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    <p>If you directly access parent classes from child objects, you create strong coupling - which is exactly what you <em>don't</em> want in a well-built system. It is best not to access the application object directly, but to use event listeners and custom events to promote changes from e.g. the navigation.</p> <p>Here's an example. First, create a custom event:</p> <pre><code>public class MyCustomEvent extends Event { public static const MENU_ITEM_SELECTED : String = "MENU_ITEM_SELECTED"; public var selectedItem:String; } </code></pre> <p>Then, let the navigation dispatch it, when a button is clicked:</p> <pre><code>public class Navigation extends Sprite () { // ... private function onButtonClicked(ev:Event) : void { ev.stopPropagation(); var custEvent:MyCustomEvent = new MyCustomEvent(MyCustomEvent.MENU_ITEM_SELECTED); custEvent.selectedItem = ev.target.name; this.dispatchEvent (custEvent); } // ... } </code></pre> <p>Finally, let the application handle the custom event and bring up a different screen: </p> <pre><code>public class App { // ... public function createNavigation () : void { navigation = new Navigation (); navigation.addEventListener (MyCustomEvent.MENU_ITEM_SELECTED, onMenuItemSelected); // ... more stuff happening } // ... private function onMenuItemSelected (ev:MyCustomEvent) : void { switchToScreen (ev.selectedItem); } private function switchToScreen (name:String) : void { // choose screen by name, etc. } } </code></pre> <p>For all of this, neither the screen, nor the navigation have to know anything about any other objects involved, so you can easily replace each one without breaking the rest of the system.</p>
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    1. COwow :D this is really a level up for me :) I guess I'll have to read this a few times to really understand. The links about inversion control and loose coupling I'll read tomorrow. Thanks for this very complete answer. Just one question; what I learned about OOP was that all the objects have their own responsibilities. I thought that the navigation then should handle ALL the code that is about navigating. The way I see it now is that the App class is actually doing some of the navigation right? or am I seeing it wrong? Thanks a million!
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    2. COSorry, I had a lot on my plate and completely forgot to answer this... You have to see it this way: The Navigation in this example is really just a bunch of buttons. It does only what it is supposed to do: process user mouse clicks and inform the application about what was clicked. Actually changing the state of the application is the responsibility of the application itself (or a dedicated controller component, but that's another story).
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