Sending a notification from another thread - objective-c

Question

I have a settings panel in my app. Whenever the user presses a button, the object is updated on another thread as the UI updates. I have a separate label on the main view that is supposed to update the object count when the object has finished updating (which I want to happen regardless of whether the settings panel is up or down). I've tried following the apple documentation regarding this very topic, but it doesn't seem to work out for me - that is, it seems that the main view controller never receives the notification for some reason. Does anyone have any suggestions on how to alert the main view controller that an object passed to another thread has finished updating? Here's the code I'm using (most of which was copied from that doc):
Object Class:
[[NSNotificationCenter defaultCenter] postNotificationName: #"ScaleCountUpdated" object: self];
Main View Controller
- (void)setUpThreadingSupport
{
if (self.notifications) {
return;
}
self.notifications = [[NSMutableArray alloc] init];
self.notificationLock = [[NSLock alloc] init];
self.notificationThread = [NSThread currentThread];
self.notificationPort = [[NSMachPort alloc] init];
[self.notificationPort setDelegate: self];
[[NSRunLoop currentRunLoop] addPort: self.notificationPort
forMode: (NSString *)kCFRunLoopCommonModes];
}
- (void)handleMachMessage:(void *)msg
{
[self.notificationLock lock];
while ([self.notifications count]) {
NSNotification *notification = [self.notifications objectAtIndex: 0];
[self.notifications removeObjectAtIndex: 0];
[self.notificationLock unlock];
[self processNotification: notification];
[self.notificationLock lock];
};
[self.notificationLock unlock];
}
- (void)processNotification:(NSNotification *)notification{
if ([NSThread currentThread] != self.notificationThread) {
// Forward the notification to the correct thread.
[self.notificationLock lock];
[self.notifications addObject: notification];
[self.notificationLock unlock];
[self.notificationPort sendBeforeDate: [NSDate date]
components: nil
from: nil
reserved: 0];
} else {
[self updateScaleCount];
}
}
- (void)updateScaleCount
{
NSLog(#"[ScalesViewController - updateScaleCount]: Scales updated from notification center.");
if([UserDefinedScales areScalesGrouped] == YES){
self.groupCountLabel.text = [NSString stringWithFormat: #"Group Count: %i", [[UserDefinedScales sortedKeys] count]];
} else {
self.groupCountLabel.text = #"Group Count: 1";
}
self.scaleCountLabel.text = [NSString stringWithFormat: #"Scale Count: %i", [UserDefinedScales scaleCount]];
}
Main View Controller - View Did Load:
[self setUpThreadingSupport];
[[NSNotificationCenter defaultCenter] addObserver: self
selector: #selector(processNotification:)
name: #"ScaleCountUpdated"
object: nil];
If you have any suggestions on how to alter this code to make it function correctly, or have another solution to offer for achieving this, it would be greatly appreciated! Thank you.

Answer 1

It looks to me like you are doing it correctly, i.e. register for notification and send it.
As far as I can see from your code and the information you give, you can basically completely forget about the setupThreadingSupport. You should definitely test it without it. Not sure what you want to achieve, but looks like overkill where probably a simple block would suffice. Is there a compelling reason to listen to the notification on a background thread? Why not let the notification center decide?
Log the sending and receiving of the notifications - addObserver and postNotification is really all this mechanism needs to work as expected.

Answer 2

I would go to a simpler implementation. The NSNotificationCenter already provides all the mechanisms you need to broadcast and receive messages across your app.
If you fire the notification from a background thread you can use a GCD dispatch_async to make make it delivered on the main thread.
Object Class
dispatch_async(dispatch_get_main_queue(), ^{
// You don't need to pass the object itself here as you are not using it later.
[[NSNotificationCenter defaultCenter] postNotificationName:#"ScaleCountUpdated"];
}
MainViewController
-(void)viewDidLoad
{
[super viewDidLoad];
// Register your controller as an observer for a specific message name
[[NSNotificationCenter defaultCenter] addObserver: self
selector: #selector(updateScaleCount)
name: #"ScaleCountUpdated"
object: nil];
}
- (void)updateScaleCount
{
NSLog(#"[ScalesViewController - updateScaleCount]: Scales updated from notification center.");
if([UserDefinedScales areScalesGrouped] == YES){
self.groupCountLabel.text = [NSString stringWithFormat: #"Group Count: %i", [[UserDefinedScales sortedKeys] count]];
} else {
self.groupCountLabel.text = #"Group Count: 1";
}
self.scaleCountLabel.text = [NSString stringWithFormat: #"Scale Count: %i", [UserDefinedScales scaleCount]];
}