Make UIKit localizable via Localizable.strings

Localizable.m

#import <objc/runtime.h>

/**
To localize `UIBarButtonSystemItemDone` use "Done" key in your localizable.strings:

"Done" = "Localized_Done";
*/

__attribute__((constructor))
static void Hack_NSBundle_localizedString () {
    typedef NSString *(*Func)(NSBundle *, SEL, NSString *, NSString *, NSString *);
    SEL sel = @selector(localizedStringForKey:value:table:);

    Method m = class_getInstanceMethod([NSBundle class], sel);
    Func imp = (Func)method_getImplementation(m);

    NSBundle *mainBundle = [NSBundle mainBundle];

#define DOUBLE_CALL
#ifdef DOUBLE_CALL
    // This approach always calls the original implementation twice.
    // First it gets the "original" value (from a system framework) and uses it
    // as a default value for `localizedStringForKey` from app's bundle.
    // For non-app strings it always makes two calls to the original implementation
    // of `localizedStringForKey:value:table:`.
    method_setImplementation(m, imp_implementationWithBlock(^NSString *(NSBundle *bundle, NSString *key, NSString *value, NSString *table) {
        NSString *localized = imp(bundle, sel, key, value, table);
        if (bundle != mainBundle) {
            localized = imp(mainBundle, sel, key, localized, nil);
        }
        return localized;
    }));
#else
    // This method looks into the app's strings first, and goes into a framework
    // only if the key was not found.
    //
    // It requires a trick with this `marker` value.
    // [[NSBundle mainBundle] localizedStringForKey:@"Non-existent-key" value:nil table:nil]
    // returns @"Non-existent-key" string but not `nil`.
    //
    // This fact makes it difficult to understand if the value for the key exists or not.
    method_setImplementation(m, imp_implementationWithBlock(^NSString *(NSBundle *bundle, NSString *key, NSString *value, NSString *table) {
        static NSString *marker = @".";
        NSString *localized = imp(mainBundle, sel, key, marker, nil);
        if (bundle != mainBundle && [localized isEqualToString:marker]) {
            localized = imp(bundle, sel, key, value, nil);
        }
        return localized;
    }));
#endif
}