iOS: How do I know if a property is KVO-compliant?

Question

In the Key-Value Observing Programming Guide, the section Registering for Key-Value Observing says \"Typically properties in Apple-supplied frameworks are only KVO-compliant

Answer 1

Based on @David DeLong's solution, this is what I came up with, and it works beautifully.

Basically, I made a category on UIWindow. And in +load, I (run-time) check whether [UIWindow instancesRespondToSelector:@selector(rootViewController)]. If not, I use class_addMethod() to dynamically add the getter & setter methods for rootViewController. Also, I use objc_getAssociatedObject and objc_setAssociatedObject to get & set the rootViewController as an instance variable of UIWindow.

// UIWindow+Additions.h

@interface UIWindow (Additions)

@end

// UIWindow+Additions.m

#import "UIWindow+Additions.h"
#include 

@implementation UIWindow (Additions)

#if __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_4_0
// Add rootViewController getter & setter.
static UIViewController *rootViewControllerKey;

UIViewController *rootViewController3(id self, SEL _cmd);
void setRootViewController3(id self, SEL _cmd, UIViewController *newRootViewController);

UIViewController *rootViewController3(id self, SEL _cmd) {
    return (UIViewController *)objc_getAssociatedObject(self, &rootViewControllerKey);
}

void setRootViewController3(id self, SEL _cmd, UIViewController *newRootViewController) {
    UIViewController *rootViewController = [self performSelector:@selector(rootViewController)];
    if (newRootViewController != rootViewController) {
        // Remove old views before adding the new one.
        for (UIView *subview in [self subviews]) {
            [subview removeFromSuperview];
        }
        objc_setAssociatedObject(self, &rootViewControllerKey, newRootViewController,
                                 OBJC_ASSOCIATION_RETAIN_NONATOMIC);
        [self addSubview:newRootViewController.view];
    }
}

+ (void)load {
    if (![UIWindow instancesRespondToSelector:@selector(rootViewController)]) {
        class_addMethod([self class], @selector(rootViewController),
                        (IMP)rootViewController3, "@@:");
        class_addMethod([self class], @selector(setRootViewController:),
                        (IMP)setRootViewController3, "v@:@");
    }
}
#endif

@end