Perfoming UI updates in secondary queue - iphone

Question

I am using the new CoreMotion framework to monitor some of the hardware devices. Here is the typical code to do that:
-(void)startAccelerometer{
self.motion.accelerometerUpdateInterval = 1/30.0f;
NSOperationQueue* accelerometerQueue = [[NSOperationQueue alloc] init];
CMAccelerometerHandler accelerometerHandler = ^(CMAccelerometerData *accelerometerData, NSError *error) {
NSLog(#"Accelerometer realtime values");
NSLog(#"x=%f", accelerometerData.acceleration.x);
NSLog(#"y=%f", accelerometerData.acceleration.y);
NSLog(#"z=%f", accelerometerData.acceleration.z);
NSLog(#" ");
};
[self.motion startAccelerometerUpdatesToQueue:accelerometerQueue withHandler:[[accelerometerHandler copy]autorelease]];
}
That works just fine. Now I want to print the values on a UILabel, but since the CoreMotion frameworks has you use blocks, this is not guaranteed to be in the main queue (and in fact isn't for my app). Is it is "wrong" to just run the label's setter on the main queue like this?
-(void)startAccelerometer{
self.motion.accelerometerUpdateInterval = 1/30.0f;
NSOperationQueue* accelerometerQueue = [[NSOperationQueue alloc] init];
CMAccelerometerHandler accelerometerHandler = ^(CMAccelerometerData *accelerometerData, NSError *error) {
dispatch_async(dispatch_get_main_queue(), ^{
self.lblAccelerometer.text = [NSString stringWithFormat:#"Accelerometer:\nx = %f\ny = %f\nz = %f",
accelerometerData.acceleration.x,
accelerometerData.acceleration.y,
accelerometerData.acceleration.z];
});
};
[self.motion startAccelerometerUpdatesToQueue:accelerometerQueue withHandler:[[accelerometerHandler copy]autorelease]];
}
It works just fine and I don't really see any reason why this would be frowned upon. Any thoughts on that?

Answer 1

This is a common method that I use in many projects. UI updates must occur on the main thread.
//Dispatch on background thread
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//background processing goes here
//Dispatch on main thread
dispatch_async(dispatch_get_main_queue(), ^{
//update UI here
});
});
In your case, your UI updates are occurring on the main thread. So I wouldn't worry about changing anything.

Answer 2

You are missunderstanding the concept of blocks, to put it simple:
Blocks are small pieces of code that can be handled as variables and be executed at a certain time or thread.
All UI updates MUST be performed on the main thread so as long as you do this it will be fine.
Codes can be executed in different threads with different priorities in sync or async mode. On your code you are doing it perfectly fine, you not only dispatch it to the Main Queue which is where uiupdates should be executed, but you are also dispatching it async which is the safest way to update send to the main queue (from your code i cannot tell if you are running this specific piece of code from the main queue or a secondary queue but if u were to dispatch a sync block from the main queue to the main queue your program would stop working)
For iOS documentation:
Use the dispatch_get_main_queue function to get the serial dispatch
queue associated with your application’s main thread. This queue is
created automatically for Cocoa applications and for applications that
either call the dispatch_main function or configure a run loop (using
either the CFRunLoopRef type or an NSRunLoop object) on the main
thread.
Read this here http://developer.apple.com/library/ios/#documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW1