延迟加载

You can listen to a ScrollController.

ScrollController has some useful information, such as the scrolloffset and a list of ScrollPosition.

In your case the interesting part is in controller.position which is the currently visible ScrollPosition. Which represents a segment of the scrollable.

ScrollPosition contains informations about it's position inside the scrollable. Such as extentBefore and extentAfter. Or it's size, with extentInside.

Considering this, you could trigger a server call based on extentAfter which represents the remaining scroll space available.

Here's an basic example using what I said.

class MyHome extends StatefulWidget {
@override
_MyHomeState createState() => _MyHomeState();
}


class _MyHomeState extends State<MyHome> {
ScrollController controller;
List<String> items = List.generate(100, (index) => 'Hello $index');


@override
void initState() {
super.initState();
controller = ScrollController()..addListener(_scrollListener);
}


@override
void dispose() {
controller.removeListener(_scrollListener);
super.dispose();
}


@override
Widget build(BuildContext context) {
return Scaffold(
body: Scrollbar(
child: ListView.builder(
controller: controller,
itemBuilder: (context, index) {
return Text(items[index]);
},
itemCount: items.length,
),
),
);
}


void _scrollListener() {
print(controller.position.extentAfter);
if (controller.position.extentAfter < 500) {
setState(() {
items.addAll(List.generate(42, (index) => 'Inserted $index'));
});
}
}
}

You can clearly see that when reaching the end of the scroll, it scrollbar expends due to having loaded more items.

Thanks for Rémi Rousselet's approach, but it does not solve all the problem. Especially when the ListView has scrolled to the bottom, it still calls the scrollListener a couple of times. The improved approach is to combine Notification Listener with Remi's approach. Here is my solution:

bool _handleScrollNotification(ScrollNotification notification) {
if (notification is ScrollEndNotification) {
if (_controller.position.extentAfter == 0) {
loadMore();
}
}
return false;
}


@override
Widget build(BuildContext context) {
final Widget gridWithScrollNotification = NotificationListener<
ScrollNotification>(
onNotification: _handleScrollNotification,
child: GridView.count(
controller: _controller,
padding: EdgeInsets.all(4.0),
// Create a grid with 2 columns. If you change the scrollDirection to
// horizontal, this would produce 2 rows.
crossAxisCount: 2,
crossAxisSpacing: 2.0,
mainAxisSpacing: 2.0,
// Generate 100 Widgets that display their index in the List
children: _documents.map((doc) {
return GridPhotoItem(
doc: doc,
);
}).toList()));
return new Scaffold(
key: _scaffoldKey,
body: RefreshIndicator(
onRefresh: _handleRefresh, child: gridWithScrollNotification));
}

The solution use ScrollController and I saw comments mentioned about page.
I would like to share my finding about package incrementally_loading_listview https://github.com/MaikuB/incrementally_loading_listview.
As packaged said : This could be used to load paginated data received from API requests.

Basically, when ListView build last item and that means user has scrolled down to the bottom.
Hope it can help someone who have similar questions.

For purpose of demo, I have changed example to let a page only include one item and add an CircularProgressIndicator.

...
bool _loadingMore;
bool _hasMoreItems;
int  _maxItems = 30;
int  _numItemsPage = 1;
...
_hasMoreItems = items.length < _maxItems;
...
return IncrementallyLoadingListView(
hasMore: () => _hasMoreItems,
itemCount: () => items.length,
loadMore: () async {
// can shorten to "loadMore: _loadMoreItems" but this syntax is used to demonstrate that
// functions with parameters can also be invoked if needed
await _loadMoreItems();
},
onLoadMore: () {
setState(() {
_loadingMore = true;
});
},
onLoadMoreFinished: () {
setState(() {
_loadingMore = false;
});
},
loadMoreOffsetFromBottom: 0,
itemBuilder: (context, index) {
final item = items[index];
if ((_loadingMore ?? false) && index == items.length - 1) {
return Column(
children: <Widget>[
ItemCard(item: item),
Card(
child: Padding(
padding: const EdgeInsets.all(16.0),
child: Column(
children: <Widget>[
Row(
crossAxisAlignment:
CrossAxisAlignment.start,
children: <Widget>[
Container(
width: 60.0,
height: 60.0,
color: Colors.grey,
),
Padding(
padding: const EdgeInsets.fromLTRB(
8.0, 0.0, 0.0, 0.0),
child: Container(
color: Colors.grey,
child: Text(
item.name,
style: TextStyle(
color: Colors.transparent),
),
),
)
],
),
Padding(
padding: const EdgeInsets.fromLTRB(
0.0, 8.0, 0.0, 0.0),
child: Container(
color: Colors.grey,
child: Text(
item.message,
style: TextStyle(
color: Colors.transparent),
),
),
)
],
),
),
),
Center(child: CircularProgressIndicator())
],
);
}
return ItemCard(item: item);
},
);

full example https://github.com/MaikuB/incrementally_loading_listview/blob/master/example/lib/main.dart

Package use ListView index = last item and loadMoreOffsetFromBottom to detect when to load more.

    itemBuilder: (itemBuilderContext, index) {
if (!_loadingMore &&
index ==
widget.itemCount() -
widget.loadMoreOffsetFromBottom -
1 &&
widget.hasMore()) {
_loadingMore = true;
_loadingMoreSubject.add(true);
}

Use lazy_load_scrollview: 1.0.0 package that use same concept behind the scenes that panda world answered here. The package make it easier to implement.

The solutions posted don't solve the issue if you want to achieve lazy loading in up AND down direction. The scrolling would jump here, see this thread.

If you want to do lazy loading in up and down direction, the library bidirectional_listview could help.

Example (Source):

static const double kItemHeight = 30.0;
BidirectionalScrollController controller;
double oldScrollPosition = 0.0;


@override
void initState() {
super.initState();


for (int i = -10; i <= 10; i++) {
items[i] = "Item " + i.toString();
}


controller = new BidirectionalScrollController()
..addListener(_scrollListener);
}
@override
void dispose() {
controller.removeListener(_scrollListener);
super.dispose();
}


@override
void build() {
// ...
List<int> keys = items.keys.toList();
keys.sort();


return new BidirectionalListView.builder(
controller: controller,
physics: AlwaysScrollableScrollPhysics(),
itemBuilder: (context, index) {
return Container(
child: Text(items[index]),
height: kItemHeight,
},
itemCount: keys.first,
negativeItemCount: keys.last.abs(),
);
// ...
}


// Reload new items in up and down direction and update scroll boundaries
void _scrollListener() {
bool scrollingDown = oldScrollPosition < controller.position.pixels;
List<int> keys = items.keys.toList();
keys.sort();
int negativeItemCount = keys.first.abs();
int itemCount = keys.last;


double positiveReloadBorder = (itemCount * kItemHeight - 3 * kItemHeight);
double negativeReloadBorder =
(-(negativeItemCount * kItemHeight - 3 * kItemHeight));


// reload items
bool rebuildNecessary = false;
if (scrollingDown && controller.position.pixels > positiveReloadBorder)
{
for (int i = itemCount + 1; i <= itemCount + 20; i++) {
items[i] = "Item " + i.toString();
}
rebuildNecessary = true;
} else if (!scrollingDown &&
controller.position.pixels < negativeReloadBorder) {
for (int i = -negativeItemCount - 20; i < -negativeItemCount; i++) {
items[i] = "Item " + i.toString();
}
rebuildNecessary = true;
}


// set new scroll boundaries
try {
BidirectionalScrollPosition pos = controller.position;
pos.setMinMaxExtent(
-negativeItemCount * kItemHeight, itemCount * kItemHeight);
} catch (error) {
print(error.toString());
}
if (rebuildNecessary) {
setState(({});
}


oldScrollPosition = controller.position.pixels;
}

I hope that this helps a few people :-)

here is my approach which is inspired by answers above,

NotificationListener(onNotification: _onScrollNotification, child: GridView.builder())


bool _onScrollNotification(ScrollNotification notification) {
if (notification is ScrollEndNotification) {
final before = notification.metrics.extentBefore;
final max = notification.metrics.maxScrollExtent;


if (before == max) {
// load next page
// code here will be called only if scrolled to the very bottom
}
}
return false;
}

here is my solution for find end of listView

_scrollController.addListener(scrollListenerMilli);




if (_scrollController.position.pixels == _scrollController.position.maxScrollExtent) {
getMoreData();
}

If you want to load more data when 1/2 or 3/4 of a list view size, then use this way.

if (_scrollController.position.pixels == (_scrollController.position.maxScrollExtent * .75)) {//.5
getMoreData();
}

Additional -> Make sure you called getMore API only one time when reaching to the bottom. You can solve this in many ways, This is one of the ways to solve this by boolean variable.

bool loadMore = false;


if (_scrollController.position.pixels == _scrollController.position.maxScrollExtent && !loadMore) {
loadMore = true;
getMoreData().then(() => loadMore = false);
}

The accepted answer is correct but you can also do as follows,

Timer _timer;


Widget chatMessages() {
_timer = new Timer(const Duration(milliseconds: 300), () {
_scrollController.animateTo(
_scrollController.position.maxScrollExtent,
curve: Curves.easeOut,
duration: const Duration(milliseconds: 300),
);
});
return StreamBuilder(
stream: chats,
builder: (context, snapshot) {
return snapshot.hasData
? ListView.builder(
// physics: NeverScrollableScrollPhysics(),
controller: _scrollController,
shrinkWrap: true,
reverse: false,
itemCount: snapshot.data.documents.length,
itemBuilder: (context, index) {
return MessageTile(
message: snapshot.data.documents[index].data["message"],
sendByMe: widget.sendByid ==
snapshot.data.documents[index].data["sendBy"],
);
})
: Container();
},
);
}

There is also this package, taking away the boilerplate: https://pub.dev/packages/lazy_load_scrollview

There is a much simpler solution than working with Scroll Controllers and Notifications. Just use the built in lazy loading feature of ListView Builders:

I suggest (and tested) to just wrap two FutureBuilders within each other and let them handle everything for you. Alternatively, the outer FutureBuilder can be replaced by loading the values in the initState.

1. Create FutureBuilder to retrieve the most compact version of your data. Best a url or an id of the data items to be displayed

2. Create a ListView.builder, which according to the flutter doc Flutter Lists Codebook, already takes care of the lazy loading part

   The standard ListView constructor works well for small lists. To work with lists that contain a large number of items, it’s best to
   use the ListView.builder constructor.

   In contrast to the default ListView constructor, which requires creating all items at once, the ListView.builder() constructor
   creates items as they’re scrolled onto the screen.

3. Within the ListView builder, add another FutureBuilder, which fetches the individual content.

4. You're done

Have a look at this example code.

  @override
Widget build(BuildContext context) {
return FutureBuilder(
future: <get a short list of ids to fetch from the web>,
builder: (BuildContext context, AsyncSnapshot<List<int>> snapshot) {
if (snapshot.hasData) {
return Expanded(
child: ListView.builder(
itemCount: snapshot.data!.length,
itemBuilder: (BuildContext context, final int index) {
final int recordId = snapshot.data![index];
return FutureBuilder(
future: <get the record content from the web>,
builder: (BuildContext context,
AsyncSnapshot<Issue?> snapshot) {
if (snapshot.hasData) {
final Record? record = snapshot.data;
if (issue != null) {
return ListTile(
isThreeLine: true,
horizontalTitleGap: 0,
title: <build record widget>,
);
}
}
return ListTile(
isThreeLine: true,
horizontalTitleGap: 0,
title: const Text("Loading data..."));
});
}),
);
}
return const Text("Loading data...",
style: TextStyle(color: Colors.orange));
});

Let me know what you think. Performance was great when I've tried it, I'm wondering what you experienced with this. Sure, this needs some clean up, I know :D

This is an old question and the current answer is to use the ListView.builder method.

Same is true for the GridView.builder, please refer to the example below.

GridView.builder(
// ask GridView to cache and avoid redundant callings of Futures
cacheExtent: 100,
    

shrinkWrap: true,
  

itemCount: c.thumbnails.length,
  

// Define this as you like
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
crossAxisCount: 3,
mainAxisSpacing: 0.0,
crossAxisSpacing: 0.0,
childAspectRatio: 1.0,
),
itemBuilder: (BuildContext context, int index) {


return FutureBuilder<Image>(builder: (ctx, snap) {
if (!snap.hasData) {
return const SizedBox.expand(); // show nothing
}
if (snap.hasError) {
return Text('An error occured ${snap.error}');
}
return snap.data!;
},
future: <YOUR THUMBNAIL FUTURE>,
);
}
);

You can handle it by knowing the current page and the last page By using listview builder

itemBuilder: (context, index) {
if(list.length - 1 == index && currentPage! < lastPage!){
currentPage = currentPage! + 1;
/// Call your api here to update the list
return Progress();
}
return ///element widget here.
},