all: [API] replace unit.Value with separate unit.Dp, unit.Sp types

The unit.Value is a struct and thus more inconvenient to use than its
underlying float32 type. In addition, most uses don't need a general
value, but rather a specific unit given by the context. This change
replaces unit.Value with two float32 units, Dp and Sp. It also changes
variables and parameters of unit.Value to a specific unit type matching
the context. That is, unit.Dp everywhere except for text sizes which are
in Sp.

Switching to typed float32s has multiple advantages

- They can be constants:

const touchSlop = unit.Dp(16)

- Casting untyped constants is no longer necessary:

insets := layout.UniformInset(16)

- Calculation with values is natural:

func (s ScrollbarStyle) Width() unit.Dp {
	return s.Indicator.MinorWidth + s.Track.MinorPadding + s.Track.MinorPadding
}

The main API change is that calls to gtx.Px must be replaced with either
gtx.Dp or gtx.Sp depending on the unit.

Idea by Christophe Meessen.

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

layout/context.go

@@ -52,15 +52,15 @@ func NewContext(ops *op.Ops, e system.FrameEvent) Context {
 	size := e.Size
 
 	if e.Insets != (system.Insets{}) {
-		left := e.Metric.Px(e.Insets.Left)
-		top := e.Metric.Px(e.Insets.Top)
+		left := e.Metric.Dp(e.Insets.Left)
+		top := e.Metric.Dp(e.Insets.Top)
 		op.Offset(image.Point{
 			X: left,
 			Y: top,
 		}).Add(ops)
 
-		size.X -= left + e.Metric.Px(e.Insets.Right)
-		size.Y -= top + e.Metric.Px(e.Insets.Bottom)
+		size.X -= left + e.Metric.Dp(e.Insets.Right)
+		size.Y -= top + e.Metric.Dp(e.Insets.Bottom)
 	}
 
 	return Context{
@@ -72,9 +72,14 @@ func NewContext(ops *op.Ops, e system.FrameEvent) Context {
 	}
 }
 
-// Px maps the value to pixels.
-func (c Context) Px(v unit.Value) int {
-	return c.Metric.Px(v)
+// Dp converts v to pixels.
+func (c Context) Dp(v unit.Dp) int {
+	return c.Metric.Dp(v)
+}
+
+// Sp converts v to pixels.
+func (c Context) Sp(v unit.Sp) int {
+	return c.Metric.Sp(v)
 }
 
 // Events returns the events available for the key. If no

layout/doc.go

@@ -16,7 +16,7 @@ For example, to add space above a widget:
 	var gtx layout.Context
 
 	// Configure a top inset.
-	inset := layout.Inset{Top: unit.Dp(8), ...}
+	inset := layout.Inset{Top: 8, ...}
 	// Use the inset to lay out a widget.
 	inset.Layout(gtx, func() {
 		// Lay out widget and determine its size given the constraints

layout/example_test.go

@@ -6,7 +6,6 @@ import (
 
 	"gioui.org/layout"
 	"gioui.org/op"
-	"gioui.org/unit"
 )
 
 func ExampleInset() {
@@ -19,7 +18,7 @@ func ExampleInset() {
 	}
 
 	// Inset all edges by 10.
-	inset := layout.UniformInset(unit.Dp(10))
+	inset := layout.UniformInset(10)
 	dims := inset.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 		// Lay out a 50x50 sized widget.
 		dims := layoutWidget(gtx, 50, 50)

layout/layout.go

@@ -113,15 +113,15 @@ func (c Constraints) Constrain(size image.Point) image.Point {
 // constraints. The minimum constraints will be adjusted to ensure
 // they do not exceed the maximum.
 type Inset struct {
-	Top, Bottom, Left, Right unit.Value
+	Top, Bottom, Left, Right unit.Dp
 }
 
 // Layout a widget.
 func (in Inset) Layout(gtx Context, w Widget) Dimensions {
-	top := gtx.Px(in.Top)
-	right := gtx.Px(in.Right)
-	bottom := gtx.Px(in.Bottom)
-	left := gtx.Px(in.Left)
+	top := gtx.Dp(in.Top)
+	right := gtx.Dp(in.Right)
+	bottom := gtx.Dp(in.Bottom)
+	left := gtx.Dp(in.Left)
 	mcs := gtx.Constraints
 	mcs.Max.X -= left + right
 	if mcs.Max.X < 0 {
@@ -153,7 +153,7 @@ func (in Inset) Layout(gtx Context, w Widget) Dimensions {
 
 // UniformInset returns an Inset with a single inset applied to all
 // edges.
-func UniformInset(v unit.Value) Inset {
+func UniformInset(v unit.Dp) Inset {
 	return Inset{Top: v, Right: v, Bottom: v, Left: v}
 }
 
@@ -213,14 +213,14 @@ func (d Direction) Position(widget, bounds image.Point) image.Point {
 
 // Spacer adds space between widgets.
 type Spacer struct {
-	Width, Height unit.Value
+	Width, Height unit.Dp
 }
 
 func (s Spacer) Layout(gtx Context) Dimensions {
 	return Dimensions{
 		Size: image.Point{
-			X: gtx.Px(s.Width),
-			Y: gtx.Px(s.Height),
+			X: gtx.Dp(s.Width),
+			Y: gtx.Dp(s.Height),
 		},
 	}
 }