Add Mounter event interface, complements Unmounter

Provides support for a `Mount()` event handler, fired when a component
is attached to the DOM.

dom.go

@@ -41,6 +41,14 @@ type Component interface {
 	Context() *Core
 }
 
+// Mounter is an optional interface that a Component can implement in order
+// to receive component mount events.
+type Mounter interface {
+	// Mount is called after the component has been mounted, after the DOM
+	// element has been added.
+	Mount()
+}
+
 // Unmounter is an optional interface that a Component can implement in order
 // to receive component unmount events.
 type Unmounter interface {
@@ -164,6 +172,7 @@ func (h *HTML) restoreHTML(prev *HTML) {
 				continue
 			}
 			h.Node.Call("appendChild", nextChildRender.Node)
+			doMount(nextChild)
 			continue
 		}
 		prevChild := prev.children[i]
@@ -177,7 +186,9 @@ func (h *HTML) restoreHTML(prev *HTML) {
 		if doRestore(prevChild, nextChild, prevChildRender, nextChildRender) {
 			continue
 		}
+		doUnmount(prevChild)
 		replaceNode(nextChildRender.Node, prevChildRender.Node)
+		doMount(nextChild)
 	}
 	for i := len(h.children); i < len(prev.children); i++ {
 		prevChild := prev.children[i]
@@ -186,9 +197,7 @@ func (h *HTML) restoreHTML(prev *HTML) {
 			prevChildRender = prevChild.(Component).Context().prevRender
 		}
 		removeNode(prevChildRender.Node)
-		if u, ok := prevChild.(Unmounter); ok {
-			u.Unmount()
-		}
+		doUnmount(prevChild)
 	}
 }
 
@@ -261,6 +270,7 @@ func (h *HTML) Restore(old ComponentOrHTML) {
 			continue
 		}
 		h.Node.Call("appendChild", nextChildRender.Node)
+		doMount(nextChild)
 	}
 }
 
@@ -300,7 +310,9 @@ func Rerender(c Component) {
 		return
 	}
 	if prevRender != nil {
+		doUnmount(c)
 		replaceNode(nextRender.Node, prevRender.Node)
+		doMount(c)
 	}
 }
 
@@ -350,10 +362,12 @@ func RenderBody(body Component) {
 	if doc.Get("readyState").String() == "loading" {
 		doc.Call("addEventListener", "DOMContentLoaded", func() { // avoid duplicate body
 			doc.Set("body", nextRender.Node)
+			doMount(body)
 		})
 		return
 	}
 	doc.Set("body", nextRender.Node)
+	doMount(body)
 }
 
 // SetTitle sets the title of the document.
@@ -368,3 +382,17 @@ func AddStylesheet(url string) {
 	link.Set("href", url)
 	js.Global.Get("document").Get("head").Call("appendChild", link)
 }
+
+// doMount calls the Mount function on Mounter components
+func doMount(h ComponentOrHTML) {
+	if m, ok := h.(Mounter); ok {
+		m.Mount()
+	}
+}
+
+// doUnmount calls the Unmount function on Unmounter components
+func doUnmount(h ComponentOrHTML) {
+	if u, ok := h.(Unmounter); ok {
+		u.Unmount()
+	}
+}