Make value getters/setters more consistent

See #6

scrollcontainer.go

@@ -42,6 +42,10 @@ type ScrollContainer struct {
 
 	horizontalCookie event.Cookie
 	verticalCookie   event.Cookie
+
+	on struct {
+		valueChange event.FuncBroadcaster
+	}
 }
 
 // NewScrollContainer creates a new scroll container.
@@ -52,10 +56,12 @@ func NewScrollContainer (sides ScrollSide) *ScrollContainer {
 	}
 	if sides.Vertical() {
 		this.layout.vertical = NewVerticalScrollbar()
+		this.layout.vertical.OnValueChange(this.handleValueChange)
 		this.Add(this.layout.vertical)
 	}
 	if sides.Horizontal() {
 		this.layout.horizontal = NewHorizontalScrollbar()
+		this.layout.horizontal.OnValueChange(this.handleValueChange)
 		this.Add(this.layout.horizontal)
 	}
 	this.CaptureScroll(true)
@@ -103,6 +109,18 @@ func (this *ScrollContainer) SetRoot (root tomo.ContentObject) {
 	}
 }
 
+// Value returns the horizontal and vertical scrollbar values where 0 is all the
+// way to the left/top, and 1 is all the way to the right/bottom.
+func (this *ScrollContainer) Value () (x, y float64) {
+	if this.layout.horizontal != nil {
+		x = this.layout.horizontal.Value()
+	}
+	if this.layout.vertical != nil {
+		y = this.layout.vertical.Value()
+	}
+	return x, y
+}
+
 // SetValue sets the horizontal and vertical scrollbar values where 0 is all the
 // way to the left/top, and 1 is all the way to the right/bottom.
 func (this *ScrollContainer) SetValue (x, y float64) {
@@ -114,16 +132,14 @@ func (this *ScrollContainer) SetValue (x, y float64) {
 	}
 }
 
-// Value returns the horizontal and vertical scrollbar values where 0 is all the
-// way to the left/top, and 1 is all the way to the right/bottom.
-func (this *ScrollContainer) Value () (x, y float64) {
-	if this.layout.horizontal != nil {
-		x = this.layout.horizontal.Value()
-	}
-	if this.layout.vertical != nil {
-		y = this.layout.vertical.Value()
-	}
-	return x, y
+// OnValueChange specifies a function to be called when the user changes the
+// position of the horizontal or vertical scrollbars.
+func (this *ScrollContainer) OnValueChange (callback func ()) event.Cookie {
+	return this.on.valueChange.Connect(callback)
+}
+
+func (this *ScrollContainer) handleValueChange () {
+	this.on.valueChange.Broadcast()
 }
 
 func (this *ScrollContainer) handleScroll (x, y float64) {