layout: move Context to its own file

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

layout/context.go

@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package layout
+
+import (
+	"image"
+
+	"gioui.org/io/event"
+	"gioui.org/io/system"
+	"gioui.org/op"
+)
+
+// Context carries the state needed by almost all layouts and widgets.
+type Context struct {
+	// Constraints track the constraints for the active widget or
+	// layout.
+	Constraints Constraints
+	// Dimensions track the result of the most recent layout
+	// operation.
+	Dimensions Dimensions
+
+	system.Config
+	event.Queue
+	*op.Ops
+}
+
+// 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.
+func ctxLayout(gtx *Context, cs Constraints, w Widget) Dimensions {
+	saved := gtx.Constraints
+	gtx.Constraints = cs
+	gtx.Dimensions = Dimensions{}
+	w()
+	gtx.Dimensions.Size = cs.Constrain(gtx.Dimensions.Size)
+	gtx.Constraints = saved
+	return gtx.Dimensions
+}
+
+// Reset the context. The constraints' minimum and maximum values are
+// set to the size.
+func (c *Context) Reset(cfg system.Config, size image.Point) {
+	c.Constraints = RigidConstraints(size)
+	c.Dimensions = Dimensions{}
+	c.Config = cfg
+	if c.Ops == nil {
+		c.Ops = new(op.Ops)
+	}
+	c.Ops.Reset()
+}