如此简单！？一行代码搞定Flutter的PageView预加载

Android的ViewPager2在更新后的好用程度自是不用多说，我们只需要设置offScreenPageLimit，就可以轻松实现预加载的效果。但尴尬的是——我们如果不希望它有预加载，就比较困难了；与之相反的是，Flutter的PageView却没有类似的属性来设置，每当我们切换到新的页面，child都会重新走一遍build的流程，那么，我们该怎么实现类似预加载的效果呢？

前置讨论

从源码入手

• 回想一下SliverList/ListView的创建过程，我们在使用ListView.builder的时候不仅实现了按需加载，还能实现一定范围内的预加载。ListView有个cacheExtent属性，就是用来控制预加载的范围大小。
• 那么：根据我们的需求，我们是否可以创建一个可以横向滑动的列表来实现预加载效果？
• 这是我一开始预想的方法，但并未实现（可能在之后有时间会写写）。我更希望在最小的修改范围内实现需求。当我对比ListView和PageView的实现后发现：PageView在创建Viewport时，因为考虑了allowImplicitScrolling属性，实际上没办法兼顾预加载和辅助功能滑动问题
• 相关代码

    ...
  @override
  Widget build(BuildContext context) {
    final AxisDirection axisDirection = _getDirection(context);
    final ScrollPhysics physics = _ForceImplicitScrollPhysics(
      allowImplicitScrolling: widget.allowImplicitScrolling,
    ).applyTo(
      widget.pageSnapping
        ? _kPagePhysics.applyTo(widget.physics ?? widget.scrollBehavior?.getScrollPhysics(context))
        : widget.physics ?? widget.scrollBehavior?.getScrollPhysics(context),
    );
    return NotificationListener(
      onNotification: (ScrollNotification notification) {
        if (notification.depth == 0 && widget.onPageChanged != null && notification is ScrollUpdateNotification) {
          final PageMetrics metrics = notification.metrics as PageMetrics;
          final int currentPage = metrics.page!.round();
          if (currentPage != _lastReportedPage) {
            _lastReportedPage = currentPage;
            widget.onPageChanged!(currentPage);
          }
        }
        return false;
      },
      child: Scrollable(
        dragStartBehavior: widget.dragStartBehavior,
        axisDirection: axisDirection,
        controller: _controller,
        physics: physics,
        restorationId: widget.restorationId,
        scrollBehavior: widget.scrollBehavior ?? ScrollConfiguration.of(context).copyWith(scrollbars: false),
        viewportBuilder: (BuildContext context, ViewportOffset position) {
          return Viewport(
            cacheExtent: widget.allowImplicitScrolling ? 1.0 : 0.0,
            cacheExtentStyle: CacheExtentStyle.viewport,
            axisDirection: axisDirection,
            offset: position,
            clipBehavior: widget.clipBehavior,
            slivers: [
              SliverFillViewport(
                viewportFraction: _controller.viewportFraction,
                delegate: widget.childrenDelegate,
                padEnds: widget.padEnds,
              ),
            ],
          );
        },
      ),
    );
  }
    ...

AllowImplicitScrolling

• 我们在package:flutter/lib/src/widgets/scroll_physics.dart中溯源这个对象的解释，可以在注释中看到如下解释：
  • Whether a viewport is allowed to change its scroll position implicitly in response to a call to [RenderObject.showOnScreen]
  • [RenderObject.showOnScreen] is for example used to bring a text field fully on screen after it has received focus. This property controls whether the viewport associated with this object is allowed to change the scroll position to fulfill such a request.
• 实际上这个属性的设置是为了考虑辅助功能下PageView滑动时焦点的获取问题，但是正因为有了这个属性，我们就无法实现cacheExtent的自定义

解决办法

不考虑辅助功能

• 如果我们的业务中不考虑这个属性的问题，自然可以直接设置cacheExtent来设置预加载的页数。我们复制PageView的源码，新建一个PreloadPageView，对外暴露preloadPagesCount属性，修改Viewport部分，一行代码就可以满足我们的需求:
• preload_page_view.dart

import 'dart:math' as math;

import 'package:flutter/foundation.dart' show clampDouble, precisionErrorTolerance;
import 'package:flutter/gestures.dart' show DragStartBehavior;
import 'package:flutter/rendering.dart';
import 'package:flutter/widgets.dart';
const PageScrollPhysics _kPagePhysics = PageScrollPhysics();
class _ForceImplicitScrollPhysics extends ScrollPhysics {
  const _ForceImplicitScrollPhysics({
    required this.allowImplicitScrolling,
    super.parent,
  });

  @override
  _ForceImplicitScrollPhysics applyTo(ScrollPhysics? ancestor) {
    return _ForceImplicitScrollPhysics(
      allowImplicitScrolling: allowImplicitScrolling,
      parent: buildParent(ancestor),
    );
  }

  @override
  final bool allowImplicitScrolling;
}








































class PreloadPageView extends StatefulWidget {
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  PreloadPageView({
    super.key,
    this.scrollDirection = Axis.horizontal,
    this.reverse = false,
    this.preloadPagesCount = 1,
    this.controller,
    this.physics,
    this.pageSnapping = true,
    this.onPageChanged,
    List children = const [],
    this.dragStartBehavior = DragStartBehavior.start,
    this.allowImplicitScrolling = false,
    this.restorationId,
    this.clipBehavior = Clip.hardEdge,
    this.scrollBehavior,
    this.padEnds = true,
  }) : childrenDelegate = SliverChildListDelegate(children);

  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  PreloadPageView.builder({
    super.key,
    this.scrollDirection = Axis.horizontal,
    this.reverse = false,
    this.controller,
    this.physics,
    this.preloadPagesCount = 1,
    this.pageSnapping = true,
    this.onPageChanged,
    required NullableIndexedWidgetBuilder itemBuilder,
    ChildIndexGetter? findChildIndexCallback,
    int? itemCount,
    this.dragStartBehavior = DragStartBehavior.start,
    this.allowImplicitScrolling = false,
    this.restorationId,
    this.clipBehavior = Clip.hardEdge,
    this.scrollBehavior,
    this.padEnds = true,
  }) : childrenDelegate = SliverChildBuilderDelegate(
    itemBuilder,
    findChildIndexCallback: findChildIndexCallback,
    childCount: itemCount,
  );

  
  
  
  
  
  
  
  
  
  
  
  const PreloadPageView.custom({
    super.key,
    this.scrollDirection = Axis.horizontal,
    this.reverse = false,
    this.controller,
    this.physics,
    this.preloadPagesCount = 1,
    this.pageSnapping = true,
    this.onPageChanged,
    required this.childrenDelegate,
    this.dragStartBehavior = DragStartBehavior.start,
    this.allowImplicitScrolling = false,
    this.restorationId,
    this.clipBehavior = Clip.hardEdge,
    this.scrollBehavior,
    this.padEnds = true,
  });


  final int preloadPagesCount;
  
  
  
  
  
  
  
  
  
  
  
  final bool allowImplicitScrolling;

  
  final String? restorationId;

  
  
  
  
  
  
  final Axis scrollDirection;

  
  
  
  
  
  
  
  
  
  
  
  
  final bool reverse;

  
  
  final PageController? controller;

  
  
  
  
  
  
  
  
  
  
  
  
  
  final ScrollPhysics? physics;

  
  
  
  
  
  final bool pageSnapping;

  
  final ValueChanged<int>? onPageChanged;

  
  
  
  
  
  
  final SliverChildDelegate childrenDelegate;

  
  final DragStartBehavior dragStartBehavior;

  
  
  
  final Clip clipBehavior;

  
  
  
  
  
  
  
  
  
  final ScrollBehavior? scrollBehavior;

  
  
  
  
  
  
  
  
  
  final bool padEnds;

  @override
  State createState() => _PreloadPageViewState();
}

class _PreloadPageViewState extends State<PreloadPageView> {
  int _lastReportedPage = 0;

  late PageController _controller;

  @override
  void initState() {
    super.initState();
    _initController();
    _lastReportedPage = _controller.initialPage;
  }

  @override
  void dispose() {
    if (widget.controller == null) {
      _controller.dispose();
    }
    super.dispose();
  }


  void _initController() {
    _controller = widget.controller ?? PageController();
  }

  @override
  void didUpdateWidget(PreloadPageView oldWidget) {
    if (oldWidget.controller != widget.controller) {
      if (oldWidget.controller == null) {
        _controller.dispose();
      }
      _initController();
    }
    super.didUpdateWidget(oldWidget);
  }

  AxisDirection _getDirection(BuildContext context) {
    switch (widget.scrollDirection) {
      case Axis.horizontal:
        assert(debugCheckHasDirectionality(context));
        final TextDirection textDirection = Directionality.of(context);
        final AxisDirection axisDirection = textDirectionToAxisDirection(textDirection);
        return widget.reverse ? flipAxisDirection(axisDirection) : axisDirection;
      case Axis.vertical:
        return widget.reverse ? AxisDirection.up : AxisDirection.down;
    }
  }

  @override
  Widget build(BuildContext context) {
    final AxisDirection axisDirection = _getDirection(context);
    final ScrollPhysics physics = _ForceImplicitScrollPhysics(
      allowImplicitScrolling: widget.allowImplicitScrolling,
    ).applyTo(
      widget.pageSnapping
          ? _kPagePhysics.applyTo(widget.physics ?? widget.scrollBehavior?.getScrollPhysics(context))
          : widget.physics ?? widget.scrollBehavior?.getScrollPhysics(context),
    );

    return NotificationListener(
      onNotification: (ScrollNotification notification) {
        if (notification.depth == 0 && widget.onPageChanged != null && notification is ScrollUpdateNotification) {
          final PageMetrics metrics = notification.metrics as PageMetrics;
          final int currentPage = metrics.page!.round();
          if (currentPage != _lastReportedPage) {
            _lastReportedPage = currentPage;
            widget.onPageChanged!(currentPage);
          }
        }
        return false;
      },
      child: Scrollable(
        dragStartBehavior: widget.dragStartBehavior,
        axisDirection: axisDirection,
        controller: _controller,
        physics: physics,
        restorationId: widget.restorationId,
        scrollBehavior: widget.scrollBehavior ?? ScrollConfiguration.of(context).copyWith(scrollbars: false),
        viewportBuilder: (BuildContext context, ViewportOffset position) {
          return Viewport(
            
            
            
            cacheExtent: widget.preloadPagesCount < 1
                ? 0
                : (widget.preloadPagesCount == 1
                ? 1
                : widget.scrollDirection == Axis.horizontal
                ? MediaQuery.of(context).size.width *
                widget.preloadPagesCount -
                1
                : MediaQuery.of(context).size.height *
                widget.preloadPagesCount -
                1),
            cacheExtentStyle: CacheExtentStyle.viewport,
            axisDirection: axisDirection,
            offset: position,
            clipBehavior: widget.clipBehavior,
            slivers: [
              SliverFillViewport(
                viewportFraction: _controller.viewportFraction,
                delegate: widget.childrenDelegate,
                padEnds: widget.padEnds,
              ),
            ],
          );
        },
      ),
    );
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder description) {
    super.debugFillProperties(description);
    description.add(EnumProperty('scrollDirection', widget.scrollDirection));
    description.add(FlagProperty('reverse', value: widget.reverse, ifTrue: 'reversed'));
    description.add(DiagnosticsProperty('controller', _controller, showName: false));
    description.add(DiagnosticsProperty('physics', widget.physics, showName: false));
    description.add(FlagProperty('pageSnapping', value: widget.pageSnapping, ifFalse: 'snapping disabled'));
    description.add(FlagProperty('allowImplicitScrolling', value: widget.allowImplicitScrolling, ifTrue: 'allow implicit scrolling'));
  }
}

  @override
  Widget build(BuildContext context) {
    final AxisDirection axisDirection = _getDirection(context);
    final ScrollPhysics physics = _ForceImplicitScrollPhysics(
      allowImplicitScrolling: widget.allowImplicitScrolling,
    ).applyTo(
      widget.pageSnapping
          ? _kPagePhysics.applyTo(widget.physics ?? widget.scrollBehavior?.getScrollPhysics(context))
          : widget.physics ?? widget.scrollBehavior?.getScrollPhysics(context),
    );

    return NotificationListener(
      onNotification: (ScrollNotification notification) {
        if (notification.depth == 0 && widget.onPageChanged != null && notification is ScrollUpdateNotification) {
          final PageMetrics metrics = notification.metrics as PageMetrics;
          final int currentPage = metrics.page!.round();
          if (currentPage != _lastReportedPage) {
            _lastReportedPage = currentPage;
            widget.onPageChanged!(currentPage);
          }
        }
        return false;
      },
      child: Scrollable(
        dragStartBehavior: widget.dragStartBehavior,
        axisDirection: axisDirection,
        controller: _controller,
        physics: physics,
        restorationId: widget.restorationId,
        scrollBehavior: widget.scrollBehavior ?? ScrollConfiguration.of(context).copyWith(scrollbars: false),
        viewportBuilder: (BuildContext context, ViewportOffset position) {
          return Viewport(
            
            cacheExtent: widget.preloadPagesCount < 1
                ? 0
                : (widget.preloadPagesCount == 1
                ? 1
                : widget.scrollDirection == Axis.horizontal
                ? MediaQuery.of(context).size.width *
                widget.preloadPagesCount -
                1
                : MediaQuery.of(context).size.height *
                widget.preloadPagesCount -
                1),
            cacheExtentStyle: CacheExtentStyle.viewport,
            axisDirection: axisDirection,
            offset: position,
            clipBehavior: widget.clipBehavior,
            slivers: [
              SliverFillViewport(
                viewportFraction: _controller.viewportFraction,
                delegate: widget.childrenDelegate,
                padEnds: widget.padEnds,
              ),
            ],
          );
        },
      ),
    );
  }

修改页面大小

• 这个方法在我的app里并不适用，但是它可能有用。PageController中提供了viewportFraction来设置当前页面的占比大小，如果我们将其设置为一个小于1的数，两侧的页面就会被加载

...
late PageController _pageController;
...
 _pageController = PageController(viewportFraction: 0.99);

最后

• PageView的预加载问题在笔者目前的版本中仍是一个TODO，Flutter团队暂时没有找到更好的办法来解决这个问题，如果使用ListView相似的方法构建PageView，可能会解决这个问题？