scroll_snap_list

A wrapper for ListView.builder widget that allows "snaping" event to an item at the end of user-scroll.

This widget allows unrestricted scrolling (unlike other widget that only able to "snap" to left/right neighbor at a time).

  • Support horizontal & vertical list
  • Use ListView.Builder, means no Animation

You can use this widget to achieve:

  • Unrestricted scrollable Jumbotron
  • Show daily entry for Health App and shows its detail directly below the list (reduce number of click)
  • Scrollable InputSelect, like datetime picker in cupertino
Jumbotron ListHorizontal ListVertical ListDynamic SizeCustom Dynamic Size
Horizontal Jumbotron ListHorizontal List with infinite-loadingVertical list with InkWell ListItemsHorizontal with dynamic item sizeHorizontal with custom dynamic item size

Getting Started

In your flutter project pubspec.yaml add the dependency:

dependencies:
  ...
  scroll_snap_list: ^[version]

Usage example

This library doesn't use other library, so it should work out of the box.

Import scroll_snap_list.dart.

import 'package:scroll_snap_list/scroll_snap_list.dart';

Add ScrollSnapList in your build method. Make sure to enter correct itemSize (helps with "snapping" process).

Expanded(
  child: ScrollSnapList(
    onItemFocus: _onItemFocus,
    itemSize: 35,
    itemBuilder: _buildListItem,
    itemCount: data.length,
    reverse: true,
  ),
),

Full example:

import 'dart:math';

import 'package:flutter/material.dart';
import 'package:scroll_snap_list/scroll_snap_list.dart';

void main() => runApp(HorizontalListDemo());

class HorizontalListDemo extends StatefulWidget {
  @override
  _HorizontalListDemoState createState() => _HorizontalListDemoState();
}

class _HorizontalListDemoState extends State<HorizontalListDemo> {
  List<int> data = [];
  int _focusedIndex = 0;

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

    for (int i = 0; i < 30; i++) {
      data.add(Random().nextInt(100) + 1);
    }
  }

  void _onItemFocus(int index) {
    setState(() {
      _focusedIndex = index;
    });
  }

  Widget _buildItemDetail() {
    if (data.length > _focusedIndex)
      return Container(
        height: 150,
        child: Text("index $_focusedIndex: ${data[_focusedIndex]}"),
      );

    return Container(
      height: 150,
      child: Text("No Data"),
    );
  }

  Widget _buildListItem(BuildContext context, int index) {
    //horizontal
    return Container(
      width: 35,
      child: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        children: <Widget>[
          Container(
            height: data[index].toDouble()*2,
            width: 25,
            color: Colors.lightBlueAccent,
            child: Text("i:$index\n${data[index]}"),
          )
        ],
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Horizontal List Demo',
      home: Scaffold(
        appBar: AppBar(
          title: Text("Horizontal List"),
        ),
        body: Container(
          child: Column(
            children: <Widget>[
              Expanded(
                child: ScrollSnapList(
                  onItemFocus: _onItemFocus,
                  itemSize: 35,
                  itemBuilder: _buildListItem,
                  itemCount: data.length,
                  reverse: true,
                ),
              ),
              _buildItemDetail(),
              Row(
                mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                children: <Widget>[
                  RaisedButton(
                    child: Text("Add Item"),
                    onPressed: () {
                      setState(() {
                        data.add(Random().nextInt(100) + 1);
                      });
                    },
                  ),
                  RaisedButton(
                    child: Text("Remove Item"),
                    onPressed: () {
                      int index = data.length > 1
                          ? Random().nextInt(data.length - 1)
                          : 0;
                      setState(() {
                        data.removeAt(index);
                      });
                    },
                  ),
                ],
              )
            ],
          ),
        ),
      ),
    );
  }
}

Usage with dynamic item size

This feature is added since version 0.4.0.

We can customize item size with respect to distance between item and center of ScrollSnapList.

@override
Widget build(BuildContext context) {
  return MaterialApp(
    title: 'Horizontal List Demo',
    home: Scaffold(
      appBar: AppBar(
        title: Text("Horizontal List"),
      ),
      body: Container(
        child: Column(
          children: <Widget>[
            Expanded(
              child: ScrollSnapList(
                onItemFocus: _onItemFocus,
                itemSize: 150,
                itemBuilder: _buildListItem,
                itemCount: data.length,
                dynamicItemSize: true,
                // dynamicSizeEquation: customEquation, //optional
              ),
            ),
            _buildItemDetail(),
          ],
        ),
      ),
    ),
  );
}

Important parameters and explanation

ParameterExplanation
keyKey for ScrollSnapList, used to call ScrollSnapListState
listViewKeyKey for ListView inside ScrollSnapListState, often used to preserve scroll location
itemBuilderSame as ListView's itemBuilder
curveAnimation curve when snapping
durationAnimation duration
endOfListTolerancePixel tolerance to trigger onReachEnd
focusOnItemTapFocus to an item when user tap on it. Inactive if the list-item have its own onTap detector (use state-key to help focusing instead).
focusToItemMethod to manually trigger focus to an item. Call with help of GlobalKey<ScrollSnapListState>
marginContainer's margin
itemCountNumber of item in this list
itemSizeSize used is width if scrollDirection is Axis.horizontal, height if Axis.vertical. Composed of the size of each item + its margin/padding.
onItemFocusCallback function when list snaps/focuses to an item
onReachEndCallback function when user reach end of list. E.g. load more data from db
reverseSame as ListView's reverse to reverse scrollDirection
updateOnScrollCalls onItemFocus (if it exists) when ScrollUpdateNotification fires
initialIndexOptional initial position which will not snap until after the first drag
scrollDirectionListView's scrollDirection
listControllerExternal ScrollController
dynamicItemSizeScale item's size depending on distance to center
dynamicSizeEquationCustom equation to determine dynamic item scaling calculation
dynamicItemOpacityCustom opacity for offset item (single value)

Other Notice

By default, SnapScrollList set focusOnItemTap as true. Means any tap event on items will trigger snap/focus event to that item.

This behavior use GestureDetector, so if the list's items have their own detector (e.g. InkWell, Button), SnapScrollList unable to trigger snap event on its own. To handle this, we may use GlobalKey<ScrollSnapListState> to trigger the event manually.

Add GlobalKey<ScrollSnapListState> in your widget

GlobalKey<ScrollSnapListState> sslKey = GlobalKey();

Add key parameter to SnapScrollList.

ScrollSnapList(
  onItemFocus: _onItemFocus,
  itemSize: 50,
  itemBuilder: _buildListItem,
  itemCount: data.length,
  key: sslKey,
  scrollDirection: Axis.vertical,
)

In your buildItem method, call focusToItem method.

Widget _buildListItem(BuildContext context, int index) {
  //horizontal
  return Container(
    height: 50,
    child: Material(
      color: _focusedIndex == index ? Colors.lightBlueAccent : Colors.white,
      child: InkWell(
        child: Text("Index: $index | Value: ${data[index]}"),
        onTap: () {
          print("Do anything here");

          //trigger focus manually
          sslKey.currentState.focusToItem(index);
        },
      ),
    ),
  );
}

Full example for this part can be found in example/vertical_list.dart.

About

Created by Vincent (MSVCode) Email: msvcode@gmail.com

Libraries

scroll_snap_list