PageView with CacheExtent - by Simon Lightfoot - 15/01/2020

page_view_with_cache.dart

// MIT License
//
// Copyright (c) 2020 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart' show ViewportOffset;

class PageViewWithCacheExtent extends StatelessWidget {
  const PageViewWithCacheExtent({
    Key key,
    @required this.controller,
    @required this.children,
    this.onPageChanged,
    this.cachedPages = 0,
    this.physics,
  })  : assert(controller != null),
        assert(children != null),
        super(key: key);

  final PageController controller;
  final ValueChanged<int> onPageChanged;
  final List<Widget> children;
  final int cachedPages;
  final ScrollPhysics physics;

  @override
  Widget build(BuildContext context) {
    return NotificationListener<ScrollNotification>(
      onNotification: (ScrollNotification notification) {
        if (notification.depth == 0 && notification is ScrollUpdateNotification) {
          final PageMetrics metrics = notification.metrics as PageMetrics;
          onPageChanged?.call(metrics.page.round());
        }
        return false;
      },
      child: LayoutBuilder(
        builder: (BuildContext context, BoxConstraints constraints) {
          return Scrollable(
            axisDirection: AxisDirection.right,
            controller: controller,
            physics: const PageScrollPhysics().applyTo(physics),
            viewportBuilder: (BuildContext context, ViewportOffset position) {
              final cacheExtent = constraints.maxWidth * controller.viewportFraction * cachedPages;
              return Viewport(
                axisDirection: AxisDirection.right,
                offset: position,
                cacheExtent: cacheExtent,
                slivers: <Widget>[
                  SliverFillViewport(
                    viewportFraction: controller.viewportFraction,
                    delegate: SliverChildListDelegate(children),
                  ),
                ],
              );
            },
          );
        },
      ),
    );
  }
}