Flutter-实现简洁好看的PageView指示器

1.介绍

在使用PageView时，我们通常需要添加指示器，以避免用户产生只有一张图片显示的错觉，所以，添加指示器是必不可少的！但是，有时候图片一多，指示器也同样的出现多的情况，导致显示的指示器不能显示太大，并且间距也需要适当的减少，下面来看一下我实现的效果，简洁大方!

2.开始实现

首先，我们要确定传进来的参数有哪些

• PageController controller 用于绑定指示器
• int itemCount 与PageView对应，需要确认有多少页
• Color indicatorColor 指示器的颜色
• double maxSize 指示器到达中间时的大小
• double minSize 指示器两边圆点的大小
• double space 指示器两圆点之间的间距

确认上面的参数后，我们来新建一个dart文件，命名为simple_page_indicator.dart ，然后添加下面的内容

class SimplePageIndicator extends StatelessWidget {
  final PageController controller;
  final int itemCount;
  final Color indicatorColor;
  final double maxSize;
  final double minSize;
  final double space;

  const SimplePageIndicator(
      {Key key,
      this.controller,
      this.itemCount,
      this.indicatorColor,
      this.maxSize,
      this.space,
      this.minSize})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

接下来，我们需要使用AnimatedBuilder，并传入animation参数为controller

//...
  @override
  Widget build(BuildContext context) {
//    return Container();
    return AnimatedBuilder(animation: controller, builder: _buildAnimatedItem);
  }

  Widget _buildAnimatedItem(BuildContext context, Widget child) {
    return Container();
  }
//...

这样我们就能监听到PageController的值发生改变时做出指示器对应的变化，下面我们来使用CustomPaint 对指示器进行绘制,新加一个SimplePageIndicatorPainter类并将对应的值传递过去继续修改

//...
  Widget _buildAnimatedItem(BuildContext context, Widget child) {
    // return Container();
    return CustomPaint(
      painter: SimplePageIndicatorPainter(
        itemCount: itemCount,
        indicatorColor: indicatorColor,
        maxSize: maxSize,
        minSize: minSize,
        space: space,
        pageIndex: 0,
        pageOffset: .0
      ),
    );
  }
/// ...

class SimplePageIndicatorPainter extends CustomPainter {
  final int itemCount;
  final Color indicatorColor;
  final double maxSize;
  final double minSize;
  final int pageIndex;
  final double pageOffset;
  final double space;

  const SimplePageIndicatorPainter(
      {this.itemCount,
      this.indicatorColor,
      this.maxSize,
      this.space,
      this.minSize,
      this.pageIndex,
      this.pageOffset});

  @override
  void paint(Canvas canvas, Size size) {}

  @override
  bool shouldRepaint(covariant SimplePageIndicatorPainter oldDelegate) {
    return true;
  }
}

下面，我们来开始画对应的小圆点,我们默认的将指示器的位置设置为0起点和偏移量也为0，我们需要画三个点，因为当前是在0起始位，我们只需要画两个点，一个大的点在中间，另一个小点在最右边

// ...
const _kMaxCircleCount = 3; // 默认三个圆点

class SimplePageIndicatorPainter extends CustomPainter {
  final int itemCount;
  final Color indicatorColor;
  final double maxSize;
  final double minSize;
  final int pageIndex;
  final double pageOffset;
  final double space;
  final bool isStart;
  final bool isEnd;

  const SimplePageIndicatorPainter(
      {this.itemCount,
      this.indicatorColor,
      this.maxSize,
      this.space,
      this.minSize,
      this.pageIndex,
      this.pageOffset,
      this.isStart,
      this.isEnd});

  @override
  void paint(Canvas canvas, Size size) {
    //初始化画笔
    Paint mPaint = Paint()
      ..color = indicatorColor
      ..isAntiAlias = true;

    //获取中间的圆点为第几个
    final _centerCircleIndex = _kMaxCircleCount ~/ 2;
    //获取每个圆点占据的大小，把widget分为3份
    double childWidth = size.width / _kMaxCircleCount;

    //遍历画圆点
    for (int i = 0; i < _kMaxCircleCount; i++) {
      // 当页数为0的时候，也就是第一页，不画第一个圆点
      if (pageIndex == 0 && i == 0) {
        continue;
      }
      if (isEnd && i == _kMaxCircleCount - 1) {
        continue;
      }
      //画中心圆
      if (_centerCircleIndex == i) {
        canvas.drawCircle(
            Offset(
                (i * childWidth) + (childWidth / 2) - childWidth * pageOffset,
                childWidth / 2),
            maxSize - (maxSize - minSize) * pageOffset,
            mPaint..color = indicatorColor);
      }
      //画左边的圆
      else if (i < _centerCircleIndex) {
        canvas.drawCircle(
            Offset(
                (i * childWidth) + (childWidth / 2) - childWidth * pageOffset,
                childWidth / 2),
            minSize * (1 - pageOffset),
            mPaint..color = indicatorColor.withOpacity(1 - pageOffset));
      }
      //话右边的圆
      else {
        canvas.drawCircle(
            Offset(
                (i * childWidth) + (childWidth / 2) - childWidth * pageOffset,
                childWidth / 2),
            minSize + (maxSize - minSize) * pageOffset,
            mPaint..color = indicatorColor);
      }
    }
    //向左移的时候画后面的圆
    if (isStart && !isEnd) {
      canvas.drawCircle(
          Offset(
              (_kMaxCircleCount * childWidth) +
                  (childWidth / 2) -
                  childWidth * (pageOffset),
              childWidth / 2),
          minSize * pageOffset,
          mPaint..color = indicatorColor.withOpacity(pageOffset));
    }
  }

  @override
  bool shouldRepaint(covariant SimplePageIndicatorPainter oldDelegate) {
    return itemCount != oldDelegate.itemCount ||
        indicatorColor != oldDelegate.indicatorColor ||
        maxSize != oldDelegate.maxSize ||
        minSize != oldDelegate.minSize ||
        space != oldDelegate.space ||
        pageIndex != oldDelegate.pageIndex ||
        isStart != oldDelegate.isStart ||
        isEnd != oldDelegate.isEnd;
  }
}
// ...

最后我们来处理一下 pageController传过来的值即可

Widget _buildAnimatedItem(BuildContext context, Widget child) {
   // return Container();
   //当前页数
   int index;
   //偏移量
   double offset;

   //如果获取不了，则使用初始化的值,一般第一次渲染的时候无法获取到
   if (!controller.hasClients || controller.page == null) {
     index = controller.initialPage;
     offset = controller.initialPage.toDouble();
   } else {
     index = controller.page ~/ 1;
     offset = controller.page;
   }

   return CustomPaint(
     size: Size(
         maxSize * _kMaxCircleCount + space * (_kMaxCircleCount - 1), maxSize),
     painter: SimplePageIndicatorPainter(
         itemCount: itemCount,
         indicatorColor: indicatorColor,
         maxSize: maxSize,
         minSize: minSize,
         pageIndex: index,
         space: space,
         pageOffset: offset - index,
         isStart:
             (offset > index) && (index + _kMaxCircleCount - 1 < itemCount),
         isEnd: index + _kMaxCircleCount - 2 >= itemCount),
   );
 }

详细代码请查看：这里

直接使用:

dependencies:
  simple_page_indicator: ^1.0.0