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) async {
  if (_hasSnapshot && !_shouldPainting) {
    return canvas.drawImage(_snapshot!, Offset.zero, _snapshotPaint);
  }

  final PictureRecorder pictureRecorder = PictureRecorder();
  final Canvas recordCanvas = Canvas(pictureRecorder);

  if (_scaleX != 1.0 || _scaleY != 1.0) {
    recordCanvas.scale(_scaleX, _scaleY);
  }

  // This lets you create composite effects, for example making a group of drawing commands semi-transparent.
  // Without using saveLayer, each part of the group would be painted individually,
  // so where they overlap would be darker than where they do not. By using saveLayer to group them together,
  // they can be drawn with an opaque color at first,
  // and then the entire group can be made transparent using the saveLayer's paint.
  recordCanvas.saveLayer(null, _saveLayerPaint);

  // Paint last content
  if (_hasSnapshot) {
    recordCanvas.drawImage(_snapshot!, Offset.zero, _snapshotPaint);
    _disposeSnapshot();
  }

  // Paint new actions
  if (_shouldPainting) {
    context!.performActions(recordCanvas, size);
  }

  // Must pair each call to save()/saveLayer() with a later matching call to restore().
  recordCanvas.restore();

  // After calling this function, both the picture recorder
  // and the canvas objects are invalid and cannot be used further.
  final Picture picture = pictureRecorder.endRecording();
  canvas.drawPicture(picture);

  // Must flat picture to image, or raster will accept a growing command buffer.
  _snapshot = await picture.toImage(size.width.toInt(), size.height.toInt());
  // Dispose the used picture.
  picture.dispose();
}