TrailPainter
- Inheritance
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- Object
- Listenable
- CustomPainter
- TrailPainter
Constructors
TrailPainter
- PathTracker? pathTracker
Properties
pathTracker
Methods
paint
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:
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Obtain an ImageStream, for example by calling ImageProvider.resolve on an AssetImage or NetworkImage object.
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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.
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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.
shouldRepaint
- covariant CustomPainter oldDelegate
Called whenever a new instance of the custom painter delegate class is provided to the RenderCustomPaint object, or any time that a new CustomPaint object is created with a new instance of the custom painter delegate class (which amounts to the same thing, because the latter is implemented in terms of the former).
If the new instance represents different information than the old instance, then the method should return true, otherwise it should return false.
If the method returns false, then the paint call might be optimized away.
It's possible that the paint method will get called even if shouldRepaint returns false (e.g. if an ancestor or descendant needed to be repainted). It's also possible that the paint method will get called without shouldRepaint being called at all (e.g. if the box changes size).
If a custom delegate has a particularly expensive paint function such that repaints should be avoided as much as possible, a RepaintBoundary or RenderRepaintBoundary (or other render object with RenderObject.isRepaintBoundary set to true) might be helpful.
The oldDelegate argument will never be null.