EXC_BAD_ACCESS upon block execution - objective-c

Question

I have a class with a single method, that uses a URLConnection to send a serialized NSDictionary to a script at a certain URL, and then calls a completion block. Here is the code for that method:
- (void)sendDictionary:(NSDictionary *)dictionary toScript:(NSString *)scriptName completion:(void (^) (id response))completionBlock
{
...Serialize data and add it to an NSURLRequest request...
H2URLConnection *connection = [[H2URLConnection alloc]initWithRequest:request];
//Define a semaphore to block execution of later statements until the signal is received.
dispatch_semaphore_t sem = dispatch_semaphore_create(0);
[connection setCompletionBlock:[^(id obj, NSError *err)
{
if (!err) {
//Catch the server response
NSString *receivedString = [[NSString alloc] initWithData:obj encoding:NSUTF8StringEncoding];
NSLog( #"ChecklistAppNetworkManager received string: %#", receivedString);
//Convert the JSON response into an NSDictionary
NSError *otherError;
id deserializedJSON = [NSJSONSerialization JSONObjectWithData:obj options:kNilOptions error:&otherError];
if (otherError) {
NSLog(#"ChecklistAppNetworkManager JSON Error: %#", otherError.description);
}
[completionBlock invoke];
NSLog(#"ChecklistAppNetworkManager JSON Response: %#", deserializedJSON);
//Dispatch the semaphore signal so that the main thread continues.
dispatch_semaphore_signal(sem);
} else {
NSLog(#"ChecklistAppNetworkManager encountered an error connecting to the server: %#", [err description]);
}
}copy]];
//Finalize and initate the connection.
[connection start];
//Since block is dispatched to main queue, stall with a loop until the semaphore signal arrives.
while (dispatch_semaphore_wait(sem, DISPATCH_TIME_NOW)) {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:10]];
}
}
I'm trying to call this method on an instance of this class from within another class, where the completion block is defined. Here's the code where I get the EXC_BAD_ACCESS:
- (void)doSomeServerTask
{
H2ChecklistAppNetworkManager *currentNetworkManager = ((H2AppDelegate *)[[UIApplication sharedApplication]delegate]).networkManager; //Instantiate class where that method is defined
NSMutableDictionary *dictonary = [NSMutableDictionary dictionary];
...populate dictionary...
[currentNetworkManager sendDictionary:dictionary toScript:#"script.php" completion:[^(id response)
{ //THIS iS THE LINE WHERE THE BAD ACCESS OCCURS
NSLog(#"LoginViewController received response: %#", response);
} copy]];
}
Any help would be appreciated!

Answer 1

The completionBlock on that method takes one argument, but you call the block with the invoke method. More likely than not, the crash is because the runtime is trying to retain whatever garbage is in memory that should be that argument.
However, you really need to refactor this code entirely. Blocking the main event loop is bad. Running a sub-runloop is even worse on the MEL; it changes the way dispatch queue handling semantics work and can lead to pathologically bad performance or behavior.
You should move to a truly asynchronous model. If the app can't proceed until these queries are done, then put up a modal indicator that blocks progress.
To do that, you structure the code loosely as:
• put user interface into a "loading..." or some other modal state
• execute an asynchronous request for data with a completion handler
• in the completion handler, dispatch the "update UI" request to the main queue
• upon "update UI", tear down your modal "loading...." UI and update the display for the user
There is no need to block the main event loop to do any of this.