layout: add NewContext, make zero value Contexts useful

While here, unexport the Queue and Config fields. The NewContext
cosntructor is shorter, and there is no reason to expose the fields
to accidental mutation.

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

layout/context.go

@@ -4,13 +4,18 @@ package layout
 
 import (
 	"image"
+	"math"
+	"time"
 
 	"gioui.org/io/event"
 	"gioui.org/io/system"
 	"gioui.org/op"
+	"gioui.org/unit"
 )
 
 // Context carries the state needed by almost all layouts and widgets.
+// A zero value Context never returns events, map units to pixels
+// with a scale of 1.0, and returns the zero time from Now.
 type Context struct {
 	// Constraints track the constraints for the active widget or
 	// layout.
@@ -19,11 +24,18 @@ type Context struct {
 	// operation.
 	Dimensions Dimensions
 
-	system.Config
-	event.Queue
+	cfg   system.Config
+	queue event.Queue
 	*op.Ops
 }
 
+// NewContext returns a Context for an event queue.
+func NewContext(q event.Queue) *Context {
+	return &Context{
+		queue: q,
+	}
+}
+
 // layout a widget with a set of constraints and return its
 // dimensions. The widget dimensions are constrained abd the previous
 // constraints are restored after layout.
@@ -42,9 +54,35 @@ func ctxLayout(gtx *Context, cs Constraints, w Widget) Dimensions {
 func (c *Context) Reset(cfg system.Config, size image.Point) {
 	c.Constraints = RigidConstraints(size)
 	c.Dimensions = Dimensions{}
-	c.Config = cfg
+	c.cfg = cfg
 	if c.Ops == nil {
 		c.Ops = new(op.Ops)
 	}
 	c.Ops.Reset()
 }
+
+// Now returns the configuration time or the the zero time.
+func (c *Context) Now() time.Time {
+	if c.cfg == nil {
+		return time.Time{}
+	}
+	return c.cfg.Now()
+}
+
+// Px maps the value to pixels. If no configuration is set,
+// Px returns the rounded value of v.
+func (c *Context) Px(v unit.Value) int {
+	if c.cfg == nil {
+		return int(math.Round(float64(v.V)))
+	}
+	return c.cfg.Px(v)
+}
+
+// Events returns the events available for the key. If no
+// queue is configured, Events returns nil.
+func (c *Context) Events(k event.Key) []event.Event {
+	if c.queue == nil {
+		return nil
+	}
+	return c.queue.Events(k)
+}