ui: add doc.go

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

ui/unit.go

@@ -2,16 +2,24 @@
 
 package ui
 
+// Value is a value with a unit.
 type Value struct {
 	V float32
 	U Unit
 }
 
+// Unit represents a unit for a Value.
 type Unit uint8
 
 const (
+	// UnitPx represent device pixels in the resolution of
+	// the underlying display.
 	UnitPx Unit = iota
+	// UnitDp represents device independent pixels. 1 dp will
+	// have the same apparent size across platforms and
+	// display resolutions.
 	UnitDp
+	// UnitSp is like UnitDp but for font sizes.
 	UnitSp
 )
 
@@ -26,16 +34,3 @@ func Dp(v float32) Value {
 func Sp(v float32) Value {
 	return Value{V: v, U: UnitSp}
 }
-
-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")
-	}
-}