NSXPCConnection pass an object by proxy

The Daemons and Services Programming Guides tells that it is possible to return a proxy object through an open XPC connection, even as a reply block parameter.

Passing an Object By Proxy

Most of the time, it makes sense to copy objects and send them to the other side of a connection. However, this is not always desirable. In particular:

If you need to share a single instance of the data between the client application and the helper, you must pass the objects by proxy. If an object needs to call methods on other objects within your application that you cannot or do not wish to pass across the connection (such as user interface objects), then you must pass an object by proxy—either the caller, the callee (where possible), or a relay object that you construct specifically for that purpose. The downside to passing objects by proxy is that performance is significantly reduced (because every access to the object requires interprocess communication). For this reason, you should only pass objects by proxy if it is not possible to pass them by copying.

You can configure additional proxy objects similarly to the way you configured the remoteObjectInterface property of the initial connection. First, identify which parameter to a method should be passed by proxy, then specify an NSXPCInterface object that defines the interface for that object.

First questions come: how should the object to be passed by proxy be defined? As an object conforming to NSXPCProxyCreating protocol? Should remoteObjectProxy and remoteObjectProxyWithErrorHandler: method be implemented then?

An example follows, that is not clear at all to me. In particular I don't understand where should I call the NSXPCInterface method (setInterface:forSelector:argumentIndex:ofReply:) to whitelist the parameter as a proxy: in the XPC service code or in the host?

The first parameter to a method is parameter 0, followed by parameter 1, and so on.

In this case, the value NO is passed for the ofReply parameter because this code is modifying the whitelist for one of the parameters of the method itself. If you are whitelisting a class for a parameter of the method’s reply block, pass YES instead.

So the question is: can anybody provide me with a clear tutorial on how to return an object as a proxy in a block reply of a XPC method call?

Solution

I can answer my own question now: to return an object as a proxy in a block reply of a XPC method call, one should call the setInterface:forSelector:argumentIndex:ofReply: method both:

in the XPC service's endpoint, where the exportedInterface is declared
in the host, where the remoteObjectInterface is declared

I.e, common code:

// common (service/host) protocol definition
@protocol Service
@end

@protocol ServiceFactory
-(void)connectToNewService: (void (^)(id<Service>)reply;
@end

In the XPC Service:

// Implement the one method in the NSXPCListenerDelegate protocol.
-(BOOL)listener:(NSXPCListener *)listener shouldAcceptNewConnection:(NSXPCConnection*)newConnection {   

    NSXPCInterface *serviceFactoryInterface =[NSXPCInterface interfaceWithProtocol:@protocol(ServiceFactory)];
    NSXPCInterface *serviceInterface =[NSXPCInterface interfaceWithProtocol:@protocol(Service)];

    // connection has to be returned as proxy, not as a copy
    [serviceFactoryInterface setInterface: serviceInterface
                          forSelector: @selector(connectToNewService:)
                        argumentIndex: 0
                              ofReply: YES];

    newConnection.exportedInterface = serviceFactoryInterface;
    newConnection.exportedObject = self;

    [newConnection resume];
    return YES;

}

In the host code:

// in the host

- (void)openNewService
{
    NSXPCConnection *xpcConnection = [[NSXPCConnection alloc] initWithServiceName:@"eu.mycompany.servicefactory"];
    NSXPCInterface *serviceFactoryInterface =[NSXPCInterface interfaceWithProtocol:@protocol(ServiceFactory)];
    NSXPCInterface *serviceInterface =[NSXPCInterface interfaceWithProtocol:@protocol(Service)];

    // connection has to be returned as proxy, not as a copy
    [serviceFactoryInterface setInterface: serviceInterface
                          forSelector: @selector(connectToNewService:)
                        argumentIndex: 0
                          ofReply: YES];

    xpcConnection.remoteObjectInterface = serviceFactoryInterface;
    [xpcConnection resume];

    [[xpcConnection remoteObjectProxy] connectToNewService:^(id<Service> newService) {

        // here a newService is returned as NSXPCDistantObject <Service>*
        [xpcConnection invalidate];

    }];

}