[SOLVED] Detecting/Repairing NSConnection failure

Detecting/Repairing NSConnection failure

I would like to use NSConnection/NSDistributedObject for interprocess communication. I would like the client to be able to handle the case where the server is only occasionally reachable.

How can I determine if sending a message to the NSConnection will fail or has failed? Currently if my server (the process that has vended the remote object) dies, the client will crash if it sends a selector to the remote object.

Ideally I'd like to have a wrapper for the remote object that can lazily instantiate (or reinstantiate) the connection, and return a default value in the case where the connection could not be instantiated, or the connection has failed. I don't really know the correct way to do this using objective c.

Here's some pseudocode representing this logic:

if myConnection is null:
    instantiate myConnection
    if MyConnection is null:
        return defaultValue

    try
        return [myConnection someMethod]
    catch
        myConnection = null
        return defaultValue

Solution

Unfortunately the only way to detect a connection failure is to use an exception handler, as there is no reliable way to "ask" a remote object if the connection is still valid. Thankfully, this is simple:

//get the distributed object
id <YourDOProtocol> remoteObject = (id <YourDOProtocol>)[NSConnection rootProxyForConnectionWithRegisteredName:@"YourRegisteredName" host:yourHost];

//call a method on the distributed object
@try
{
    NSString* response = [remoteObject responseMethod];
    //do something with response
}
@catch(NSException* e)
{
    //the receiver is invalid, which occurs if the connection cannot be made
    //handle error here
}