ui: add doc.go

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/ui.go

@@ -8,26 +8,52 @@ import (
 	"gioui.org/ui/f32"
 )
 
+// Config contain the context for updating and
+// drawing a user interface.
 type Config struct {
+	// Device pixels per dp.
 	PxPerDp float32
+	// Device pixels per sp.
 	PxPerSp float32
-	Now     time.Time
+	// The current time for animation.
+	Now time.Time
 }
 
+// Pixels converts a value to unitless device pixels.
+func (c *Config) Pixels(v Value) float32 {
+	switch v.U {
+	case UnitPx:
+		return v.V
+	case UnitDp:
+		return c.PxPerDp * v.V
+	case UnitSp:
+		return c.PxPerSp * v.V
+	default:
+		panic("unknown unit")
+	}
+}
+
+// Op is implemented by all known drawing and control
+// operations.
 type Op interface {
 	ImplementsOp()
 }
 
+// OpLayer represents a semantic layer of UI.
 type OpLayer struct {
 	Op Op
 }
 
+// OpRedraw requests a redraw at the given time. Use
+// the zero value to request an immediate redraw.
 type OpRedraw struct {
 	At time.Time
 }
 
+// Ops is the operation for a list of ops.
 type Ops []Op
 
+// OpTransform transforms an op.
 type OpTransform struct {
 	Transform Transform
 	Op        Op