GIODrawCheckBoard.go

package main

import (
	"image/color"
	"log"
	"os"
	"strings"

	"gioui.org/app"
	"gioui.org/f32"
	"gioui.org/io/system"
	"gioui.org/layout"
	"gioui.org/op"
	"gioui.org/op/clip"
	"gioui.org/op/paint"
	"gioui.org/unit"
)

func main() {
	go func() {
		w := app.NewWindow(app.Size(unit.Dp(200), unit.Dp(200)))
		if err := loop(w); err != nil {
			log.Fatal(err)
		}
		os.Exit(0)
	}()
	app.Main()
}

func loop(w *app.Window) error {
	var ops op.Ops
	// load the desired font
	goregular, err := opentype.Parse(goregular.TTF)
	if err != nil {
		return err
	}
	for e := range w.Events() {
		switch e := e.(type) {
		case system.DestroyEvent:
			return e.Err
		case system.FrameEvent:
			gtx := layout.NewContext(&ops, e)
			// draw black background
			paint.Fill(&ops, color.NRGBA{A: 255})
			// draw red and white checkboard
			DrawCheckBoard(&ops, f32.Pt(10, 10), f32.Pt(150, 150), 10,
				color.NRGBA{R: 255, G: 255, B: 255, A: 255}, color.NRGBA{R: 255, A: 255})
			// render the drawing
			e.Frame(gtx.Ops)
		}
	}
	return nil
}

func DrawCheckBoard(ops *op.Ops, pos, size f32.Point, sqSize float32, clr1, clr2 color.NRGBA) {
	var path clip.Path
	// offset origin at pos
	defer op.Offset(pos).Push(ops).Pop()
	// set the global clipping region and paint with clr1
	path.Begin(ops)
	path.LineTo(f32.Pt(size.X, 0))
	path.LineTo(size)
	path.LineTo(f32.Pt(0, size.Y))
	path.Close()
	defer clip.Outline{Path: path.End()}.Op().Push(ops).Pop()
	paint.Fill(ops, clr1)
	// create the square path
	path.Begin(ops)
	path.LineTo(f32.Pt(sqSize, 0))
	path.LineTo(f32.Pt(sqSize, sqSize))
	path.LineTo(f32.Pt(0, sqSize))
	path.Close()
	square := clip.Outline{Path: path.End()}.Op()
	// create macro painting a square with clr2
	drawSquareRec := op.Record(ops)
	s := square.Push(ops)
	paint.Fill(ops, clr2)
	s.Pop()
	drawSquare := drawSquareRec.Stop()
	// create a macro painting a row of squares
	drawSquareRowRec := op.Record(ops)
	for x := float32(0); x < size.X+sqSize; x += 2 * sqSize {
		s := op.Offset(f32.Pt(x, 0)).Push(ops)
		drawSquare.Add(ops)
		paint.Fill(ops, clr2)
		s.Pop()
	}
	drawSquareRow := drawSquareRowRec.Stop()
	// draw all even rows
	for y := float32(0); y < size.Y+sqSize; y += 2 * sqSize {
		s := op.Offset(f32.Pt(0, y)).Push(ops)
		drawSquareRow.Add(ops)
		s.Pop()
	}
	// draw all odd rows
	for y := sqSize; y < size.Y+sqSize; y += 2 * sqSize {
		s := op.Offset(f32.Pt(sqSize, y)).Push(ops)
		drawSquareRow.Add(ops)
		s.Pop()
	}
}