Remedy #10

backend.go

@@ -33,6 +33,17 @@ type Backend interface {
 	// must reject any canvas that was not made by it.
 	NewCanvas (image.Rectangle) canvas.CanvasCloser
 
+	// SetStyle sets the style that will be used on objects. The backend is
+	// in charge of applying the style to objects. When this method is
+	// called, it must propagate a StyleChange event to all boxes it is
+	// keeping track of.
+	SetStyle (Style)
+
+	// SetIcons sets the icon set that icons will be pulled from. When this
+	// method is called, it must propagate an IconChange event to all boxes
+	// it is keeping track of.
+	SetIcons (Icons)
+
 	// Run runs the event loop until Stop() is called, or the backend
 	// experiences a fatal error.
 	Run () error

icon.go

@@ -419,5 +419,7 @@ var icons Icons
 
 // SetIcons sets the icon set.
 func SetIcons (icns Icons) {
+	assertBackend()
 	icons = icns
+	backend.SetIcons(icns)
 }

object.go

@@ -124,6 +124,8 @@ type Box interface {
 	OnScroll      (func (deltaX, deltaY float64)) event.Cookie
 	OnKeyDown     (func (key input.Key, numberPad bool)) event.Cookie
 	OnKeyUp       (func (key input.Key, numberPad bool)) event.Cookie
+	OnStyleChange (func ()) event.Cookie
+	OnIconsChange (func ()) event.Cookie
 }
 
 // CanvasBox is a box that can be drawn to.

theme.go

@@ -77,13 +77,7 @@ var style Style
 
 // SetStyle sets the style.
 func SetStyle (sty Style) {
+	assertBackend()
 	style = sty
-}
-
-// Apply applies the current style to the given object, according its role. This
-// may register event listeners with the given object; closing the returned
-// cookie will remove them.
-func Apply (object Object) event.Cookie {
-	if style == nil { return event.NoCookie { } }
-	return style.Apply(object)
+	backend.SetStyle(sty)
 }