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Customize UIKit with Method Swizzling

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Update 03/03/12: You should really use UIAppearance since this no longer works in iOS 5.0.


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:

@interface UINavigationBar (CustomBackground)
@end
@implementation UINavigationBar (CustomBackground)

- (void)drawRect:(CGRect)rect {
    [[UIColor greenColor] set];
    CGRect fillRect = CGRectMake(0.0, 0.0, self.frame.size.width, self.frame.size.height);
    CGContextFillRect(UIGraphicsGetCurrentContext(), fillRect);
}

@end

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:

@interface UINavigationBar (CustomBackground)
- (void)drawRectCustomBackground:(CGRect)rect;
@end
@implementation UINavigationBar (CustomBackground)

- (void)drawRectCustomBackground:(CGRect)rect {
    if (self.barStyle == UIBarStyleDefault) {
        [[UIColor greenColor] set];
        CGRect fillRect = CGRectMake(0.0, 0.0, self.frame.size.width, self.frame.size.height);
        CGContextFillRect(UIGraphicsGetCurrentContext(), fillRect);
        return;
    }

    // Call default implementation
    [self drawRectCustomBackground:rect];
}

@end

I then updated main.m to look like this:

#import <objc/runtime.h>
#import "UINavigationBar+CustomBackground.h"

int main(int argc, char *argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // Swizzle the nav bar
    Method drawRectCustomBackground = class_getInstanceMethod([UINavigationBar class], @selector(drawRectCustomBackground:));
    Method drawRect = class_getInstanceMethod([UINavigationBar class], @selector(drawRect:));
    method_exchangeImplementations(drawRect, drawRectCustomBackground);

    int retVal = UIApplicationMain(argc, argv, nil, @"AppDelegate");

    [pool release];
    return retVal;
}

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!