Multiple serial queues, UI not updating - ios

Question

I'm trying to run some intensive processes serially, with multiple serial queues. The code is working, however my UI update doesn't occur, even though the method is called.
Here is the code that runs several processes serially.
- (void)importProcess {
dispatch_queue_t serialQueue = dispatch_queue_create("com.cyt.importprocessqueue", DISPATCH_QUEUE_SERIAL);
NSLog(#"checking image sizes");
__block NSMutableArray *assets;
dispatch_sync(serialQueue, ^() {
assets = [self checkImageSizes];
});
dispatch_sync(serialQueue, ^() {
[self appendLogToTextView:[NSString stringWithFormat:#"%i screenshot(s) ignored due to invalid size.",(int)(self.assets.count-assets.count)]];
if(assets.count==0) {
[self showNoRunesFoundAlert];
}
else {
[self appendLogToTextView:#"Preparing to process screenshots..."];
self.assets = [NSArray arrayWithArray:assets];
}
});
NSLog(#"processing uploads");
dispatch_sync(serialQueue, ^() {
[self processUploads];
});
NSLog(#"recognizing images");
dispatch_sync(serialQueue, ^() {
[self recognizeImages];
});
NSLog(#"recognizing images");
dispatch_sync(serialQueue, ^() {
[self processRuneText];
});
//dispatch_sync(dispatch_get_main_queue(), ^ {
//});
}
Within checkImageSizes, I have another serial queue:
- (NSMutableArray *)checkImageSizes {
dispatch_queue_t serialQueue = dispatch_queue_create("com.cyt.checkimagesizequeue", DISPATCH_QUEUE_SERIAL);
NSMutableArray *assets = [NSMutableArray new];
for(int i=0;i<self.assets.count;i++) {
dispatch_sync(serialQueue, ^{
PHAsset *asset = (PHAsset *)self.assets[i];
if(asset.pixelWidth==self.screenSize.width && asset.pixelHeight==self.screenSize.height) {
[assets addObject:asset];
NSString *logText = [NSString stringWithFormat:#"Screenshot %i/%i size is OK.",i+1,(int)self.assets.count];
[self performSelectorOnMainThread:#selector(appendLogToTextView:) withObject:logText waitUntilDone:YES];
}
else {
[self appendLogToTextView:[NSString stringWithFormat:#"ERROR: Screenshot %i/%i has invalid size. Skipping...",i+1,(int)self.assets.count]];
}
});
}
return assets;
}
The appendLogToTextView method is supposed to update the UI. Here is that code:
- (void)appendLogToTextView:(NSString *)newText {
dispatch_block_t block = ^ {
self.logText = [NSString stringWithFormat:#"%#\n%#", self.logText, newText];
NSString *textViewText = [self.logText substringFromIndex:1];
[UIView setAnimationsEnabled:NO];
if(IOS9) {
[self.textView scrollRangeToVisible:NSMakeRange(0,[self.textView.text length])];
self.textView.scrollEnabled = NO;
self.textView.text = textViewText;
self.textView.scrollEnabled = YES;
}
else {
self.textView.text = textViewText;
NSRange range = NSMakeRange(self.textView.text.length - 1, 1);
[self.textView scrollRangeToVisible:range];
}
[UIView setAnimationsEnabled:YES];
};
if ([NSThread isMainThread]) {
block();
}
else {
dispatch_sync(dispatch_get_main_queue(), block);
}
}
As you can see, I have tried calling appendLogToTextView both directly and using performSelectorOnMainThread. However, I'm not getting any updates to my text view, even though I confirm that the method is being called properly.
Interestingly, the UI updating works properly when I only use a single serial queue and use iteration counts to call the next method, as shown in the code below (_serialQueue is defined in viewDidLoad). However, I do not believe that implementation is good practice, as I'm wrapping serial queues within serial queues.
- (void)checkImageSizes {
NSMutableArray *assets = [NSMutableArray new];
for(int i=0;i<self.assets.count;i++) {
dispatch_async(_serialQueue, ^{
PHAsset *asset = (PHAsset *)self.assets[i];
if(asset.pixelWidth==self.screenSize.width && asset.pixelHeight==self.screenSize.height) {
[assets addObject:asset];
[self appendLogToTextView:[NSString stringWithFormat:#"Screenshot %i/%i size is OK.",i+1,(int)self.assets.count]];
}
else {
[self appendLogToTextView:[NSString stringWithFormat:#"ERROR: Screenshot %i/%i has invalid size. Skipping...",i+1,(int)self.assets.count]];
}
//request images
if(i==self.assets.count-1) {
[self appendLogToTextView:[NSString stringWithFormat:#"%i screenshot(s) ignored due to invalid size.",(int)(self.assets.count-assets.count)]];
if(assets.count==0) {
[self showNoRunesFoundAlert];
}
else {
[self appendLogToTextView:#"Preparing to process screenshots..."];
self.assets = [NSArray arrayWithArray:assets];
[self processUploads];
}
}
});
}
}
What am I not understanding about serial queues that is causing the UI updates in this version of the code to work, but my attempt at a "cleaner" implementation to fail?

Answer 1

In the end, I just ended up using dispatch groups and completion blocks in order to solve this problem.