Locales

NSLocale.m

/*

======================================================

THIS CODE IS FOR EDUCATIONAL PURPOSES ONLY.

I'M NOT RESPONSIBLE IF YOU SHIP THIS AND IT BLOWS UP IN YOUR FACE.

IF IT DOES AND YOU COMPLAIN TO ME I WILL LAUGH AT YOU.

======================================================

This code is based on the following premise:
• You want to show things in a particular locale
• You want to honor any overrides the user has set

iOS will only format dates using the +[NSLocale currentLocale] if the app is localized to support that locale.

That means that if your app is running on:
• a device set to a language your app does not support AND
• the user has custom overrides for 12/24 hour, date/time formats, etc

THEN:

• the user will see dates formatted in another language, and WITHOUT their overrides.

This is because you end up getting a locale that's just the "default" locale for a particular language, 
with no overrides applied.

Enter this code.

This code attempts to work around this (admittedly rather edge case) scenario.

The overrides on a locale are stored inside the "_prefs" field. For the current locale, 
this is basically copied out of NSUserDefaults and is how the current locale knows about the 
12/24 hour override, among other things.

For custom locales, this is NULL. And there is no way to change it.

Also working against us is the fact that _prefs IS NOT AN OBJC IVAR FIELD. NSLocale is toll-free
bridged to CFLocaleRef, which means we're really dealing with pointers to C structs, and the fields
on those structs are not described to the Objective-C runtime. So the Runtime doesn't even help us.

So instead, we get to drop down to the raw memory layout and fudge things around.

Please don't use this in shipping code.

*/

@interface NSLocale (Voodoo)

// hide all the nastiness behind a nice, pleasant method
+ (NSLocale *)localeIncludingOverridesWithLocaleIdentifier:(NSString *)localeIdentifier;

@end


// this is the (rough) layout of a CFLocaleRef
// I figured this out based on:
//   - https://github.com/apple/swift-corelibs-foundation/blob/155f1ce1965effe55289477507a6f9fbdc8fe333/CoreFoundation/Locale.subproj/CFLocale.c#L144-L151
//   - too much time in the debugger

struct __DDLocale {
    void *_base; // The CF version of an "isa" pointer
    void *_field1; // maybe the locale identifier? 
    void *_field2; // maybe a cache
    void *_field3; // no idea
    CFDictionaryRef _prefs; // The Secret Sauce
    void *_lock; // maybe some thread safety thing
    Boolean _nullLocale; // who knows
};

@implementation NSLocale (Voodoo)

+ (NSLocale *)localeIncludingOverridesWithLocaleIdentifier:(NSString *)localeIdentifier {

    // First, rip apart the locale identifier in to its components
    // Why? because if you create a locale w/ an identifier, and another locale instance 
    // already exists that has the same identifier, you'll just get a pointer back
    // to the pre-existing object. For this class, that's probably what you want
    // most of the time. For this scenario, it's not.
    NSMutableDictionary *components = [[NSLocale componentsFromLocaleIdentifier:localeIdentifier] mutableCopy];
    
    // So instead we'll "force" a new instance of a locale by creating a never-before-seen
    // locale identifier, thanks to the improbable magic of NSUUID
    [components setObject:[[NSUUID UUID] UUIDString] forKey:@"custom"];
    
    // Turn the components back into a locale identifier. It's now "guaranteed"
    // to be unique
    NSString *newID = [NSLocale localeIdentifierFromComponents:components];
    
    // Construct a brand-spanking-new NSLocale
    NSLocale *copy = [[NSLocale alloc] initWithLocaleIdentifier:newID];
    
    // If, for some reason, it failed, early return
    // This is because we're about to do pointer magic, and doing pointer
    // magic with a NULL pointer is a great way to crash. Let's not crash.
    if (copy == nil) { return nil; }

    // Cast (lie to the compiler) that these pointers are actually pointers
    // to our struct definition that we worked out before
    struct __DDLocale* this = (struct __DDLocale *)[NSLocale currentLocale];
    struct __DDLocale* that = (struct __DDLocale *)copy;
    
    // Copy over the dictionary of overrides from the +currentLocale
    // to our new copy. This is how our copy will get to know about the
    // overrides as well
    that->_prefs = CFDictionaryCreateMutableCopy(NULL, 0, this->_prefs);
    
    // Share And Enjoy.
    return copy;
}

@end