davedelong · October 15, 2022 08:02 · Oct 12, 2019 · Oct 12, 2019
diff --git a/NSLocale.m b/NSLocale.m
@@ -62,7 +62,7 @@ + (NSLocale *)localeIncludingOverridesWithLocaleIdentifier:(NSString *)localeIde
 
 struct __DDLocale {
     void *_base; // The CF version of an "isa" pointer
-    void *_field1;    // maybe the locale identifier? 
+    void *_field1; // maybe the locale identifier? 
     void *_field2; // maybe a cache
     void *_field3; // no idea
     CFDictionaryRef _prefs; // The Secret Sauce
@@ -82,10 +82,10 @@ + (NSLocale *)localeIncludingOverridesWithLocaleIdentifier:(NSString *)localeIde
     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 improbably magic of NSUUID
+    // 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 not "guaranteed"
+    // Turn the components back into a locale identifier. It's now "guaranteed"
     // to be unique
     NSString *newID = [NSLocale localeIdentifierFromComponents:components];
 

diff --git a/NSLocale.m b/NSLocale.m
@@ -0,0 +1,114 @@
+/*
+
+======================================================
+
+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 improbably magic of NSUUID
+    [components setObject:[[NSUUID UUID] UUIDString] forKey:@"custom"];
+
+    // Turn the components back into a locale identifier. It's not "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