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Android?Flutter實現上拉加載組件的示例代碼_Android

作者：JulyYu ? 更新時間： 2022-10-24 編程語言

前言

在此之前對列表下拉刷新做了調整方案，具體介紹可以閱讀下拉刷新組件交互調整。既然列表有下拉刷新外當然還有上拉加載更多操作了，本次就來介紹如何為列表增加上拉加載更多的交互實現。

實現方案

上拉刷新實現形式上可以有多種實現方式，若不帶交互形式可采用NotificationListener組件監聽滑動來實現上拉加載更多；如果對操作交互上有一定要求期望上拉刷新帶有直觀動畫可操作性就需要實現一定樣式來實現了。

監聽NotificationListener實現

NotificationListener(
      onNotification: (scrollNotification) {
        if (scrollNotification.metrics.pixels >=
                scrollNotification.metrics.maxScrollExtent - size.height &&
            scrollNotification.depth == 0) {
          if (!isLoading) {
            isLoading = true;
            Future.delayed(Duration(seconds: 1), () {
              isLoading = false;
              length += 10;
              Scaffold.of(context).showSnackBar(SnackBar(
                content: Text('下拉加載更多了！！！'),
                duration: Duration(milliseconds: 700),
              ));
              setState(() {});
            });
          }
        }
        return false;
      }
  ......
  )

NotificationListener增加樣式

     SliverToBoxAdapter(
       child: Center(
         child: Text(
           length < 30 ? "加載更多...." : "沒有更多",
           style: TextStyle(fontSize: 25),
         ),
       ),
     )

ScrollPhysics調整

BouncingScrollPhysics是iOS帶有阻尼效果滑動交互，在下拉刷新中帶有回彈阻尼效果是比較好的交互，但在上拉加載更多獲取交互上這種效果或許有點多余。因此需要定制下拉刷新帶有回彈阻尼效果，上拉加載沒有回彈阻尼效果的。ScrollPhysics

CustomScrollView(
   physics: BouncingScrollPhysics(),
   slivers: <Widget>[]
)

具體實現代碼如下所示：

class CustomBouncingScrollPhysics extends ScrollPhysics {
  const CustomBouncingScrollPhysics({ ScrollPhysics parent }) : super(parent: parent);

  @override
  CustomBouncingScrollPhysics applyTo(ScrollPhysics ancestor) {
    return CustomBouncingScrollPhysics(parent: buildParent(ancestor));
  }

  double frictionFactor(double overscrollFraction) => 0.52 * math.pow(1 - overscrollFraction, 2);



  /// 阻尼參數計算
  @override
  double applyPhysicsToUserOffset(ScrollMetrics position, double offset) {
    assert(offset != 0.0);
    assert(position.minScrollExtent <= position.maxScrollExtent);

    if (!position.outOfRange)
      return offset;

    final double overscrollPastStart = math.max(position.minScrollExtent - position.pixels, 0.0);
    final double overscrollPastEnd = math.max(position.pixels - position.maxScrollExtent, 0.0);
    final double overscrollPast = math.max(overscrollPastStart, overscrollPastEnd);
    final bool easing = (overscrollPastStart > 0.0 && offset < 0.0)
        || (overscrollPastEnd > 0.0 && offset > 0.0);
    final double friction = easing
    // Apply less resistance when easing the overscroll vs tensioning.
        ? frictionFactor((overscrollPast - offset.abs()) / position.viewportDimension)
        : frictionFactor(overscrollPast / position.viewportDimension);
    final double direction = offset.sign;

    return direction * _applyFriction(overscrollPast, offset.abs(), friction);
  }

  static double _applyFriction(double extentOutside, double absDelta, double gamma) {
    assert(absDelta > 0);
    double total = 0.0;
    if (extentOutside > 0) {
      final double deltaToLimit = extentOutside / gamma;
      if (absDelta < deltaToLimit)
        return absDelta * gamma;
      total += extentOutside;
      absDelta -= deltaToLimit;
    }
    return total + absDelta;
  }


  /// 邊界條件 復用ClampingScrollPhysics的方法 保留列表在底部的邊界判斷條件
  @override
  double applyBoundaryConditions(ScrollMetrics position, double value){
    if (position.maxScrollExtent <= position.pixels && position.pixels < value) // overscroll
      return value - position.pixels;
    if (position.pixels < position.maxScrollExtent && position.maxScrollExtent < value) // hit bottom edge
      return value - position.maxScrollExtent;
    return 0.0;
  }

  @override
  Simulation createBallisticSimulation(ScrollMetrics position, double velocity) {
    final Tolerance tolerance = this.tolerance;
    if (velocity.abs() >= tolerance.velocity || position.outOfRange) {
      return BouncingScrollSimulation(
        spring: spring,
        position: position.pixels,
        velocity: velocity * 0.91, // TODO(abarth): We should move this constant closer to the drag end.
        leadingExtent: position.minScrollExtent,
        trailingExtent: position.maxScrollExtent,
        tolerance: tolerance,
      );
    }
    return null;
  }

  @override
  double get minFlingVelocity => kMinFlingVelocity * 2.0;

  @override
  double carriedMomentum(double existingVelocity) {
    return existingVelocity.sign *
        math.min(0.000816 * math.pow(existingVelocity.abs(), 1.967).toDouble(), 40000.0);
  }

  @override
  double get dragStartDistanceMotionThreshold => 3.5;
}

但是直接會發生錯誤日志,由于回調中OverscrollIndicatorNotification是沒有metrics對象。對于

The following NoSuchMethodError was thrown while notifying listeners for AnimationController:
Class 'OverscrollIndicatorNotification' has no instance getter 'metrics'.

由于上滑沒有阻尼滑動到底部回調Notification發生了變化，因此需要在NotificationListener中增加判斷對超出滑動范圍回調進行過濾處理避免異常情況發生。

   if(scrollNotification is OverscrollIndicatorNotification){
     return false;
   }

結果展示

演示代碼看這里

原文鏈接：https://juejin.cn/post/7136948689628561444

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