Add documentation for Slider

scrollbar.go

@@ -33,12 +33,12 @@ func newScrollbar (orient string) *Scrollbar {
 
 	this.Add(this.handle)
 	this.SetFocusable(true)
-	
+
 	this.CaptureDND(true)
 	this.CaptureMouse(true)
 	this.CaptureScroll(true)
 	this.CaptureKeyboard(true)
-	
+
 	this.OnKeyDown(this.handleKeyDown)
 	this.OnMouseDown(this.handleMouseDown)
 	this.OnMouseUp(this.handleMouseUp)
@@ -80,6 +80,13 @@ func (this *Scrollbar) SetValue (value float64) {
 // right/bottom.
 func (this *Scrollbar) Value () float64 {
 	// TODO
+	return 0
+}
+
+// OnValueChange specifies a function to be called when the position of the
+// scrollbar changes.
+func (this *Slider) OnValueChange (callback func ()) event.Cookie {
+	return this.on.valueChange.Connect(callback)
 }
 
 func (this *Scrollbar) handleKeyDown (key input.Key, numpad bool) {