widget: use caller-provided buffers for reading out text

This commit alters the textView API to give callers the option to provide
their own buffers for reading text. This enables some widget usecases to
be zero-allocation if a widget simply needs to examine the contents of the
text without returning it as a string.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

widget/selectable.go

@@ -50,8 +50,11 @@ func (s stringSource) ReplaceRunes(byteOffset, runeCount int64, str string) {
 
 // Selectable holds text selection state.
 type Selectable struct {
-	initialized  bool
-	source       stringSource
+	initialized bool
+	source      stringSource
+	// scratch is a buffer reused to efficiently read text out of the
+	// textView.
+	scratch      []byte
 	lastValue    string
 	text         textView
 	focused      bool
@@ -127,7 +130,8 @@ func (l *Selectable) SetCaret(start, end int) {
 // SelectedText returns the currently selected text (if any) from the editor.
 func (l *Selectable) SelectedText() string {
 	l.initialize()
-	return l.text.SelectedText()
+	l.scratch = l.text.SelectedText(l.scratch)
+	return string(l.scratch)
 }
 
 // ClearSelection clears the selection, by setting the selection end equal to
@@ -140,7 +144,8 @@ func (l *Selectable) ClearSelection() {
 // Text returns the contents of the label.
 func (l *Selectable) Text() string {
 	l.initialize()
-	return l.text.Text()
+	l.scratch = l.text.Text(l.scratch)
+	return string(l.scratch)
 }
 
 // SetText updates the text to s if it does not already contain s. Updating the
@@ -325,7 +330,8 @@ func (e *Selectable) command(gtx layout.Context, k key.Event) {
 		e.text.MoveEnd(selAct)
 	// Copy or Cut selection -- ignored if nothing selected.
 	case "C", "X":
-		if text := e.text.SelectedText(); text != "" {
+		e.scratch = e.text.SelectedText(e.scratch)
+		if text := string(e.scratch); text != "" {
 			clipboard.WriteOp{Text: text}.Add(gtx.Ops)
 		}
 	// Select all