go-widgets/painter

▶ Live demo: https://go-widgets.github.io/painter/

The drawing seam of go-widgets: one Painter interface that every widget draws through, and two painters that turn the same calls into different things:

• PixelPainter writes into a caller-owned RGBA []byte, which a native window, a browser <canvas>, an Android surface or an image encoder then presents;
• CellPainter writes into a grid of terminal cells (rune, foreground, background) with a 24-bit ANSI serialiser.

A widget never learns which one it was handed. That is what lets go-widgets/toolkit, whose Widget.Draw has taken a painter.Painter since its v0.6, run unchanged in a window, a browser and a terminal.

The interface

type Painter interface {
	FillRect(r Rect, c RGBA)
	StrokeRect(r Rect, c RGBA, lineW int)
	FillRoundRect(r Rect, radius int, c RGBA)
	StrokeRoundRect(r Rect, radius int, c RGBA, lineW int)
	PutPixel(x, y int, c RGBA)
	Text(x, y int, s string, ink RGBA)
	Size() (w, h int)
}

Seven methods, all of which every back-end must implement — so the contract says what happens where a back-end cannot: a cell grid ignores lineW, and draws a rounded rectangle square.

Optional capabilities

What not every surface can do is not in the base interface. A widget that needs it type-asserts, and draws something sensible when the answer is no:

Interface	Methods	File
Clipper	PushClip, PopClip	clip.go
Translator	PushTranslate, PopTranslate	translate.go
PathPainter	FillPath, StrokePath	path.go, pathpaint.go
ImagePainter	DrawImage	image.go
MaskPainter	DrawMask	mask.go
FacePainter	TextFace	face.go

if c, ok := p.(painter.Clipper); ok {
	c.PushClip(bounds)
	defer c.PopClip()
}

PixelPainter implements all of them. Its anti-aliased paths are rasterised by go-gfx/gfx/vector, in Go; that is the module's one dependency.

Try it

# pixels, written as a PNG — what a browser canvas or a window would get
go run ./cmd/wui-demo --out demo.png
go run ./cmd/wui-demo --out demo.png --theme dark

# cells, written to stdout as 24-bit ANSI
go run ./cmd/tui-demo
go run ./cmd/tui-demo --theme dark

# pixels in a browser
task serve                                      # http://localhost:8091/

The three demos draw the same three sample widgets (widget.go: Label, two Buttons, a ProgressBar) through the same code; only the painter changes. Those widgets and the 5×7 font in font.go exist for the demos. The real widget set, and its full font support, is go-widgets/toolkit.

Status

The production seam of go-widgets/toolkit and go-widgets/tui. 100% statement coverage, CGO_ENABLED=0, cross-built for amd64, arm64, riscv64, loong64, ppc64le and s390x, and GOOS=js GOARCH=wasm.

License

BSD 3-Clause. See LICENSE.