MultiChildSliverObserver class
Properties
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axis
→ Axis
-
isRevealed may use this to calculate the trailing offset for an item
no setterinherited
-
firstLayoutFinished
→ bool
-
if the first layout is completed.
it should be true as long as doFinishLayout is invoked at least once.
once firstLayoutFinished is true, indicating that all operations on this
observer could be completed safely.
no setterinherited
-
hashCode
→ int
-
The hash code for this object.
no setterinherited
-
isActive
→ bool
-
indicates whether a RenderSliver is being observed by this observer.
If false, it would throw errors to report illegal usage,
only when
isActive is true, the observer could work normally.
no setterinherited
-
isObserving
→ bool
-
no setterinherited
-
itemCount
↔ int?
-
getter/setter pairinherited
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items
→ Map<int, ItemScrollExtent>
-
the items whose ItemScrollExtent have been observed after doFinishLayout
no setterinherited
-
mainAxisExtent
→ double
-
the available painting extent along the main axis for a viewport
no setterinherited
-
origin
↔ RevealedOffset?
-
the origin of RenderObject that is being observed
getter/setter pairinherited
-
renderObject
→ RenderSliver?
-
RenderObject is being observed
no setterinherited
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renderToTargetIndex
↔ IndexConverter?
-
sometimes, the render index may not be the target index to which users want to scroll.
by setting
renderToTargetIndex, users could define how to convert the render index to the target index.
getter/setter pairinherited
-
renderVisible
→ bool
-
if the observing RenderSliver is visible in its closest ancestor RenderViewportBase
no setterinherited
-
runtimeType
→ Type
-
A representation of the runtime type of the object.
no setterinherited
-
shouldDoFinishLayout
↔ bool
-
once onLayout is invoked, it would be true to indicate
that it require invoking doFinishLayout at least once.
Users typically do not set shouldDoFinishLayout directly.
getter/setter pairinherited
-
shouldUpdateOrigin
→ bool
-
no setterinherited
-
sizes
→ Map<int, Size>
-
the Size of laid out children, the key may be its index for SliverScrollObserver,
or ParentData.hashCode for BoxScrollObserver
no setterinherited
-
targetToRenderIndex
↔ IndexConverter?
-
sometimes, the target index to which users want to scroll may not be same as the current render index,
by using
targetToRenderIndex, users could define how to map the target index to a render index.
e.g., when using ListView.separated, the item index may not be equal to its render index,
since separators would be also counted as the children of RenderSliver.
getter/setter pairinherited
Methods
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alignVisibleOffset(double alignment)
→ RevealedOffset?
-
Calculate the position of renderObject using the given alignment
See also:
inherited
-
animateToIndex(int index, {required ScrollPosition position, required Duration duration, required Curve curve, bool closeToEdge = true, double alignment = 0.0})
→ Future<bool>
-
animate to
index based on the given position.
inherited
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checkFirstLayoutFinished()
→ void
-
ensure
firstLayoutFinished is true to guarantee legal usage for estimateScrollOffset before continuing
inherited
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clear()
→ void
-
inherited
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doFinishLayout()
→ void
-
active items have been laid out, and some of them are painting,
at this time, we know the layout for renderObject must be completed,
so we could doFinishLayout safely
override
-
estimateScrollOffset(int target, {required ScrollExtent scrollExtent})
→ double
-
estimate the scroll offset for
target based on either its ItemScrollExtent or
_averageExtentForEachIndex.
Use _first or _last to indicate if scroll up or scroll down.
The final estimation would be clamped between ScrollExtent.min and ScrollExtent.max
to avoid over scrolling.
inherited
-
getItemScrollExtent(int index)
→ ItemScrollExtent?
-
get the observed ItemScrollExtent for
index
inherited
-
getItemSize(int index)
→ Size?
-
Different observers may override
getItemSize base on how they store sizes of items.
e.g., BoxScrollObserver would override this method since sizes's key is ParentData.hashCode
instead of its index
inherited
-
getMainAxisExtent(int index)
→ double
-
get the
index's main axis extent based on axis
inherited
-
getTrailingEdgeFromScroll(ScrollExtent scrollExtent)
→ double
-
if the viewport is overlapped by the previous RenderSliver,
the trailing edge should subtract the overlapped area
inherited
-
getVisibleItems({required ScrollExtent scrollExtent, VisibilityStrategy strategy = VisibilityStrategy.tolerance})
→ List<int>
-
inherited
-
isDesiredParentData(ParentData? parentData)
→ bool
-
check if
parentData is desired for an observer.
it should be implemented by the subclasses of LayoutObserver
override
-
isDesiredRenderObject(RenderObject renderObject)
→ bool
-
inherited
-
isRevealed(int index, {required ScrollExtent scrollExtent, VisibilityStrategy strategy = VisibilityStrategy.tolerance, double tolerance = 0.1, bool shouldNormalized = true, bool shouldConvert = false})
→ bool
-
if
index is revealed in its closest ancestor RenderSliver.
typically, index must have been observed before checking isRevealed.
inherited
-
jumpToIndex(int index, {required ScrollPosition position, bool closeToEdge = true, double alignment = 0.0})
→ void
-
jump to
index based on the given position.
this observer and position should be associated/attached to the same ScrollController.
if closeToEdge is true, we would try scrolling index to the edge of ScrollView.reverse if not over scrolling.
alignment only takes effects when the observed RenderObject is not visible on the screen,
if the RenderObject has been visible on the screen, alignment is ignored.
inherited
-
normalizeIndex(int index)
→ int
-
inherited
-
noSuchMethod(Invocation invocation)
→ dynamic
-
Invoked when a nonexistent method or property is accessed.
inherited
-
onLayout(RenderObject value, {required Size size, ParentData? parentData})
→ void
-
If it observes multiple scrollable children
set renderObject if both are not identical,
and mark shouldUpdateOrigin as true to do
_updateRenderOrigin.
Meanwhile, it should mark shouldDoFinishLayout as true since some items are laid out
override
-
pause()
→ void
-
sometimes, ObserverProxy may not be descendants of RenderSliver temporarily,
e.g., ReorderableListView is ordering.
using pause to stop observing temporarily.
inherited
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relativePositionInViewport(int index, {required ScrollExtent scrollExtent, double alignment = 0.0, bool shouldNormalized = true, bool shouldConvert = false})
→ double?
-
inherited
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resume()
→ void
-
using resume to continue observing.
inherited
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showInViewport(ViewportOffset offset, {double alignment = 0.0, Duration duration = Duration.zero, Curve curve = Curves.ease})
→ void
-
make its observed RenderObject visible in its closest ancestor RenderViewportBase
we guarantee that RenderAbstractViewport.of would find an RenderAbstractViewport ancestor,
since the scrollable content is always wrapped in a kind of RenderAbstractViewport.
alignment indicates how you want to align the RenderObject on the screen when RenderObject is visible.
if alignment is 0.0, RenderObject would try closing to ScrollPosition.pixels as much as possible;
if alignment is 1.0, RenderObject would try closing to ScrollPosition.maxScrollExtent as much as possible;
if alignment is 0.5, RenderObject would try closing to the center between ScrollPosition.pixels and ScrollPosition.maxScrollExtent
as much as possible.
inherited
-
toString()
→ String
-
A string representation of this object.
inherited
-
updateEstimation(double totalExtent, int count)
→ void
-
average
_averageExtentForEachIndex based on the current observed totalExtent,
maybe we should have a better way to summarize the previous estimation: _averageExtentForEachIndex
and the current exact extent: totalExtent
inherited
-
updateMaxCrossCount({required int maxCrossCount, required double maxCrossOffset, required double crossAxisExtent})
→ void
-
maxCrossOffset is cross axis offset of the left/bottom most item
maxCrossCount increments when maxCrossOffset increments
inherited
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updateRange(int? first, int? last)
→ void
-
record those children that have been laid out
inherited
-
visibleRatioInViewport(ScrollExtent scrollExtent)
→ double
-
calculate the visible part of the RenderObject relative to the main axis extent of its viewport.
for example, the observed RenderObject has 200 pixels painted on the screen, while the viewport has
500 pixels painting extent. The ratio would be 200/500 = 0.4.
The result would never over 1.0, since the visible part can not be greater than the viewport's painting extent.
typically, it is used for multi slivers in a custom scroll view to see if how many ratio of which the sliver is visible in the viewport
inherited
