diff --git a/elements/basic/textbox.go b/elements/basic/textbox.go
index 2d9ba2d..387178c 100644
--- a/elements/basic/textbox.go
+++ b/elements/basic/textbox.go
@@ -7,6 +7,7 @@ import "git.tebibyte.media/sashakoshka/tomo/artist"
 import "git.tebibyte.media/sashakoshka/tomo/textmanip"
 import "git.tebibyte.media/sashakoshka/tomo/elements/core"
 
+// TextBox is a single-line text input.
 type TextBox struct {
 	*core.Core
 	core core.CoreControl
@@ -19,9 +20,14 @@ type TextBox struct {
 	text        []rune
 	placeholderDrawer artist.TextDrawer
 	valueDrawer       artist.TextDrawer
+	onKeyDown         func (tomo.Key, tomo.Modifiers, bool) (bool)
+	onChange          func ()
 }
 
-func NewTextBox (placeholder, text string) (element *TextBox) {
+// NewTextBox creates a new text box with the specified placeholder text, and
+// a value. When the value is empty, the placeholder will be displayed in gray
+// text.
+func NewTextBox (placeholder, value string) (element *TextBox) {
 	element = &TextBox { enabled: true }
 	element.Core, element.core = core.NewCore(element)
 	element.placeholderDrawer.SetFace(theme.FontFaceRegular())
@@ -29,7 +35,7 @@ func NewTextBox (placeholder, text string) (element *TextBox) {
 	element.placeholder = placeholder
 	element.placeholderDrawer.SetText([]rune(placeholder))
 	element.updateMinimumSize()
-	element.SetText(text)
+	element.SetValue(value)
 	return
 }
 
@@ -52,6 +58,10 @@ func (element *TextBox) HandleKeyDown (
 	modifiers tomo.Modifiers,
 	repeated bool,
 ) {
+	if element.onKeyDown != nil && element.onKeyDown(key, modifiers, repeated) {
+		return
+	}
+
 	altered := true
 	switch {
 	case key == tomo.KeyBackspace:
@@ -60,6 +70,7 @@ func (element *TextBox) HandleKeyDown (
 			element.text,
 			element.cursor,
 			modifiers.Control)
+		element.runOnChange()
 			
 	case key == tomo.KeyDelete:
 		if len(element.text) < 1 { break }
@@ -67,6 +78,7 @@ func (element *TextBox) HandleKeyDown (
 			element.text,
 			element.cursor,
 			modifiers.Control)
+		element.runOnChange()
 			
 	case key == tomo.KeyLeft:
 		element.cursor = textmanip.MoveLeft (
@@ -85,6 +97,7 @@ func (element *TextBox) HandleKeyDown (
 			element.text,
 			element.cursor,
 			rune(key))
+		element.runOnChange()
 			
 	default:
 		altered = false
@@ -168,10 +181,11 @@ func (element *TextBox) updateMinimumSize () {
 		theme.Padding() * 2)
 }
 
-func (element *TextBox) SetText (text string) {
+func (element *TextBox) SetValue (text string) {
 	// if element.text == text { return }
 
 	element.text = []rune(text)
+	element.runOnChange()
 	element.valueDrawer.SetText(element.text)
 	if element.cursor > element.valueDrawer.Length() {
 		element.cursor = element.valueDrawer.Length()
@@ -187,6 +201,30 @@ func (element *TextBox) Value () (value string) {
 	return string(element.text)
 }
 
+func (element *TextBox) Filled () (filled bool) {
+	return len(element.text) > 0
+}
+
+func (element *TextBox) OnKeyDown (
+	callback func (
+		key tomo.Key, modifiers tomo.Modifiers, repeated bool,
+	) (
+		handled bool,
+	),
+) {
+	element.onKeyDown = callback
+}
+
+func (element *TextBox) OnChange (callback func ()) {
+	element.onChange = callback
+}
+
+func (element *TextBox) runOnChange () {
+	if element.onChange != nil {
+		element.onChange()
+	}
+}
+
 func (element *TextBox) draw () {
 	bounds := element.core.Bounds()
 
diff --git a/examples/input/main.go b/examples/input/main.go
index e0530c6..f16bdbd 100644
--- a/examples/input/main.go
+++ b/examples/input/main.go
@@ -12,32 +12,45 @@ func main () {
 
 func run () {
 	window, _ := tomo.NewWindow(2, 2)
-	window.SetTitle("Approaching")
+	window.SetTitle("Enter Details")
 	container := basic.NewContainer(layouts.Vertical { true, true })
 	window.Adopt(container)
 
+	// create inputs
 	firstName    := basic.NewTextBox("First name", "")
 	lastName     := basic.NewTextBox("Last name", "")
 	fingerLength := basic.NewTextBox("Length of fingers", "")
 	button       := basic.NewButton("Ok")
 
-	lastName.SetEnabled(false)
+	button.SetEnabled(false)
 	button.OnClick (func () {
+		// create a dialog displaying the results
 		popups.NewDialog (
 			popups.DialogKindInfo,
 			"Profile",
-			firstName.Value() + " [REDACTED]'s fingers\n" +
-			"measure in at " + fingerLength.Value() + " feet.")
+			firstName.Value() + " " + lastName.Value() +
+			"'s fingers\nmeasure in at " + fingerLength.Value() +
+			" feet.")
 	})
-	
+
+	// enable the Ok button if all three inputs have text in them
+	check := func () {
+		button.SetEnabled (
+			firstName.Filled() &&
+			lastName.Filled() &&
+			fingerLength.Filled())
+	}
+	firstName.OnChange(check)
+	lastName.OnChange(check)
+	fingerLength.OnChange(check)
+
+	// add elements to container
 	container.Adopt(basic.NewLabel("Choose your words carefully.", false), true)
 	container.Adopt(firstName, false)
 	container.Adopt(lastName, false)
 	container.Adopt(fingerLength, false)
 	container.Adopt(basic.NewSpacer(true), false)
 	container.Adopt(button, false)
-
-	firstName.Select()
 	
 	window.OnClose(tomo.Stop)
 	window.Show()