How do I track NSError objects across threads? - objective-c

Question

I have a set of asynchronous calls being spawned using NSInvocationOperation:
- (void)listRequestQueue:(StoreDataListRequest *)request {
[openListRequests addObject:request];
NSInvocationOperation *requestOp = [[NSInvocationOperation alloc]
initWithTarget:self
selector:#selector(listRequestStart:)
object:request];
[opQueue addOperation:requestOp];
[requestOp release];
}
- (void)listRequestStart:(StoreDataListRequest *)request {
if(self.resourceData == nil) {
//TODO fail appropriately...
return;
}
StoreDataListResponse *response = [self newListResponseForProductID:request.productID];
[self performSelectorOnMainThread:#selector(listRequestFinish:)
withObject:response waitUntilDone:NO];
[response release];
[self performSelectorOnMainThread:#selector(cleanUpListRequest:)
withObject:request waitUntilDone:NO];
}
- (void)listRequestFinish:(StoreDataListResponse *)response {
[self.delegate storeData:self didReceiveListResponse:response];
}
- (StoreDataListResponse *)newListResponseForProductID:(NSString *)productID {
CollectionData *data = [self.resourceData dataForProduct:productID];
if(data == nil) {
//TODO do something
}
StoreDataListResponse *response = [[StoreDataListResponse alloc] init];
response.productID = productID;
if(productID != data.productID) {
//TODO fail; remove product from list
}
response.name = NSLocalizedString(#"Loading...", #"Loading message");
response.blurb = NSLocalizedString(#"Waiting for response from server", #"Waiting for website to respond");
return response;
}
For each of the TODOs in the above code, I should resolve the issue and let any handlers know that things have failed and why. Looking at the NSError class and documentation, it appears to be the appropriate answer but I am having trouble figuring out how to get to work with NSInvocationOperation and performSelectorOnMainThread:withObject:waitUntilDone:. I can certainly get the NSErrors out of the newListResponseForProductID: method by changing it to something like this:
- (StoreDataListResponse *)newListResponseForProductID:(NSString *)productID error:(NSError **)error;
How do I get the error generated back into the main thread so I can deal with the failed request?

Answer 1

The easiest way to run any code on the main thread is to use GCD and blocks on iOS 4 and above. Like this:
dispatch_async(dispatch_get_main_queue(), ^(void) {
// Your code to run on the main thread here,
// any variables in scope like NSError is captured.
});
Or use dispatch_sync() if you know you are on a background thread and want the block to complete on the main thread before you continue.