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

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

widget/text.go

@@ -338,11 +338,19 @@ func (e *textView) Len() int {
 	return e.closestToRune(math.MaxInt).runes
 }
 
-// Text returns the contents of the editor.
-func (e *textView) Text() string {
+// Text returns the contents of the editor. If the provided buf is large enough, it will
+// be filled and returned. Otherwise a new buffer will be allocated.
+// Callers can guarantee that buf is large enough by giving it capacity e.Len()*utf8.UTFMax.
+func (e *textView) Text(buf []byte) []byte {
+	size := e.rr.Size()
+	if cap(buf) < int(size) {
+		buf = make([]byte, size)
+	}
+	buf = buf[:size]
 	e.Seek(0, io.SeekStart)
-	b, _ := io.ReadAll(e)
-	return string(b)
+	n, _ := io.ReadFull(e, buf)
+	buf = buf[:n]
+	return buf
 }
 
 func (e *textView) ScrollBounds() image.Rectangle {
@@ -632,19 +640,26 @@ func (e *textView) SetCaret(start, end int) {
 	e.caret.end = e.closestToRune(end).runes
 }
 
-// SelectedText returns the currently selected text (if any) from the editor.
-func (e *textView) SelectedText() string {
+// SelectedText returns the currently selected text (if any) from the editor,
+// filling the provided byte slice if it is large enough or allocating and
+// returning a new byte slice if the provided one is insufficient.
+// Callers can guarantee that the buf is large enough by providing a buffer
+// with capacity e.SelectionLen()*utf8.UTFMax.
+func (e *textView) SelectedText(buf []byte) []byte {
 	startOff := e.runeOffset(e.caret.start)
 	endOff := e.runeOffset(e.caret.end)
 	start := min(startOff, endOff)
 	end := max(startOff, endOff)
-	buf := make([]byte, end-start)
+	if cap(buf) < end-start {
+		buf = make([]byte, end-start)
+	}
+	buf = buf[:end-start]
 	n, _ := e.rr.ReadAt(buf, int64(start))
 	// There is no way to reasonably handle a read error here. We rely upon
 	// implementations of textSource to provide other ways to signal errors
 	// if the user cares about that, and here we use whatever data we were
 	// able to read.
-	return string(buf[:n])
+	return buf[:n]
 }
 
 func (e *textView) updateSelection(selAct selectionAction) {