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CHAPTER 9: Writing an Application 283To handle custom objects within a collection, you would provide some additions to the classwhitelist for the NSXPCInterface defining the receiver of the relevant message. The same appliesif the collection is being provided to a caller-passed reply block. The code excerpted inListing 9-15 shows how this would normally be achieved.Listing 9-15. Passing Custom Objects in an XPC Collection@interface MyCustomObject...@end@protocol MyFilter - (void) filterObjects: (NSArray *) objectsreply: (void (^)(NSArray * filtered)) reply;@end...- (void) listener: (NSXPCListener *) listenershouldAcceptConnection: (NSXPCConnection *) connection{NSXPCInterface * myInterface =[NSXPCInterface interfaceWithProtocol: @protocol(MyFilter)];// define the class whitelistNSSet * expectedClasses = [NSSet setWithObject: [MyCustomObject class]];// set this list for the input collection...[myInterface setClasses: expectedClassesforSelector: @selector(filterObjects:reply:)argumentIndex: 0 // the first parameterofReply: NO]; // an argument of the method itself// ...and also on the collection passed to the reply block[myInterface setClasses: expectedClassesforSelector: @selector(filterObjects:reply:)argumentIndex: 0 // the first parameterofReply: YES];}connection.exportedInterface = myInterface;...In your application, you won’t be sending custom objects across the XPC connection. You will,however, be vending objects by proxy. This has a slightly different implementation—rather thanspecifying a class whitelist for a message parameter, you specify an NSXPCInterface describingthe object to be proxied. Once you have the interface defined, you inform your main interface bycalling a method very similar to the one used to update the class whitelist.In the primary interface defined by the APRemoteAddressBookBrowser protocol there are two suchobjects. The first is the APAddressBook-conforming object passed into -vendAddressBook:www.it-ebooks.info