等不及的小伙伴可以提前食用，特别是图片列表内存过大而导致闪退的小伙伴可以先看demo,先解决掉一直折磨大家的问题
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