Invoke model method with block that will run on the main thread - iphone

Question

one of the central tenets of the architecture of my latest app is that I'm going to call methods on the app's model which will be async and accept failure and success scenario blocks.
i.e., The UI calls the model method with 2 blocks, one for success and one for failure.
This is great because the context of the original call is retained, however, the block itself is called on the background thread. Is there anyway of calling a block on the main thread??
Hopefully I have explianed it ok, if not, basically, my model methods are async, return immediately and create a new thread on which to run the op. Once the op returns I will invoke a block which will postprocess the returned data, THEN i need to call the block for the success scenario defined by the called inside the UI. However, the success and failure scenario blocks defined in the UI should be called in the main thread because I need to interact with UI elements which should only be done on the main thread I believe.
many thanks

Answer 1

Something like this is probably what you're after:
- (void) doSomethingWhichTakesAgesWithArg: (id) theArg
resultHandler: (void (^)(BOOL, id, NSError *)) handler
{
// run in the background, on the default priority queue
dispatch_async( dispatch_get_global_queue(0, 0), ^{
id someVar = [theArg computeSomething];
NSError * anError = nil;
[someVar transmuteSomehowUsing: self error: &anError];
// call the result handler block on the main queue (i.e. main thread)
dispatch_async( dispatch_get_main_queue(), ^{
// running synchronously on the main thread now -- call the handler
handler( (error == nil), theArg, anError );
});
});
}

Answer 2

If you are using GCD, you can use the "get main queue":
dispatch_queue_t dispatch_get_main_queue()
Call this inside an async dispatch. i.e.
dispatch_async(dispatch_get_main_queue(), ^{
/* Do somthing here with UIKit here */
})
The example block above could be running in an async background queue and the example code would send the UI work off to the main thread.

Answer 3

Similar approach works also with NSOperationQueue:
NSBlockOperation *aOperation = [NSBlockOperation blockOperationWithBlock:^
{
if ( status == FAILURE )
{
// Show alert -> make sure it runs on the main thread
[[NSOperationQueue mainQueue] addOperationWithBlock:^
{
UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:#"Alert" message:#"Your action failed!" delegate:nil
cancelButtonTitle:#"Ok" otherButtonTitles:nil] autorelease];
[alert show];
}];
}
}];
// myAsyncOperationQueue is created somewhere else
[myAsyncOperationQueue addOperation:aOperation];

Answer 4

NSObject has a method:
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait
Create a method that takes a NSDictionary parameter in a convenient class that will always be around (like your app delegate, or a singleton object), package up the block and its parameters into a NSDictionary or NSArray, and call
[target performSelectorOnMainThread:#selector(doItSelector) withObject:blockAndParameters waitUntilDone:waitOrNot];