LR iOS Developers Group

Well, a miracle happened on Thursday, April 25th and I managed to score a ticket for WWDC 2013.  I had prepared for the announcement like any other Apple and iOS obsessed nerd would.  I wrote several applications whose sole purpose in life was to alert me when the event was announced.  Apps written for my Mac, my iPhone and web based apps who would quietly check on that special WWDC page every minute of every day since Feb of this year.  In the end, all that work, those countless hours of coding in Obj-C, VB.Net and PHP were wasted with the announcement on Tuesday that tickets would be available that following Thursday.  While this was much more fair than the old way of making tickets available on announcement day, it also took away all the special advantage I had.  Oh well, at least I was one of the first to be notified of the announcement  In any event, I reformulated, hatched new and complex plans to avenge those lost hours and get my ticket anyway.  In other words, I prayed and got as prepared as I could for Thursday 12PM Central Time.  When that time came, I blazed through the screens of the ticket purchase, my mind having already gone through the scenario hundreds of times.  My hands were robotic and instinctive on the mouse and keyboard.  Me, the keyboard, mouse and web-site were fused together in a beautiful ballet of keyboard and mouse clicks.  Less than 90 seconds later I finished the maze of the iTunes purchase process.  That 90 seconds was a blur, but the final screen was anyone but.  Congrats to my fellow developers who also made it through with success and my condolences to those that didn’t make it.

Blocks are all the rage today.  They greatly simplify code and eliminate many callbacks and delegate methods from having to be coded.  The lines of code advantage is big, but the performance and efficiency advantages really make this a great choice.

To illustrate, I will show what I used to use to pull web content (in this case, an image stored on a public web server) down to my application against what this same process requires when using a class method and blocks.

Before:
In the Interface file, set up the properties needed:

@property (weak, nonatomic) IBOutlet UIImageView *myImageView;
@property(nonatomic,retain) NSMutableData *incomingImageData;

And then in the Implmentation file, synthesize these and setup the methods needed to pull the image from the web server:

@synthesize myImageView;
@synthesize incomingImageData;

#pragma mark - My Methods
-(void)reloadImage {
    NSString *urlString = @"http://mywebserver/myimage.jpg";

    MBProgressHUD *hud = [MBProgressHUD showHUDAddedTo:self.view animated:YES];
    hud.labelText = @"Loading Image...";

    self.incomingImageData = [[NSMutableData alloc] init];

    NSURLRequest *request = [[NSURLRequest alloc]
 							 initWithURL: [NSURL URLWithString:urlString]
 							 cachePolicy: NSURLRequestReloadIgnoringLocalCacheData
 							 timeoutInterval: 10
 							 ];

    NSURLConnection *connection = [[NSURLConnection alloc]
 								   initWithRequest:request
 								   delegate:self
 								   startImmediately:YES];

}

And then the delegate methods needed for NSURLConnection:

#pragma mark - NSURLConnection Delegate Methods
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    [self.incomingImageData appendData:data];
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    self.incomingImageData = nil;
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
   [MBProgressHUD hideHUDForView:self.view animated:YES];
    // Set appIcon and clear temporary data/image

    if (!incomingImageData) {
        NSString *errorImage = 
        [[NSBundle mainBundle] pathForResource:@"ErrorImage" ofType:@"png"]; 
        NSData *errorImageData = [NSData dataWithContentsOfFile:errorImage];
        UIImage *image = [[UIImage alloc] initWithData:errorImageData];
        myImageView.image = image;
    }
    else {
        UIImage *image = [[UIImage alloc] initWithData:self.incomingImageData];
        myImageView.image = image;
    }

       self.incomingImageData = nil;

}

What all this does is load the image asynchronously so the UI thread isn’t blocked. It works and works pretty fast.

Doing this same thing in a block (contained in a class method I wrote) requires this:
Interface file:

@property (weak, nonatomic) IBOutlet UIImageView *myImageView;

Implementation file:

@synthesize myImageView;

#pragma mark - My Methods
-(void)reloadImage {
    NSString *urlString = @"http://mywebserver/myimage.jpg";

    MBProgressHUD *hud = [MBProgressHUD showHUDAddedTo:self.view animated:YES];
    hud.labelText = @"Loading Image...";

    [WebImageOperations processImageDataWithURLString:urlString andBlock:^(NSData *imageData) {
        if (self.view.window) {
            UIImage *image = [UIImage imageWithData:imageData];

            myImageView.image = image;
            [MBProgressHUD hideHUDForView:self.view animated:YES];
        }

    }];

}

That’s it, no delegate methods needed and much less code written. In case you wanted to see it, here is the class method I wrote to take in an NSString (url) and a block and process the image using Grand Central Dispatch:

+ (void)processImageDataWithURLString:(NSString *)urlString andBlock:(void (^)(NSData *imageData))processImage
{
    NSURL *url = [NSURL URLWithString:urlString];

    dispatch_queue_t callerQueue = dispatch_get_current_queue();
	dispatch_queue_t downloadQueue = dispatch_queue_create("com.myapplication.processimagedataqueue", NULL);
	dispatch_async(downloadQueue, ^{
		NSData * imageData = [NSData dataWithContentsOfURL:url];

        dispatch_async(callerQueue, ^{
		    processImage(imageData);
		});
	});
	dispatch_release(downloadQueue);
}

There ya go, much less code and faster! I did the benchmarking on two different phones. One (an iPhone 4S) running the old way of doing it (NSURLConnection) and the other (an iPhone 3GS) running the new way of doing it (blocks and GCD). I had both phones using 3G (no WiFi) so the time difference could be seen better. The new way (3GS) outperformed the old way 8 times out of 10. Both methods were kicked off at the same time and the difference was visible. This wasn’t even a fair comparison as the 4S is much faster at processing and GPU output than the 3GS, but the 3GS won 80% of the time.