add ability to abort from controller::onunload

docs/change-log.md

@@ -2,6 +2,10 @@
 
 [v0.1.16](/mithril/archive/v0.1.6) - maintenance
 
+### News:
+
+-	controller::onunload now receives an event parameter so that the unloading can be aborted [#135](https://github.com/lhorie/mithril.js/issues/135)
+
 ### Bug Fixes:
 
 -	prevent route change when only hash changes in non-hash mode [#107](https://github.com/lhorie/mithril.js/issues/107)

docs/mithril.module.md

@@ -129,6 +129,17 @@ m.module(document, module2); // logs "unloading module 1"
 
 This mechanism is useful to clear timers and unsubscribe event handlers. If you have a hierarchy of components, you can recursively call `onunload` on all the components in the tree or use a [pubsub](http://microjs.com/#pubsub) library to unload specific components on demand.
 
+You can also use this event to prevent a module from being unloaded (e.g. to alert a user to save their changes before navigating away from a page)
+
+```javascript
+var module1 = {}
+module1.controller = function() {
+	this.onunload = function(e) {
+		if (!confirm("are you sure you want to leave this page?")) e.preventDefault()
+	}
+}
+```
+
 ---
 
 ### Signature
@@ -140,7 +151,8 @@ void module(DOMElement rootElement, Module module)
 
 where:
 	Module :: Object { Controller, void view(Object controllerInstance) }
-	Controller :: void controller() | void controller() { prototype: void unload() }
+	Controller :: void controller() | void controller() { prototype: void unload(UnloadEvent e) }
+	UnloadEvent :: Object {void preventDefault()}
 ```
 
 -	**DOMElement rootElement**

mithril.js

@@ -297,14 +297,22 @@ Mithril = m = new function app(window) {
 
 	var roots = [], modules = [], controllers = [], now = 0, lastRedraw = 0, lastRedrawId = 0, computePostRedrawHook = null
 	m.module = function(root, module) {
-		m.startComputation()
 		var index = roots.indexOf(root)
 		if (index < 0) index = roots.length
-		roots[index] = root
-		modules[index] = module
-		if (controllers[index] && typeof controllers[index].onunload == "function") controllers[index].onunload()
-		controllers[index] = new module.controller
-		m.endComputation()
+		var isPrevented = false
+		if (controllers[index] && typeof controllers[index].onunload == "function") {
+			var event = {
+				preventDefault: function() {isPrevented = true}
+			}
+			controllers[index].onunload(event)
+		}
+		if (!isPrevented) {
+			m.startComputation()
+			roots[index] = root
+			modules[index] = module
+			controllers[index] = new module.controller
+			m.endComputation()
+		}
 	}
 	m.redraw = function() {
 		now = window.performance && window.performance.now ? window.performance.now() : new window.Date().getTime()