1、Observing child widgets those are being displayed in the ScrollView

Usage

Parameter description of ListViewObserver:

Parameter	Required	Description
child	yes	Create [ListView] as a child of [ListViewObserver]
sliverListContexts	no	In this callback, we need to return all [BuildContext] of the [ListView] those needs to be observed. This property is only used when [BuildContext] needs to be specified exactly
onObserve	no	This callback can listen for information about the child widgets those are currently being displayed in the current first [Sliver]
onObserveAll	no	This callback can listen for information about all the children of slivers that are currently being displayed. This callback is only needed when there are more than one [Sliver]
leadingOffset	no	The offset of the head of scroll view. Find the first child start at this offset.
dynamicLeadingOffset	no	This is a callback that provides [leadingOffset], used when the leading offset in the head of the scroll view is dynamic. It has a higher priority than [leadingOffset]
toNextOverPercent	no	When the percentage of the first child widget is blocked reaches this value, the next child widget will be the first child that is displaying. The default value is 1
autoTriggerObserveTypes	no	Used to set types those can trigger observe automatically
triggerOnObserveType	no	Used to set the prerequisite for triggering [onObserve] callback and [onObserveAll] callback

Method 1: General (Recommended)

It is relatively simple to use and has a wide application range. In general, only this method is needed

Build ListViewObserver and pass the ListView instance to the child parameter

ListViewObserver(
  child: _buildListView(),
  onObserve: (resultModel) {
    print('firstChild.index -- ${resultModel.firstChild?.index}');
    print('displaying -- ${resultModel.displayingChildIndexList}');
  },
)

By default, ListView relevant data will only be observed when scrolling.

If needed, you can use ListObserverController triggered an observation manually.

// Create an instance of [ListObserverController]
ListObserverController controller = ListObserverController();
...

// Pass the controller instance to the 'controller' parameter of 'ListViewObserver'
ListViewObserver(
  ...
  controller: controller,
  ...
)
...

// Trigger an observation manually.
controller.dispatchOnceObserve();

Definition of dispatchOnceObserve method:

Take dispatchOnceObserve of ListObserverController as an example.

Future<ListViewOnceObserveNotificationResult> dispatchOnceObserve({
  BuildContext? sliverContext,
  bool isForce = false,
  bool isDependObserveCallback = true,
})

Parameter description of dispatchOnceObserve:

Parameter	Required	Description
sliverContext	no	The BuildContext of the scrollView is only used when CustomScrollView has multiple Sliver
isForce	no	Whether to enforce observation, equivalent to ObserverTriggerOnObserveType.directly
isDependObserveCallback	no	Whether it depends on judging whether onObserve and other callbacks are implemented. If true is passed, even if the corresponding callback is not implemented, the observation result can still be obtained

Its return value can directly get the result of observation!

Method 2: Specify BuildContext for Sliver

Relatively complex to use, the scope of application is small, there are more than one Sliver is possible to use this method

Detailed instructions

BuildContext? _sliverListViewContext;

Create a ListView and record BuildContext in its builder callback

ListView _buildListView() {
  return ListView.separated(
    itemBuilder: (ctx, index) {
      if (_sliverListViewContext != ctx) {
        _sliverListViewContext = ctx;
      }
      ...
    },
    ...
  );
}

Create ListViewObserver

ListViewObserver(
  child: _buildListView(),
  sliverListContexts: () {
    return [if (_sliverListViewContext != null) _sliverListViewContext!];
  },
  onObserve: (resultMap) {
    final model = resultMap[_sliverListViewContext];
    if (model == null) return;

    // Prints the first child widget index that is currently being displayed
    print('firstChild.index -- ${model.firstChild?.index}');

    // Prints the index of all child widgets those are currently being displayed
    print('displaying -- ${model.displayingChildIndexList}');
  },
)

By default, ListView relevant data will only be observed when scrolling.

If needed, you can use ListViewOnceObserveNotification triggered an observation manually.

ListViewOnceObserveNotification().dispatch(_sliverListViewContext);

1.1、Parameter autoTriggerObserveTypes

Used to set types those can trigger observe automatically, defined as follows:

final List<ObserverAutoTriggerObserveType>? autoTriggerObserveTypes;

enum ObserverAutoTriggerObserveType {
  scrollStart,
  scrollUpdate,
  scrollEnd,
}

Defaults to [.scrollStart, .scrollUpdate, .scrollEnd].

Description of enum values as follow：

value	Desc
scrollStart	when a [Scrollable] widget has started scrolling.
scrollUpdate	when a [Scrollable] widget has changed its scroll position.
scrollEnd	when a [Scrollable] widget has stopped scrolling.

1.2、Parameter triggerOnObserveType

Used to set the prerequisite for triggering [onObserve] callback and [onObserveAll] callback, defined as follows:

final ObserverTriggerOnObserveType triggerOnObserveType;

enum ObserverTriggerOnObserveType {
  directly,
  displayingItemsChange,
}

Defaults to .displayingItemsChange.

Description of enum values as follow：

value	Desc
directly	The callback onObserve will be called directly when getting observed result for scrollView.
displayingItemsChange	The callback onObserve will be called when child widget comes in and out or when the number of child widget changes.

1.3、Callback onObserveViewport

Only CustomScrollView is supported.

Used to observe which specified slivers are being displayed in the Viewport of CustomScrollView.

SliverViewObserver(
  child: _buildScrollView(),
  sliverContexts: () {
    return [
      if (grid1Context != null) grid1Context!,
      if (swipeContext != null) swipeContext!,
      if (grid2Context != null) grid2Context!,
    ];
  },
  onObserveViewport: (result) {
    firstChildCtxInViewport = result.firstChild.sliverContext;
    if (firstChildCtxInViewport == grid1Context) {
      debugPrint('current first sliver in viewport - gridView1');
    } else if (firstChildCtxInViewport == swipeContext) {
      debugPrint('current first sliver in viewport - swipeView');
    } else if (firstChildCtxInViewport == grid2Context) {
      debugPrint('current first sliver in viewport - gridView2');
    }
  },
)

1.4、Callback customTargetRenderSliverType

Only ListViewObserver and GridViewObserver are supported.

While maintaining the original observation logic, tell the package the RenderSliver to be observed, in order to support the observation of the list built by the third-party package.

customTargetRenderSliverType: (renderObj) {
  // Here you tell the package what type of RenderObject it needs to observe.
  return renderObj is ExtendedRenderSliverList;
},

1.5、Callback customHandleObserve

This callback is used to customize the observation logic and is used when the built-in observation logic does not meet your needs.

customHandleObserve: (context) {
  // Here you can customize the observation logic.
  final _obj = ObserverUtils.findRenderObject(context);
  if (_obj is RenderSliverList) {
    ObserverCore.handleListObserve(context: context);
  }
  if (_obj is RenderSliverGrid || _obj is RenderSliverWaterfallFlow) {
    return ObserverCore.handleGridObserve(context: context);
  }
  return null;
},

1.6、Callback extendedHandleObserve

Only SliverViewObserver is supported.

This callback is used to supplement the original observation logic, which originally only dealt with RenderSliverList, RenderSliverFixedExtentList and RenderSliverGrid.

extendedHandleObserve: (context) {
  // An extension of the original observation logic.
  final _obj = ObserverUtils.findRenderObject(context);
  if (_obj is RenderSliverWaterfallFlow) {
    return ObserverCore.handleGridObserve(context: context);
  }
  return null;
},

Model Property

ObserveModel

The base class of the observing data.

Property	Type	Desc
sliver	RenderSliver	The target sliver.
visible	bool	Whether this sliver should be painted.
displayingChildIndexList	List<int>	Stores index list for children widgets those are displaying.
axis	Axis	The axis of sliver.
scrollOffset	double	The scroll offset of sliver.

ListViewObserveModel

A special observing models which inherits from the ObserveModel for ListView and SliverList.

Property	Type	Desc
sliver	RenderSliverMultiBoxAdaptor	The target sliverList.
firstChild	ListViewObserveDisplayingChildModel	The observing data of the first child widget that is displaying.
displayingChildModelList	List<ListViewObserveDisplayingChildModel>	Stores observing model list of displaying children widgets.

GridViewObserveModel

A special observing models which inherits from the ObserveModel for GridView and SliverGrid.

Property	Type	Desc
sliverGrid	RenderSliverGrid	The target sliverGrid.
firstGroupChildList	List<GridViewObserveDisplayingChildModel>	The observing datas of first group displaying child widgets.
displayingChildModelList	List<GridViewObserveDisplayingChildModel>	Stores observing model list of displaying children widgets.

ObserveDisplayingChildModel

Data information about the child widget that is currently being displayed.

Property	Type	Desc
sliver	RenderSliver	The target sliverList.
index	int	The index of child widget.
renderObject	RenderBox	The renderObject [RenderBox] of child widget.

ObserveDisplayingChildModelMixin

The currently displayed child widgets data information, is for ObserveDisplayingChildModel supplement.

Property	Type	Desc
axis	Axis	The axis of sliver.
size	Size	The size of child widget.
mainAxisSize	double	The size of child widget on the main axis.
scrollOffset	double	The scroll offset of sliver.
layoutOffset	double	The layout offset of child widget.
leadingMarginToViewport	double	The margin from the top of the child widget to the viewport.
trailingMarginToViewport	double	The margin from the bottom of the child widget to the viewport.
displayPercentage	double	The display percentage of the current widget.