Sam Soffes

Have you ever wanted to override some functionality in UIKit that was in a hard to reach place? A lot of applications on the App Store have custom UINavigationBar's. I really wanted to do this one of my company's upcoming apps.

A popular solution for this creating a category and overriding the method you want to change in it. For this the example, we'll make a UINavigationBar green instead (obviously you could do something cool here instead). The category way would look something like this:

This approach works very well, but there are two issues with it: overriding methods with a category is an Objective-C no no and if you need to call the default implementation, you can't.

In my app, I wanted to change most of the navigation bars in all of the navigation controllers. I am using a UIImagePickerController in part of the app and it was customized to. I really wanted to keep it the translucent style instead of the style for the rest of my app.

I decided any UINavigationBar with UIBarStyleDefault as its style, I want to override and everything else leave alone. There is no way to do this with the category approach. You can't call [self drawRect:rect] because it would infinitely call itself since you replaced it with the method you are calling it from.

Method swizzling

After some googling and some help from #macdev on Freenode, I changed my solution to use method swizzling. Method swizzling, in short, is switching methods at runtime. So you can say for UINavigationBar don't use the standard drawRect:, but instead swap it with a different one. (This is kinda confusing, but hang in there. It's not that hard.)

I updated my category to look like this:

I then updated main.m to look like this:

So this kinda hurt my head when I was first looking at all of this. In main.m, before the application starts, I swizzle the UINavigationBar methods. method_exchangeImplementations() switches my drawRectCustomBackground: with drawRect: in UINavigationBar. When I call the default implementation in drawRectCustomBackground:, it looks like I'm calling the same method, but I am actually calling the default implementation because it swapped them.

This is pretty crazy and a little confusing (especially with someone new to Objective-C), but really powerful. You can use this approach to customize a lot of things Apple didn't intend for you to mess with. Go out and make something cool!