I’m using the composite pattern for a Flex app. I’m drawing a series of nodes which are connected to one another (it represents a flow diagram). In the requirement, nodes have children but those children can also be parents of the node. How would I achieve something like this. Have you come across any examples? Maybe I’m approaching it wrong altogether. Maybe there should only be a parent/child relationship and there should be some other mechanism to achieve this relationship?

Ben

Advantage: You can go through the whole objects of the objekt-tree, e.g. to get just one sort ob objects.

I found a java-example in the head-first dp<book. But i have no idea how do this in action-script. There a lot of recursive stuff, and they work with the java-build-in-iteraor ;-(

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removeChild(cv);
trace("removed component: " + cv);
_cvChildren.splice(i,1);
break;

You are correct. Simply removing the composite should suffice for garbage collection, as long as there are no external references to any of its child components.

How would I remove a single, specific component from a composite? This is the closest I’ve come:

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override public function remove(cv:CountryComponent):void {
	for (var i:uint=0; i < _cvChildren.length; i++) {
		if (_cvChildren[i] == cv) {
			_cvChildren.splice(i,1);
			removeChild(cv);
			trace("removed component: " + cv);
		}
	}
}

Also – re: your last reply, if we want to implement a “remove all” we can just remove the composite, correct? or do I have to script a removeAll that loops through _cvChildren and runs remove() for each component?

A good example is the display list in Flash as it implements a composite pattern. The component nodes in the display list inherit from the DisplayObject class. The composite nodes inherit from the DisplayObjectContainer class. The Sprite, Loader and Stage classes extend the DisplayObjectContainer class as we want them to have child components as in the case of nested sprites. In contrast, the Bitmap, Shape and StaticText classes extend the DisplayObject class and are true leaf nodes as for example, we don’t want to have nested objects inside a StaticText field.

You are absolutely correct on the second point. I erred on the side of caution as I wasn’t quite sure of how (and when) Flash garbage collection worked. I wish I’d looked at Grant Skinner’s article on mark sweeping which, clearly shows that an isolated collection of objects referencing each other will also get garbage collected.

http://gskinner.com/blog/archives/2006/06/as3_resource_ma.html

I was a little paranoid about leaving a node branch out there with nothing to do.

Glad you like the book.

-Chandima

I have two questions for you re: the Composite Pattern – one about the pattern itself, one more Actionscript related.

First, I am creating a data structure that’s essentially like a DOM-tree and I don’t know at node-creation-time if the node “is” a branch or a leaf; so I’m making all my nodes branches (composites). I figured that a leaf is a leaf by simple fact that it has no children, but then I was afraid I’d need to “convert” it to composite if the user decided later they wanted to add child nodes to it.

For example, take a bulleted list (…). Most of the time the list item will just contain text (and be a leaf), but some of the time, the list item will contain lots of child nodes as well – a table, another list inside a table (which is wrapped in and and )…

Is this a common scenario with the Composite Pattern? I’ve not read anyone else comment on this problem so I assume either I’ve got it wrong, it’s not a common problem, or all my nodes are REALLY Composite nodes (even if they don’t have Children) simply because they CAN have Children.

Any thoughts?

My second question regards removing nodes. You recursively loop through the child axis to remove nodes to release memory. I find this confusing. If I simple remove reference to a child node from it’s parent (in the parent’s child nodes array), does this not destroy the children in this axis?

I mean, do you really need to go through and kill every child node individually to release memory? If so, can you offer any more detail about this? I was surprised to hear I needed to do the recursive loop (although maybe I just don’t understand something there too).

Many thanks – again, terrific book (and BLOG!).
-Ted

http://www.adobe.com/devnet/actionscript/articles/ora_as3_design_patterns.html

How about a part three dealing with handling events in the composite pattern? Would be really helpful.