gistfile1.m

@protocol APRemoteAddressBook <NSObject>
- (void) allPeople: (void (^)(NSArray *, NSError *)) reply;
- (void) mailingAddressesForPersonWithIdentifier: (NSString *) identifier
                                           reply: (void (^)(NSArray *, NSError *)) reply;
- (void) emailAddressesForPersonWithIdentifier: (NSString *) identifier
                                         reply: (void (^)(NSArray *, NSError *)) reply;
- (void) phoneNumbersForPersonWithIdentifier: (NSString *) identifier
                                       reply: (void (^)(NSArray *, NSError *)) reply;
@end

@protocol APRemoteCommandHandler <NSObject>
- (void) handleCommand: (NSDictionary *) command
       returningResult: (void (^)(NSDictionary *packagedResult)) handler;
@end

@protocol APRemoteAddressBookBrowser <NSObject>
- (void) setCommandHandler: (id<APRemoteCommandHandler>) commandHandler
              errorHandler: (void (^)(NSError *)) errorHandler;
- (void) availableServiceNames: (void (^)(NSArray *, NSError *)) reply;
- (void) connectToServiceWithName: (NSString *) name
                     replyHandler: (void (^)(id<APRemoteAddressBook>, NSError *)) replyHandler;
@end

@interface APRemoteAddressBook : NSObject <APRemoteAddressBook>
// ...
@end

@interface APRemoteAddressBookBrowser : NSObject <APRemoteAddressBookBrowser>
// ...
@end

@implementation APRemoteAddressBookBrowser

// ...

- (void) connectToServiceWithName: (NSString *) name
                     replyHandler: (void (^)(id<APRemoteAddressBook>, NSError *)) replyHandler
{
    // copy the reply block onto the heap
    void (^replyCopy)(id<APRemoteAddressBook>, NSError *) = [replyHandler copy];
    
    // ...
    
    NSLog(@"Resolved successfully");
    APRemoteAddressBook * book = [[APRemoteAddressBook alloc] initWithResolvedService: selected];
    replyCopy(book, nil);  // this is definitely the returned invocation
}

// ...

@end

// XPC endpoint set up like so:

- (BOOL) listener: (NSXPCListener *) listener shouldAcceptNewConnection: (NSXPCConnection *) newConnection
{
    NSXPCInterface * myInterface = [NSXPCInterface interfaceWithProtocol: @protocol(APRemoteAddressBookBrowser)];
    
    // add proxy details for the return value of -connectToServiceWithName:
    NSXPCInterface * bookInterface = [NSXPCInterface interfaceWithProtocol: @protocol(APRemoteAddressBook)];
    // first argument of the reply block is to be sent as a proxy
    [myInterface setInterface: bookInterface
                  forSelector: @selector(connectToServiceWithName:replyHandler:)
                argumentIndex: 0
                      ofReply: YES];
    
    // proxy details for the commandHandler object sent to -setCommandHandler:
    NSXPCInterface * commandHandlerInterface = [NSXPCInterface interfaceWithProtocol: @protocol(APRemoteCommandHandler)];
    // first argument to the function is a proxy object
    [myInterface setInterface: commandHandlerInterface
                  forSelector: @selector(setCommandHandler:errorHandler:)
                argumentIndex: 0
                      ofReply: NO];
    
    newConnection.exportedInterface = myInterface;
    newConnection.exportedObject = self;
    [newConnection resume];
    
    return ( YES );
}

// when I attempt to actually return an APRemoteAddressBook proxy, the receiving
//  application complains that it doesn't expect it. If I change the interface declaration
//  above, it fails to encode, because APRemoteAddressBook doesn't implement NSSecureCoding.
//  So it's definitely the right thing being passed as a proxy, so that's fine.
// What else is going wrong? The error here is damned near *useless*.

/*
2012-07-30 14:45:06.924 Core Data Contacts[7388:8623] Warning: Exception caught during decoding of received reply to message 'connectToServiceWithName:replyHandler:', dropping incoming message and calling failure block.

Exception: Exception while decoding argument 1 of invocation:
<NSInvocation: 0x101b76740>
return value: {v} void
target: {@?} 0x0 (block)
argument 1: {@} 0x0
argument 2: {@} 0x0

Exception: value for key '(no key, possibly an argument to a message)' was of unexpected class '_NSXPCDistantObject'. Allowed classes are '{(
)}'.
*/