feat: Style.Transform for altering strings at render time

get.go

@@ -408,6 +408,12 @@ func (s Style) GetFrameSize() (x, y int) {
 	return s.GetHorizontalFrameSize(), s.GetVerticalFrameSize()
 }
 
+// GetTransform returns the transform set on the style. If no transform is set
+// nil is returned.
+func (s Style) GetTransform() func(string) string {
+	return s.getAsTransform(transformKey)
+}
+
 // Returns whether or not the given property is set.
 func (s Style) isSet(k propKey) bool {
 	_, exists := s.rules[k]
@@ -469,6 +475,17 @@ func (s Style) getBorderStyle() Border {
 	return noBorder
 }
 
+func (s Style) getAsTransform(k propKey) func(string) string {
+	v, ok := s.rules[k]
+	if !ok {
+		return nil
+	}
+	if fn, ok := v.(func(string) string); ok {
+		return fn
+	}
+	return nil
+}
+
 // Split a string into lines, additionally returning the size of the widest
 // line.
 func getLines(s string) (lines []string, widest int) {

set.go

@@ -544,6 +544,18 @@ func (s Style) StrikethroughSpaces(v bool) Style {
 	return s
 }
 
+// Transform applies a given function to a string at render time, allowing for
+// the string being rendered to be manipuated.
+//
+// Example:
+//
+//	s := NewStyle().Transform(strings.ToUpper)
+//	fmt.Println(s.Render("raow!") // "RAOW!"
+func (s Style) Transform(fn func(string) string) Style {
+	s.set(transformKey, fn)
+	return s
+}
+
 // Renderer sets the renderer for the style. This is useful for changing the
 // renderer for a style that is being used in a different context.
 func (s Style) Renderer(r *Renderer) Style {

style.go

@@ -73,6 +73,8 @@ const (
 	tabWidthKey
 	underlineSpacesKey
 	strikethroughSpacesKey
+
+	transformKey
 )
 
 // A set of properties.
@@ -225,6 +227,8 @@ func (s Style) Render(strs ...string) string {
 
 		// Do we need to style spaces separately?
 		useSpaceStyler = underlineSpaces || strikethroughSpaces
+
+		transform = s.getAsTransform(transformKey)
 	)
 
 	if len(s.rules) == 0 {
@@ -401,6 +405,10 @@ func (s Style) Render(strs ...string) string {
 		str = strings.Join(lines[:min(maxHeight, len(lines))], "\n")
 	}
 
+	if transform != nil {
+		return transform(str)
+	}
+
 	return str
 }
 

style_test.go

@@ -3,6 +3,7 @@ package lipgloss
 import (
 	"io"
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/muesli/termenv"
@@ -374,6 +375,42 @@ func TestTabConversion(t *testing.T) {
 	requireEqual(t, "[\t]", s.Render("[\t]"))
 }
 
+func TestStringTransform(t *testing.T) {
+	for i, tc := range []struct {
+		input    string
+		fn       func(string) string
+		expected string
+	}{
+		{
+			"raow",
+			strings.ToUpper,
+			"RAOW",
+		},
+		{
+			"The quick brown 狐 jumped over the lazy 犬",
+			func(s string) string {
+				n := 0
+				rune := make([]rune, len(s))
+				for _, r := range s {
+					rune[n] = r
+					n++
+				}
+				rune = rune[0:n]
+				for i := 0; i < n/2; i++ {
+					rune[i], rune[n-1-i] = rune[n-1-i], rune[i]
+				}
+				return string(rune)
+			},
+			"犬 yzal eht revo depmuj 狐 nworb kciuq ehT",
+		},
+	} {
+		res := NewStyle().Transform(tc.fn).Render(tc.input)
+		if res != tc.expected {
+			t.Errorf("Test #%d:\nExpected: %q\nGot: %q", i+1, tc.expected, res)
+		}
+	}
+}
+
 func BenchmarkStyleRender(b *testing.B) {
 	s := NewStyle().
 		Bold(true).