paint method

  1. @override
void paint(
  1. Canvas canvas,
  2. Size size
)
override

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) {
  // 背景区域大小
  final backgroundRect = Offset.zero & size;
  final cutoutPath = Path()
    // 添加扫描窗口的路径
    ..addRRect(
      RRect.fromRectAndCorners(
        scanWindow,
        topLeft: Radius.circular(borderRadius),
        topRight: Radius.circular(borderRadius),
        bottomLeft: Radius.circular(borderRadius),
        bottomRight: Radius.circular(borderRadius),
      ),
    );

  // 绘制半透明背景，遮盖除扫描窗口以外的区域
  final backgroundPaint = Paint()
    ..color = Colors.black.withValues(alpha: 0.5)
    ..blendMode = BlendMode.srcOver;
  // 背景路径
  final backgroundPath = Path()..addRect(backgroundRect);
  // 背景路径减去扫描窗口路径，形成镂空效果
  final backgroundWithCutout = Path.combine(
    PathOperation.difference,
    backgroundPath,
    cutoutPath,
  );
  // 绘制背景
  canvas.drawPath(backgroundWithCutout, backgroundPaint);
  // 阴影最大高度为扫描窗口高度的80%
  final maxShadowHeight = scanWindow.height * 0.8;
  // 扫描线进度
  final progress = (scanLinePosition - scanWindow.top) / scanWindow.height;
  double shadowHeight;
  if (isReverse) {
    // 反向时阴影高度计算
    shadowHeight = maxShadowHeight * (1 - (-progress));
  } else {
    // 正向时阴影高度计算
    shadowHeight = maxShadowHeight * (progress + 0.4);
  }

  final shadowRect = Rect.fromLTRB(
    scanWindow.left,
    // 计算阴影的顶部位置
    scanWindow.top + scanLinePosition,
    scanWindow.right,
    // 计算阴影的底部位置
    min(scanWindow.bottom, scanWindow.top + scanLinePosition + shadowHeight),
  );

  final shadowGradient = LinearGradient(
    begin: Alignment.topCenter,
    end: Alignment.bottomCenter,
    colors: [
      Colors.greenAccent.withValues(alpha: .5),
      Colors.greenAccent.withValues(alpha: 0),
    ],
  );
  // 阴影路径
  final shadowPath = Path()..addRect(shadowRect);
  final shadowClippedPath = Path.combine(
    PathOperation.intersect,
    shadowPath,
    cutoutPath,
  );

  // 渐变效果绘制阴影
  canvas.drawPath(
    shadowClippedPath,
    Paint()..shader = shadowGradient.createShader(shadowRect),
  );

  // 绘制扫描线
  final scanLinePaint = Paint()
    ..color = Colors.greenAccent
    ..strokeWidth = 2.0;

  // 扫描线Y轴位置
  final scanLineY = scanWindow.top + scanLinePosition;
  canvas.drawLine(
    Offset(scanWindow.left, scanLineY),
    Offset(scanWindow.right, scanLineY),
    scanLinePaint,
  );
}