paintBorder function

void paintBorder(
  1. Canvas canvas,
  2. Rect rect, {
  3. BorderSide top = BorderSide.none,
  4. BorderSide right = BorderSide.none,
  5. BorderSide bottom = BorderSide.none,
  6. BorderSide left = BorderSide.none,
})

Paints a border around the given rectangle on the canvas.

The four sides can be independently specified. They are painted in the order top, right, bottom, left. This is only notable if the widths of the borders and the size of the given rectangle are such that the border sides will overlap each other. No effort is made to optimize the rendering of uniform borders (where all the borders have the same configuration); to render a uniform border, consider using Canvas.drawRect directly.

See also:

  • paintImage, which paints an image in a rectangle on a canvas.
  • Border, which uses this function to paint its border when the border is not uniform.
  • BoxDecoration, which describes its border using the Border class.

Implementation

void paintBorder(
  Canvas canvas,
  Rect rect, {
  BorderSide top = BorderSide.none,
  BorderSide right = BorderSide.none,
  BorderSide bottom = BorderSide.none,
  BorderSide left = BorderSide.none,
}) {

  // We draw the borders as filled shapes, unless the borders are hairline
  // borders, in which case we use PaintingStyle.stroke, with the stroke width
  // specified here.
  final Paint paint = Paint()
    ..strokeWidth = 0.0;

  final Path path = Path();

  switch (top.style) {
    case BorderStyle.solid:
      paint.color = top.color;
      path.reset();
      path.moveTo(rect.left, rect.top);
      path.lineTo(rect.right, rect.top);
      if (top.width == 0.0) {
        paint.style = PaintingStyle.stroke;
      } else {
        paint.style = PaintingStyle.fill;
        path.lineTo(rect.right - right.width, rect.top + top.width);
        path.lineTo(rect.left + left.width, rect.top + top.width);
      }
      canvas.drawPath(path, paint);
    case BorderStyle.none:
      break;
  }

  switch (right.style) {
    case BorderStyle.solid:
      paint.color = right.color;
      path.reset();
      path.moveTo(rect.right, rect.top);
      path.lineTo(rect.right, rect.bottom);
      if (right.width == 0.0) {
        paint.style = PaintingStyle.stroke;
      } else {
        paint.style = PaintingStyle.fill;
        path.lineTo(rect.right - right.width, rect.bottom - bottom.width);
        path.lineTo(rect.right - right.width, rect.top + top.width);
      }
      canvas.drawPath(path, paint);
    case BorderStyle.none:
      break;
  }

  switch (bottom.style) {
    case BorderStyle.solid:
      paint.color = bottom.color;
      path.reset();
      path.moveTo(rect.right, rect.bottom);
      path.lineTo(rect.left, rect.bottom);
      if (bottom.width == 0.0) {
        paint.style = PaintingStyle.stroke;
      } else {
        paint.style = PaintingStyle.fill;
        path.lineTo(rect.left + left.width, rect.bottom - bottom.width);
        path.lineTo(rect.right - right.width, rect.bottom - bottom.width);
      }
      canvas.drawPath(path, paint);
    case BorderStyle.none:
      break;
  }

  switch (left.style) {
    case BorderStyle.solid:
      paint.color = left.color;
      path.reset();
      path.moveTo(rect.left, rect.bottom);
      path.lineTo(rect.left, rect.top);
      if (left.width == 0.0) {
        paint.style = PaintingStyle.stroke;
      } else {
        paint.style = PaintingStyle.fill;
        path.lineTo(rect.left + left.width, rect.top + top.width);
        path.lineTo(rect.left + left.width, rect.bottom - bottom.width);
      }
      canvas.drawPath(path, paint);
    case BorderStyle.none:
      break;
  }
}