最近Flutter项目开发中，用到了PageView的上下滚动，但是正常的PageView的使用，onPageChanged的回调，在页面滑到中间的时候，就会把下一个页面的pageIndex值传递过来。但是实际需求中，我需要知道什么时候页面滑动结束，这时候才去执行页面完全展示的方法。所以就需要对PageView进行自定义改造。
一、尝试了使用系统自带的PageController进行滑动监听，失败
结果发现，返回的offset和page是Double类型的，更坑爹的是，在某些手机或者滑动距离过长时，返回的数据不够精准，没办法准确判断是否滑动完成。一看不能用，没办法在系统的api里面找到解决办法，就只能看源码了
二、查看PageView的源码，自定义pageView
没办法，只能查看系统源码，看看为什么会出现onPageChanged在中间的时候就返回回调了。在查看源码过程中，发现以下神奇的地方
根据源码我们能看到，PageView在实际上也是通过NotificationListener进行的监听，而且在onPageChanged的返回前，判断了notification is ScrollUpdateNotification。
ScrollUpdateNotification是什么呢？这就需要我们看一下NotificationListener的回调监听了，也就是返回的ScrollNotification这个类。
点击进去看到ScrollUpdateNotification继承ScrollNotification，那是不是还有其他的监听回调类型，可以实现我们的需求呢？果然最后源码中发现，还有ScrollStartNotification、OverscrollNotification、ScrollEndNotification和UserScrollNotification四种滑动回调类型！
既然我们想要的是滑动完成的回调，就只需要在NotificationListener判断ScrollEndNotification这个滑动类型就行了。
全部代码如下：
```
// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'dart:async';
import 'dart:math' as math;
import 'package:flutter/cupertino.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/gestures.dart' show DragStartBehavior;
import 'package:flutter/foundation.dart' show precisionErrorTolerance;
///自定义的PageView
///新增了页面加载开始和结束的回调
final PageController _defaultPageController =PageController();
const PageScrollPhysics _kPagePhysics =PageScrollPhysics();
class CustomPageViewextends StatefulWidget {
/// Creates a scrollable list that works page by page from an explicit [List]
/// of widgets.
///
/// This constructor is appropriate for page views with a small number of
/// children because constructing the [List] requires doing work for every
/// child that could possibly be displayed in the page view, instead of just
/// those children that are actually visible.
CustomPageView({
Key key,
this.scrollDirection = Axis.horizontal,
this.reverse =false,
PageController controller,
this.physics,
this.pageSnapping =true,
this.onPageChanged,
this.onPageEndChanged,
this.onPageStartChanged,
List children =const [],
this.dragStartBehavior = DragStartBehavior.start,
}) :controller = controller ?? _defaultPageController,
childrenDelegate =SliverChildListDelegate(children),
super(key: key);
/// Creates a scrollable list that works page by page using widgets that are
/// created on demand.
///
/// This constructor is appropriate for page views with a large (or infinite)
/// number of children because the builder is called only for those children
/// that are actually visible.
///
/// Providing a non-null [itemCount] lets the [CustomPageView] compute the maximum
/// scroll extent.
///
/// [itemBuilder] will be called only with indices greater than or equal to
/// zero and less than [itemCount].
///
/// [CustomPageView.builder] by default does not support child reordering. If
/// you are planning to change child order at a later time, consider using
/// [CustomPageView] or [CustomPageView.custom].
CustomPageView.builder({
Key key,
this.scrollDirection = Axis.horizontal,
this.reverse =false,
PageController controller,
this.physics,
this.pageSnapping =true,
this.onPageChanged,
this.onPageEndChanged,
this.onPageStartChanged,
@required IndexedWidgetBuilder itemBuilder,
int itemCount,
this.dragStartBehavior = DragStartBehavior.start,
}) :controller = controller ?? _defaultPageController,
childrenDelegate =
SliverChildBuilderDelegate(itemBuilder, childCount: itemCount),
super(key: key);
CustomPageView.custom({
Key key,
this.scrollDirection = Axis.horizontal,
this.reverse =false,
PageController controller,
this.physics,
this.pageSnapping =true,
this.onPageChanged,
this.onPageEndChanged,
this.onPageStartChanged,
@required this.childrenDelegate,
this.dragStartBehavior = DragStartBehavior.start,
}) :assert(childrenDelegate !=null),
controller = controller ?? _defaultPageController,
super(key: key);
/// The axis along which the page view scrolls.
///
/// Defaults to [Axis.horizontal].
final AxisscrollDirection;
/// Whether the page view scrolls in the reading direction.
///
/// For example, if the reading direction is left-to-right and
/// [scrollDirection] is [Axis.horizontal], then the page view scrolls from
/// left to right when [reverse] is false and from right to left when
/// [reverse] is true.
///
/// Similarly, if [scrollDirection] is [Axis.vertical], then the page view
/// scrolls from top to bottom when [reverse] is false and from bottom to top
/// when [reverse] is true.
///
/// Defaults to false.
final boolreverse;
/// An object that can be used to control the position to which this page
/// view is scrolled.
final PageControllercontroller;
/// How the page view should respond to user input.
///
/// For example, determines how the page view continues to animate after the
/// user stops dragging the page view.
///
/// The physics are modified to snap to page boundaries using
/// [PageScrollPhysics] prior to being used.
///
/// Defaults to matching platform conventions.
final ScrollPhysicsphysics;
/// Set to false to disable page snapping, useful for custom scroll behavior.
final boolpageSnapping;
/// Called whenever the page in the center of the viewport changes.
final ValueChangedonPageChanged;
final ValueChangedonPageEndChanged;
final ValueChangedonPageStartChanged;
/// A delegate that provides the children for the [CustomPageView].
///
/// The [CustomPageView.custom] constructor lets you specify this delegate
/// explicitly. The [CustomPageView] and [CustomPageView.builder] constructors create a
/// [childrenDelegate] that wraps the given [List] and [IndexedWidgetBuilder],
/// respectively.
final SliverChildDelegatechildrenDelegate;
/// {@macro flutter.widgets.scrollable.dragStartBehavior}
final DragStartBehaviordragStartBehavior;
@override
_CustomPageViewStatecreateState() =>_CustomPageViewState();
}
class _CustomPageViewStateextends State {
int_lastReportedPage =0;
int_currentReportedPage =0;
@override
void initState() {
super.initState();
_lastReportedPage =widget.controller.initialPage;
}
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;
}
return null;
}
@override
Widgetbuild(BuildContext context) {
final AxisDirection axisDirection = _getDirection(context);
final ScrollPhysics physics =widget.pageSnapping
? _kPagePhysics.applyTo(widget.physics)
:widget.physics;
return NotificationListener(
onNotification: (ScrollNotification notification) {
///滑动开始
if (notificationis ScrollStartNotification) {
if (widget.onPageStartChanged !=null) {
widget.onPageStartChanged(_currentReportedPage);
}
//print('返回的开始的页码=' + _currentReportedPage.toString());
}
///滑动中
if (notification.depth ==0 &&
notificationis ScrollUpdateNotification) {
final PageMetrics metrics = notification.metrics;
final int currentPage = metrics.page.round();
_currentReportedPage = currentPage;
//print('外部返回的页码=' + currentPage.toString());
//print('外部返回的pixels=' + metrics.pixels.toString());
if (currentPage !=_lastReportedPage) {
_lastReportedPage = currentPage;
if (widget.onPageChanged !=null) {
widget.onPageChanged(currentPage);
}
//print('返回的页码=' + currentPage.toString());
}
}
///滑动结束
if (notification.depth ==0 && notificationis ScrollEndNotification) {
if (widget.onPageEndChanged !=null) {
widget.onPageEndChanged(_currentReportedPage);
}
print('返回的结束的页码=' +_currentReportedPage.toString());
}
return false;
},
child:Scrollable(
dragStartBehavior:widget.dragStartBehavior,
axisDirection: axisDirection,
controller:widget.controller,
physics: physics,
viewportBuilder: (BuildContext context, ViewportOffset position) {
return Viewport(
cacheExtent:0.0,
axisDirection: axisDirection,
offset: position,
slivers: [
SliverFillViewport(
viewportFraction:widget.controller.viewportFraction,
delegate:widget.childrenDelegate,
),
],
);
},
),
);
}
@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', widget.controller,
showName:false));
description.add(DiagnosticsProperty(
'physics', widget.physics,
showName:false));
description.add(FlagProperty('pageSnapping',
value:widget.pageSnapping, ifFalse:'snapping disabled'));
}
}
```
