ui,ui/app: convert Config to an interface

To keep the interface slim, remove the helper methods and shorten
the essential method, Pixels, to Px.

Add and use unexported Config implementation in the app package.

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

ui/app/app.go

@@ -4,8 +4,10 @@ package app
 
 import (
 	"image"
+	"math"
 	"os"
 	"strings"
+	"time"
 
 	"gioui.org/ui"
 )
@@ -15,7 +17,7 @@ type Event interface {
 }
 
 type DrawEvent struct {
-	Config ui.Config
+	Config Config
 	Size   image.Point
 	// Insets is the window space taken up by
 	// system decoration such as translucent
@@ -131,3 +133,34 @@ func init() {
 func DataDir() (string, error) {
 	return dataDir()
 }
+
+// Config implements the ui.Config interface.
+type Config struct {
+	// Device pixels per dp.
+	pxPerDp float32
+	// Device pixels per sp.
+	pxPerSp float32
+	now     time.Time
+}
+
+func (c *Config) Now() time.Time {
+	return c.now
+}
+
+func (c *Config) Px(v ui.Value) int {
+	var r float32
+	switch v.U {
+	case ui.UnitPx:
+		r = v.V
+	case ui.UnitDp:
+		r = c.pxPerDp * v.V
+	case ui.UnitSp:
+		r = c.pxPerSp * v.V
+	default:
+		panic("unknown unit")
+	}
+	if math.IsInf(float64(r), +1) {
+		return ui.Inf
+	}
+	return int(math.Round(float64(r)))
+}