Add layout components 001-020 with demos and behavior tests

layout/screen.go

@@ -0,0 +1,38 @@
+package layout
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type ScreenOptions struct {
+	// Vignette dithers Shade into the edges.
+	Vignette bool
+	Padding  int
+}
+
+// Screen fills the window with Back; children form a column.
+func Screen(c *ui.Context, o ScreenOptions, children func()) *ui.Element {
+	k := pixel.K(c)
+	pad := orDef("Screen Padding", o.Padding, 0)
+	return ui.Column(c).Fill().Padding(k.D(pad)).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		g.Rect(0, 0, g.W, g.H, pixel.Back)
+		if o.Vignette {
+			vignette(g)
+		}
+	}).Children(children)
+}
+
+// vignette darkens four bands toward the edges.
+func vignette(g pixel.Canvas) {
+	band := max(2, min(g.W, g.H)/32)
+	for i := range 4 {
+		in, lv := i*band, 8-2*i
+		g.Veil(in, in, g.W-2*in, band, pixel.Shade, lv)
+		g.Veil(in, g.H-in-band, g.W-2*in, band, pixel.Shade, lv)
+		g.Veil(in, in+band, band, g.H-2*in-2*band, pixel.Shade, lv)
+		g.Veil(g.W-in-band, in+band, band, g.H-2*in-2*band, pixel.Shade, lv)
+	}
+}