flutter_zxing/lib/src/ui/scanner_overlay.dart

import 'package:flutter/material.dart';

class ScannerOverlay extends ShapeBorder {
  const ScannerOverlay({
    this.borderColor = Colors.red,
    this.borderWidth = 3.0,
    this.overlayColor = const Color.fromRGBO(0, 0, 0, 40),
    this.borderRadius = 0,
    this.borderLength = 40,
    this.cutOutSize = 250,
  }) : assert(borderLength <= cutOutSize / 2 + borderWidth * 2,
            "Border can't be larger than ${cutOutSize / 2 + borderWidth * 2}");

  final Color borderColor;
  final double borderWidth;
  final Color overlayColor;
  final double borderRadius;
  final double borderLength;
  final double cutOutSize;

  @override
  EdgeInsetsGeometry get dimensions => const EdgeInsets.all(10);

  @override
  Path getInnerPath(Rect rect, {TextDirection? textDirection}) {
    return Path()
      ..fillType = PathFillType.evenOdd
      ..addPath(getOuterPath(rect), Offset.zero);
  }

  @override
  Path getOuterPath(Rect rect, {TextDirection? textDirection}) {
    Path getLeftTopPath(Rect rect) {
      return Path()
        ..moveTo(rect.left, rect.bottom)
        ..lineTo(rect.left, rect.top)
        ..lineTo(rect.right, rect.top);
    }

    return getLeftTopPath(rect)
      ..lineTo(
        rect.right,
        rect.bottom,
      )
      ..lineTo(
        rect.left,
        rect.bottom,
      )
      ..lineTo(
        rect.left,
        rect.top,
      );
  }

  @override
  void paint(Canvas canvas, Rect rect, {TextDirection? textDirection}) {
    final double width = rect.width;
    final double borderWidthSize = width / 2;
    final double height = rect.height;
    final double borderOffset = borderWidth / 2;
    final double newBorderLength =
        borderLength > cutOutSize / 2 + borderWidth * 2
            ? borderWidthSize / 2
            : borderLength;
    final double newCutOutSize =
        cutOutSize < width ? cutOutSize : width - borderOffset;

    final Paint backgroundPaint = Paint()
      ..color = overlayColor
      ..style = PaintingStyle.fill;

    final Paint borderPaint = Paint()
      ..color = borderColor
      ..style = PaintingStyle.stroke
      ..strokeWidth = borderWidth;

    final Paint boxPaint = Paint()
      ..color = borderColor
      ..style = PaintingStyle.fill
      ..blendMode = BlendMode.dstOut;

    final Rect cutOutRect = Rect.fromLTWH(
      rect.left + width / 2 - newCutOutSize / 2 + borderOffset,
      rect.top + height / 2 - newCutOutSize / 2 + borderOffset,
      newCutOutSize - borderOffset * 2,
      newCutOutSize - borderOffset * 2,
    );

    canvas
      ..saveLayer(
        rect,
        backgroundPaint,
      )
      ..drawRect(
        rect,
        backgroundPaint,
      )
      // Draw top right corner
      ..drawRRect(
        RRect.fromLTRBAndCorners(
          cutOutRect.right - newBorderLength,
          cutOutRect.top,
          cutOutRect.right,
          cutOutRect.top + newBorderLength,
          topRight: Radius.circular(borderRadius),
        ),
        borderPaint,
      )
      // Draw top left corner
      ..drawRRect(
        RRect.fromLTRBAndCorners(
          cutOutRect.left,
          cutOutRect.top,
          cutOutRect.left + newBorderLength,
          cutOutRect.top + newBorderLength,
          topLeft: Radius.circular(borderRadius),
        ),
        borderPaint,
      )
      // Draw bottom right corner
      ..drawRRect(
        RRect.fromLTRBAndCorners(
          cutOutRect.right - newBorderLength,
          cutOutRect.bottom - newBorderLength,
          cutOutRect.right,
          cutOutRect.bottom,
          bottomRight: Radius.circular(borderRadius),
        ),
        borderPaint,
      )
      // Draw bottom left corner
      ..drawRRect(
        RRect.fromLTRBAndCorners(
          cutOutRect.left,
          cutOutRect.bottom - newBorderLength,
          cutOutRect.left + newBorderLength,
          cutOutRect.bottom,
          bottomLeft: Radius.circular(borderRadius),
        ),
        borderPaint,
      )
      ..drawRRect(
        RRect.fromRectAndRadius(
          cutOutRect,
          Radius.circular(borderRadius),
        ),
        boxPaint,
      )
      ..restore();
  }

  @override
  ShapeBorder scale(double t) {
    return ScannerOverlay(
      borderColor: borderColor,
      borderWidth: borderWidth,
      overlayColor: overlayColor,
    );
  }
}