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) {
  // Set up the paint for the chart line
  var paint = Paint();
  paint.color = color ?? const Color(0xFFF63E02);
  paint.style = PaintingStyle.stroke;
  paint.strokeWidth = strokeWidth;

  // Set up the paint for the chart fill
  var fillPaint = Paint();
  fillPaint.style = PaintingStyle.fill;

  // Set up the paint for the axes
  var axisPaint = Paint()
    ..color = Colors.grey
    ..style = PaintingStyle.stroke
    ..strokeWidth = 1.0;

  // Draw X axis
  canvas.drawLine(
      Offset(0, size.height), Offset(size.width, size.height), axisPaint);

  // Draw Y axis
  canvas.drawLine(const Offset(0, 0), Offset(0, size.height), axisPaint);

  // Create paths for the chart line and fill
  var path = Path();
  var fillPath = Path();

  // Check if there are enough values to draw the chart
  if (xValues.length > 1 && yValues.isNotEmpty) {
    // Calculate some initial values
    final maxValue = yValues.last.values.last;
    final firstValueHeight =
        size.height * (xValues.first.values.first / maxValue);

    // Initialize the paths with the first point
    path.moveTo(0.0, size.height - firstValueHeight);
    fillPath.moveTo(0.0, size.height);
    fillPath.lineTo(0.0, size.height - firstValueHeight);

    // Calculate the distance between each x value
    final itemXDistance = size.width / (xValues.length - 1);

    // Loop through the x values and draw the chart line and fill
    for (var i = 1; i < xValues.length; i++) {
      final x = itemXDistance * i;
      final valueHeight = size.height -
          strokeWidth -
          ((size.height - strokeWidth) *
              (xValues[i].values.elementAt(0) / maxValue));
      final previousValueHeight = size.height -
          strokeWidth -
          ((size.height - strokeWidth) *
              (xValues[i - 1].values.elementAt(0) / maxValue));

      // Draw a quadratic bezier curve between each point
      path.quadraticBezierTo(
        x - (itemXDistance / 2) - (itemXDistance / 8),
        previousValueHeight,
        x - (itemXDistance / 2),
        valueHeight + ((previousValueHeight - valueHeight) / 2),
      );
      path.quadraticBezierTo(
        x - (itemXDistance / 2) + (itemXDistance / 8),
        valueHeight,
        x,
        valueHeight,
      );

      // Draw the fill path using the same quadratic bezier curves
      fillPath.quadraticBezierTo(
        x - (itemXDistance / 2) - (itemXDistance / 8),
        previousValueHeight,
        x - (itemXDistance / 2),
        valueHeight + ((previousValueHeight - valueHeight) / 2),
      );
      fillPath.quadraticBezierTo(
        x - (itemXDistance / 2) + (itemXDistance / 8),
        valueHeight,
        x,
        valueHeight,
      );
    }

    // Close the fill path
    fillPath.lineTo(size.width, size.height);
    fillPath.close();
  }

  // Create a gradient for the fill
  LinearGradient gradient = LinearGradient(
    colors: gradientColors,
    stops: gradientStops,
    begin: Alignment.topCenter,
    end: Alignment.bottomCenter,
  );
  Rect rect = Rect.fromLTWH(0, 0, size.width, size.height);
  fillPaint.shader = gradient.createShader(rect);

  // Draw the fill path with the gradient
  canvas.drawPath(fillPath, fillPaint);

  // Draw the chart line
  canvas.drawPath(path, paint);

  // Draw X axis labels
  for (int i = 0; i < xValues.length; i++) {
    double x = size.width * i / (xValues.length - 1);
    var textPainter = TextPainter(
      text:
      TextSpan(text: xValues[i].keys.elementAt(0), style: labelTextStyle),
      textDirection: TextDirection.ltr,
    );
    textPainter.layout();
    textPainter.paint(
        canvas, Offset(x - textPainter.width / 2, size.height + 2));
  }

  // Draw Y axis labels
  for (int i = 0; i < yValues.length; i++) {
    double y = size.height * i / (yValues.length - 1);
    double labelValue = yValues.last.values.elementAt(0) *
        (yValues.length - i - 1) /
        (yValues.length - 1);
    var textPainter = TextPainter(
      text: TextSpan(
          text: labelValue.toStringAsFixed(0), style: labelTextStyle),
      textDirection: TextDirection.ltr,
    );
    textPainter.layout();
    textPainter.paint(
        canvas, Offset(-textPainter.width - 2, y - textPainter.height / 2));
  }
}