Increasingly I’ve been encountering software developed for iOS which was written by someone who cut their eye-teeth on writing UI software for the web in Javascript. And I’ve noticed a common pattern which arises from that code which makes their code overly complex and hard to maintain on iOS.

Basically, they treat UIView-based objects as passive objects.

And if you started live writing software on the web using Javascript, this makes sense. After all, the view structure represented in the DOM hierarchy is by and large a passive participant in the user interface. When you want to present a list of items in a scrolling region, for example, you would create the scrolling region, then populate the items in that region, setting the attributes of each of the <div> sections and <li> sections with the appropriate CSS styles for overflow and font information.

(In something like GWT it’s not hard to create your own custom view objects, just as you can create custom view objects in Javascript. But at the bottom of the stack, the Menu class or the Table class you built simply declares a <div> and manipulates the subset of the DOM within that <div> tag. The view hierarchy is still a passive participant; you simply wrap part of that passive content into something with some logic.)

The problem is when you move all this onto iOS or onto Android or onto Windows or onto MacOS X, you lose one of the most powerful elements of those platforms–and that is Views are active objects rather than passive participants.

Meaning that a child class of a UITableViewCell in iOS can actively know how to present a data record passed to it; you do not need to put the code to populate the table view cell within the UITableViewController class. You can build an Image View which knows how make a network call and load and cache an image without putting image loading and caching code into the UIViewController. You can create a single view which has complex behavior–without having to put the behavior code inside the view controller.

And this allows you to create very complex user interface elements and reuse them throughout your code.

Of course this also needs to be balanced with the available tools. For example, it makes no sense to create a custom UIButton which presents a button with a different font, when you can set the font and appearance of a default button within the NIB.

But it does indicate that in many cases the proper thing to do is to push functionality down into the view, rather than make the view controller responsible for that logic.

You can’t do this in Javascript. You cannot create a new <myTableTag> which takes special arguments and presents the contents in a unique way, which can be reused throughout your site.

And there is nothing inherently “wrong” with putting all the logic for your button’s behavior, the way table view cells are laid out, and the responsibility for loading images in the background into your view controller.

It just makes life far more complex than it has to be, because you’re leaving one of the most important tools in your arsenal on the floor.