4 files changed, 171 insertions, 0 deletions
diff --git a/layout/doc.go b/layout/doc.go
new file mode 100644
index 0000000..bae4673
--- /dev/null
+++ b/layout/doc.go
@@ -0,0 +1,15 @@
+/*
+Package layout provides means of partitioning screen space.
+
+A layout exists in a parent Env, and has one or more child Envs. It
+acts as a multiplexer for the children. The parent Env may be a window,
+another layout, or whatever... Several layers of layouts can be composed.
+
+A layout allocates screen area to its children by intercepting Resize
+events from the parent Env. Upon reception by the layout, a Resize event
+is transformed for each child, and forwarded to them.
+
+Draw calls from the children are intercepted and translated onto their
+respective areas before being forwarded to the parent Env.
+*/
+package layout
diff --git a/layout/region.go b/layout/region.go
new file mode 100644
index 0000000..1e92455
--- /dev/null
+++ b/layout/region.go
@@ -0,0 +1,63 @@
+package layout
+
+import (
+	"image"
+	"image/draw"
+
+	"github.com/faiface/gui"
+)
+
+// Region is a layout with a single child Env that occupies a sub-area of its parent.
+type Region struct {
+	events <-chan gui.Event
+	draw   chan<- func(draw.Image) image.Rectangle
+}
+
+// NewRegion creates a region layout that occupies part of the parent env's area, as determined by the resize function.
+// Resize takes the area of the parent and returns the area of the region.
+func NewRegion(env gui.Env, resize func(image.Rectangle) image.Rectangle) gui.Env {
+	events := make(chan gui.Event)                     // to child
+	drw := make(chan func(draw.Image) image.Rectangle) // from child
+
+	go func(events chan<- gui.Event, drw <-chan func(draw.Image) image.Rectangle) {
+		// Forward first resize event to child
+		event := <-env.Events()
+		area := resize(event.(gui.Resize).Rectangle) // first event guaranteed to be Resize
+		events <- gui.Resize{area}
+
+		for {
+			select {
+			case event := <-env.Events():
+				switch event := event.(type) {
+				case gui.Resize:
+					area = resize(event.Rectangle)
+					events <- gui.Resize{area}
+				default:
+					events <- event
+				}
+			case f, ok := <-drw:
+				if !ok {
+					close(events)
+					close(env.Draw())
+					return
+				}
+				env.Draw() <- drawRegion(f, area)
+			}
+		}
+	}(events, drw)
+
+	return Region{events, drw}
+}
+
+// Translate a draw call to the given area.
+func drawRegion(f func(draw.Image) image.Rectangle, area image.Rectangle) func(draw.Image) image.Rectangle {
+	return func(img draw.Image) image.Rectangle {
+		return f(subimage(img, area))
+	}
+}
+
+// Events implements the Env interface.
+func (r Region) Events() <-chan gui.Event { return r.events }
+
+// Draw implements the Env interface.
+func (r Region) Draw() chan<- func(draw.Image) image.Rectangle { return r.draw }
diff --git a/layout/subimage.go b/layout/subimage.go
new file mode 100644
index 0000000..ecbe931
--- /dev/null
+++ b/layout/subimage.go
@@ -0,0 +1,17 @@
+package layout
+
+import (
+	"image"
+	"image/draw"
+)
+
+type subimager interface {
+	SubImage(image.Rectangle) image.Image
+}
+
+// Subimage returns an image representing the portion of the image m visible through r.
+// The returned value shares pixels with the original.
+// Panics if the concrete image type does not have a SubImage() method.
+func subimage(m draw.Image, r image.Rectangle) draw.Image {
+	return m.(subimager).SubImage(r).(draw.Image)
+}
diff --git a/test/region.go b/test/region.go
new file mode 100644
index 0000000..afd739e
--- /dev/null
+++ b/test/region.go
@@ -0,0 +1,76 @@
+package main
+
+import (
+	"image"
+	"image/draw"
+	"time"
+
+	"github.com/faiface/gui"
+	"github.com/faiface/gui/layout"
+	"github.com/faiface/gui/win"
+	"github.com/faiface/mainthread"
+)
+
+func main() {
+	mainthread.Run(run)
+}
+
+func run() {
+	w, err := win.New(win.Title("Grid Layout Test"), win.Resizable())
+	if err != nil {
+		panic(err)
+	}
+
+	mux, env := gui.NewMux(w)
+
+	// Create region in bottom-right quadrant of window
+	region := layout.NewRegion(mux.MakeEnv(), func(r image.Rectangle) image.Rectangle {
+		return image.Rect(r.Min.X+r.Dx()/2, r.Min.Y+r.Dy()/2, r.Max.X, r.Max.Y)
+	})
+	go blinker(region)
+
+	for event := range env.Events() {
+		switch event.(type) {
+		case win.WiClose:
+			close(env.Draw())
+			return
+		}
+	}
+}
+
+// Blinker is a widget that blinks three times when it is clicked.
+func blinker(env gui.Env) {
+	redraw := func(visible bool) func(draw.Image) image.Rectangle {
+		return func(img draw.Image) image.Rectangle {
+			if visible {
+				draw.Draw(img, img.Bounds(), image.White, image.ZP, draw.Src)
+			} else {
+				draw.Draw(img, img.Bounds(), image.Black, image.ZP, draw.Src)
+			}
+			return img.Bounds()
+		}
+	}
+
+	area := (<-env.Events()).(gui.Resize).Rectangle // first event guaranteed to be Resize
+	env.Draw() <- redraw(true)
+
+	for event := range env.Events() {
+		switch event := event.(type) {
+		case gui.Resize:
+			area = event.Rectangle
+			env.Draw() <- redraw(true)
+		case win.MoDown:
+			if event.Point.In(area) {
+				go func() {
+					for i := 0; i < 3; i++ {
+						env.Draw() <- redraw(false)
+						time.Sleep(time.Second / 3)
+						env.Draw() <- redraw(true)
+						time.Sleep(time.Second / 3)
+					}
+				}()
+			}
+		}
+	}
+	close(env.Draw())
+}