paint method

@override void paint(Canvas canvas, Size size)

Called whenever the object needs to paint. The given Canvas has its coordinate space configured such that the origin is at the top left of the box. The area of the box is the size of the size argument.

Paint operations should remain inside the given area. Graphical operations outside the bounds may be silently ignored, clipped, or not clipped. It may sometimes be difficult to guarantee that a certain operation is inside the bounds (e.g., drawing a rectangle whose size is determined by user inputs). In that case, consider calling Canvas.clipRect at the beginning of paint so everything that follows will be guaranteed to only draw within the clipped area.

Implementations should be wary of correctly pairing any calls to Canvas.save/Canvas.saveLayer and Canvas.restore, otherwise all subsequent painting on this canvas may be affected, with potentially hilarious but confusing results.

To paint text on a Canvas, use a TextPainter.

To paint an image on a Canvas:

1. Obtain an ImageStream, for example by calling ImageProvider.resolve on an AssetImage or NetworkImage object.

2. Whenever the ImageStream's underlying ImageInfo object changes (see ImageStream.addListener), create a new instance of your custom paint delegate, giving it the new ImageInfo object.

3. In your delegate's paint method, call the Canvas.drawImage, Canvas.drawImageRect, or Canvas.drawImageNine methods to paint the ImageInfo.image object, applying the ImageInfo.scale value to obtain the correct rendering size.

Implementation

@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!));
  }
}