1 files changed, 63 insertions, 0 deletions
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 }