Visiting Categories

You're all familiar with the visitor pattern (wikipedia). One of its limitations is that it's only really useful for classes you are writing yourself. That is, unless you have a runtime that allows you to add methods to existing classes like the Objective-C runtime allows via categories (Apple).

Preparing to Visit

Say for example you want to add the ability to traverse your UIView hierarchy and visit each view. The first step is to provide the visitor type that you would like to have visit your views.

@interface ViewVisitor : NSObject
- (void)visitView:(UIView *)view;
- (void)visitLabel:(UILabel *)label;
- (void)visitButton:(UIButton *)button;
...
@end

Now add an acceptVisitor: method to UIView.

@interface UIView (AcceptsViewVisitor)
- (void)acceptVisitor:(ViewVisitor *)visitor;
@end

Now for UIView and each of it's subclasses, implement the acceptVisitor:.

@implementation UIView (AcceptsViewVisitor)
- (void)acceptVisitor:(ViewVisitor *)visitor {
  [visitor visitView:self];
}
@end

You should provide an implementation of this category for each visit___: method you provide in your ViewVisitor class. If you don't provide an implementation for a particular UIView subclass then it's nearest parent class's implementation of the AcceptsViewVisitor will invoke that parent class's associated visit___: method. For example if you provide a visitScrollView: method and implement the AcceptsViewVisitor category for UIScrollView, then calling acceptVisitor: on a UITableView will result in visitScrollView: being called on your visitor with that UITableView instance.

Paying a Visit

Now you can pass your useful subclass of ViewVisitor and pass an instance to each of the views you would like to visit.

MyNSLoggingVisitor *visitor = [MyNSLoggingVisitor new];
for (UIView *subview in view.subviews)
  [subview acceptVisitor:visitor];
}];

Making an Appearance

One fun application of this approach is to style your UI using UIKit's Appearance Proxies (NSHipster).

It turns out that you can set custom setters on class' UIAppearanceProxies and these setters (which must have an object as the first parameter and start with 'set') will be invoked at the appropriate time for instances of that class. Leveraging that we can style a whole UIViewController's contents with a single ViewVisitor instance by providing a setter in a UIView category with the following implementation:

- (void)setAppearanceVisitor:(ViewVisitor *)visitor {
  [self acceptVisitor:visitor];
}

Now for your WickedAwesomeViewController just add this code to your + (void)initialize method or invoke it somewhere before your view controller appears:

MakeItPrettyViewVisitor *visitor = [MakeItPrettyViewVisitor new];
[[UIView appearanceWhenContainedIn:
  [WickedAwesomeViewController class], nil]
    setAppearanceVisitor:visitor];

Check out the sample code on github!