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/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) {