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(final Canvas canvas, final Size size) {
  for (final arrow in arrows) {
    final (sourceRow, sourceCol) = getIndexFromSquare(arrow.startSquare);
    final (destRow, destCol) = getIndexFromSquare(arrow.endSquare);
    final blockSize = size.width / 8;

    // Calculate centers of the source and destination squares
    final sourceX = sourceCol * blockSize + (blockSize / 2);
    final sourceY = sourceRow * blockSize + (blockSize / 2);
    final destX = destCol * blockSize + (blockSize / 2);
    final destY = destRow * blockSize + (blockSize / 2);

    // Vector calculation between source and destination
    final dx = destX - sourceX;
    final dy = destY - sourceY;
    final distance = sqrt(dx * dx + dy * dy);

    // Define lengths for offsets and arrowhead sides
    final sourceThreshold = blockSize * 0.35;
    final destinationThreshold = blockSize * 0.272;
    final arrowheadSideLength = blockSize * 0.2;

    // Calculate the main line's adjusted endpoint (just before the base of the arrowhead triangle)
    final adjustedSourceX = sourceX + (dx / distance) * sourceThreshold;
    final adjustedSourceY = sourceY + (dy / distance) * sourceThreshold;
    final adjustedDestX =
        destX - (dx / distance) * (arrowheadSideLength * cos(pi / 6));
    final adjustedDestY =
        destY - (dy / distance) * (arrowheadSideLength * cos(pi / 6));
    final strokeDestX = destX - (dx / distance) * destinationThreshold;
    final strokeDestY = destY - (dy / distance) * destinationThreshold;

    // Draw the main arrow line
    final paint = Paint()
      ..color = arrow.color!
      ..strokeWidth = blockSize * 0.16;
    canvas.drawLine(Offset(adjustedSourceX, adjustedSourceY),
        Offset(strokeDestX, strokeDestY), paint);

    // Calculate points for the equilateral triangle arrowhead
    final angle = atan2(dy, dx);

    // Arrowhead points: tip at destination center, and two base points at the adjusted end of the line
    final tipX = destX;
    final tipY = destY;
    final baseX1 = adjustedDestX - arrowheadSideLength * cos(angle - pi / 3);
    final baseY1 = adjustedDestY - arrowheadSideLength * sin(angle - pi / 3);
    final baseX2 = adjustedDestX - arrowheadSideLength * cos(angle + pi / 3);
    final baseY2 = adjustedDestY - arrowheadSideLength * sin(angle + pi / 3);

    // Draw the equilateral triangle arrowhead
    final arrowheadPath = Path()
      ..moveTo(tipX, tipY)
      ..lineTo(baseX1, baseY1)
      ..lineTo(baseX2, baseY2)
      ..close();

    // Fill the arrowhead triangle
    final arrowheadPaint = Paint()..color = arrow.color!;
    canvas.drawPath(arrowheadPath, arrowheadPaint);
  }
}