add Target assignments to widgets

This allows to define widgets inside the main UI in the
natural order.
Widgets that are referenced from other widgets can do so
by Target pointers.

This is an example implementation for mjl-/duit#9

examples/alert/alert.go

@@ -17,16 +17,17 @@ func main() {
 	dui, err := duit.NewDUI("ex/alert", &duit.DUIOpts{Dimensions: "400x300"})
 	check(err, "new dui")
 
-	field := &duit.Field{
-		Text: "type an alert message here",
-	}
+	var field *duit.Field
 
 	dui.Top.UI = &duit.Box{
 		Padding: duit.SpaceXY(6, 4),
 		Margin:  image.Pt(6, 4),
 		Valign:  duit.ValignMiddle,
 		Kids: duit.NewKids(
-			field,
+			&duit.Field{
+				Target: &field,
+				Text:   "type an alert message here",
+			},
 			&duit.Button{
 				Text: "click me to create an alert",
 				Click: func() (e duit.Event) {

examples/basic/main.go

@@ -18,7 +18,8 @@ func main() {
 
 	// assign some UIs to variables, so we can reference
 	// and modify them in the button click handler.
-	status := &duit.Label{Text: "status: not logged in yet"}
+	var status *duit.Label
+
 	username := &duit.Field{}
 	password := &duit.Field{Password: true}
 	login := &duit.Button{
@@ -57,7 +58,7 @@ func main() {
 		Margin:  image.Pt(6, 4),     // space between kids in this box
 		// duit.NewKids is a convenience function turning UIs into Kids
 		Kids: duit.NewKids(
-			status,
+			&duit.Label{Target: &status, Text: "status: not logged in yet"},
 			&duit.Grid{
 				Columns: 2,
 				Padding: []duit.Space{

examples/demo/main.go

@@ -30,7 +30,7 @@ func main() {
 	}
 
 	count := 0
-	counter := &duit.Label{Text: fmt.Sprintf("%d", count)}
+	var counter *duit.Label
 	tick := make(chan struct{}, 0)
 	go func() {
 		for {
@@ -39,6 +39,8 @@ func main() {
 		}
 	}()
 
+	// We cannot define the RadiobuttonGroup inside the main UI.
+	// It cannot be assigned to a Target, as it is just a slice.
 	radio1 := &duit.Radiobutton{
 		Selected: true,
 		Value:    1,
@@ -116,7 +118,7 @@ func main() {
 					Margin:  image.Pt(6, 4),
 					Kids: duit.NewKids(
 						&duit.Label{Text: "counter:"},
-						counter,
+						&duit.Label{Target: &counter, Text: fmt.Sprintf("%d", count)},
 						&duit.Button{
 							Text:     "button1",
 							Colorset: &dui.Primary,

field.go

@@ -12,6 +12,7 @@ import (
 
 // Field is a single line text field. The cursor is always visible, and determines which part of the text is shown.
 type Field struct {
+	Target          **Field                              `json:"-"` // Reference to itself.
 	Text            string                               // Current text.
 	Placeholder     string                               // Text displayed in lighter color as example.
 	Disabled        bool                                 // If disabled, mouse and key input have no effect.
@@ -80,6 +81,10 @@ func (ui *Field) removeSelection() {
 func (ui *Field) Layout(dui *DUI, self *Kid, sizeAvail image.Point, force bool) {
 	dui.debugLayout(self)
 
+	if ui.Target != nil {
+		*ui.Target = ui
+	}
+
 	ui.size = image.Point{sizeAvail.X, ui.font(dui).Height + 2*ui.space(dui).Y}
 	self.R = rect(ui.size)
 	return

label.go

@@ -12,9 +12,10 @@ import (
 //	cmd-c, copy text
 //	\n, like button1 click, calls the Click function
 type Label struct {
-	Text  string           // Text to draw, wrapped at glyph boundary.
-	Font  *draw.Font       `json:"-"` // For drawing text.
-	Click func() (e Event) `json:"-"` // Called on button1 click.
+	Target **Label          `json:"-"`
+	Text   string           // Text to draw, wrapped at glyph boundary.
+	Font   *draw.Font       `json:"-"` // For drawing text.
+	Click  func() (e Event) `json:"-"` // Called on button1 click.
 
 	lines []string
 	size  image.Point
@@ -30,6 +31,10 @@ func (ui *Label) font(dui *DUI) *draw.Font {
 func (ui *Label) Layout(dui *DUI, self *Kid, sizeAvail image.Point, force bool) {
 	dui.debugLayout(self)
 
+	if ui.Target != nil {
+		*ui.Target = ui
+	}
+
 	font := ui.font(dui)
 	ui.lines = []string{}
 	s := 0

radiobutton.go

@@ -8,6 +8,7 @@ import (
 
 // Radiobutton is typically part of a group of radiobuttons, with exactly one of them selected. Labels are not part of the radiobutton itself.
 type Radiobutton struct {
+	Target   **Radiobutton `json:"-"`
 	Selected bool
 	Disabled bool             // If set, cannot be selected.
 	Group    RadiobuttonGroup // Other radiobuttons as part of this group. If a radiobutton is selected, others in the group are unselected.
@@ -54,6 +55,10 @@ func (ui *Radiobutton) innerDim(dui *DUI) int {
 func (ui *Radiobutton) Layout(dui *DUI, self *Kid, sizeAvail image.Point, force bool) {
 	dui.debugLayout(self)
 
+	if ui.Target != nil {
+		*ui.Target = ui
+	}
+
 	hit := image.Point{0, 1}
 	size := pt(2*BorderSize + 7*dui.Display.DefaultFont.Height/10).Add(hit)
 	self.R = rect(size)