main.go

// Problem:
// The green and red area (and the intersection) receive
// click events (press and release).
// The green area also receives hover events (enter and leave),
// but the red one does not.

package main

import (
	"image"
	"image/color"
	"log"

	"gioui.org/app"
	"gioui.org/f32"
	"gioui.org/io/pointer"
	"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()
		if err := loop(w); err != nil {
			log.Fatal(err)
		}
	}()
	app.Main()
}


func loop(w *app.Window) error {
	var ops op.Ops
	var btn Button
	for {
		e := <-w.Events()
		switch e := e.(type) {
		case system.DestroyEvent:
			return e.Err
		case system.FrameEvent:
			gtx := layout.NewContext(&ops, e.Queue, e.Config, e.Size)
			btn.Layout(gtx)
			e.Frame(gtx.Ops)
		}
	}
}

type Button struct {
	active bool
	hover bool
}

func (b *Button) Layout(gtx layout.Context) layout.Dimensions {
	for _, ev := range gtx.Queue.Events(b) {
		log.Println("event", ev)
		if x, ok := ev.(pointer.Event); ok {
			switch x.Type {
			case pointer.Enter:
				b.hover = true
			case pointer.Leave:
				b.hover = false
			case pointer.Press:
				b.active = true
			case pointer.Release:
				b.active = false
			}
		}
	}
	defer op.Push(gtx.Ops).Pop()

	rect := f32.Rect(100, 100, 400, 400)
	rr := unit.Px(50)
	clip.Rect{Rect: rect, SE: rr.V, SW: rr.V, NE: rr.V, NW: rr.V}.Add(gtx.Ops)

	col := color.RGBA{B: 100, A: 0xff}
	if b.hover {
		col = color.RGBA{B: 150, A: 0xff}
	}
	if b.active {
		col = color.RGBA{B: 200, A: 0xff}
	}

	paint.ColorOp{Color: col}.Add(gtx.Ops)
	paint.PaintOp{Rect: rect}.Add(gtx.Ops)

	func() {
		defer op.Push(gtx.Ops).Pop()
		drawBorderRect(gtx, f32.Rect(150, 100, 350, 400), color.RGBA{R: 0xFF, A: 0xFF})
		pointer.Rect(image.Rect(150, 100, 350, 400)).Add(gtx.Ops)
		pointer.InputOp{Tag: b, Types: pointer.Enter | pointer.Leave | pointer.Press | pointer.Release}.Add(gtx.Ops)
	}()
	func() {
		defer op.Push(gtx.Ops).Pop()
		drawBorderRect(gtx, f32.Rect(100, 150, 400, 350), color.RGBA{G: 0xFF, A: 0xFF})
		pointer.Rect(image.Rect(100, 150, 400, 350)).Add(gtx.Ops)
		pointer.InputOp{Tag: b, Types: pointer.Enter | pointer.Leave | pointer.Press | pointer.Release}.Add(gtx.Ops)
	}()

	return layout.Dimensions{
		Size: image.Point{500, 500},
	}
}

func drawBorderRect(gfx layout.Context, rect f32.Rectangle, fill color.RGBA) {
	border := unit.Px(2).V
	paint.ColorOp{Color: fill}.Add(gfx.Ops)
	paint.PaintOp{Rect: f32.Rect(rect.Min.X, rect.Min.Y, rect.Min.X+border, rect.Max.Y)}.Add(gfx.Ops) // left
	paint.PaintOp{Rect: f32.Rect(rect.Max.X - border, rect.Min.Y, rect.Max.X, rect.Max.Y)}.Add(gfx.Ops) // right
	paint.PaintOp{Rect: f32.Rect(rect.Min.X, rect.Min.Y, rect.Max.X, rect.Min.Y+border)}.Add(gfx.Ops) // top
	paint.PaintOp{Rect: f32.Rect(rect.Min.X, rect.Max.Y - border, rect.Max.X, rect.Max.Y)}.Add(gfx.Ops) // bottom
}