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/editor.go

@@ -56,7 +56,10 @@ type Editor struct {
 	// all characters are allowed.
 	Filter string
 
-	buffer       *editBuffer
+	buffer *editBuffer
+	// scratch is a byte buffer that is reused to efficiently read portions of text
+	// from the textView.
+	scratch      []byte
 	eventKey     int
 	blinkStart   time.Time
 	focused      bool
@@ -320,8 +323,9 @@ func (e *Editor) processKey(gtx layout.Context) {
 			}
 			if !e.ReadOnly && e.Submit && (ke.Name == key.NameReturn || ke.Name == key.NameEnter) {
 				if !ke.Modifiers.Contain(key.ModShift) {
+					e.scratch = e.text.Text(e.scratch)
 					e.events = append(e.events, SubmitEvent{
-						Text: e.text.Text(),
+						Text: string(e.scratch),
 					})
 					continue
 				}
@@ -358,8 +362,9 @@ func (e *Editor) processKey(gtx layout.Context) {
 				if e.text.Changed() {
 					e.events = append(e.events, ChangeEvent{})
 				}
+				e.scratch = e.text.Text(e.scratch)
 				e.events = append(e.events, SubmitEvent{
-					Text: e.text.Text(),
+					Text: string(e.scratch),
 				})
 			}
 		// Complete a paste event, initiated by Shortcut-V in Editor.command().
@@ -450,7 +455,8 @@ func (e *Editor) command(gtx layout.Context, k key.Event) {
 		}
 	// 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)
 			if k.Name == "X" && !e.ReadOnly {
 				e.Delete(1)
@@ -686,7 +692,8 @@ func (e *Editor) Len() int {
 // Text returns the contents of the editor.
 func (e *Editor) Text() string {
 	e.initBuffer()
-	return e.text.Text()
+	e.scratch = e.text.Text(e.scratch)
+	return string(e.scratch)
 }
 
 func (e *Editor) SetText(s string) {
@@ -954,7 +961,8 @@ func (e *Editor) SetCaret(start, end int) {
 // SelectedText returns the currently selected text (if any) from the editor.
 func (e *Editor) SelectedText() string {
 	e.initBuffer()
-	return e.text.SelectedText()
+	e.scratch = e.text.SelectedText(e.scratch)
+	return string(e.scratch)
 }
 
 // ClearSelection clears the selection, by setting the selection end equal to