Saving image from a URL asynchronously - ios

Question

So i have already gone through almost all questions on SO, and I have put them all together to create a method that accepts two paramaters, first is the URL of the image to download and display in UIImageView and second is the placeholder image for that UIImageView. I want to save the image so that it won't be downloaded every time. I have used SDWebImage to download the image, however i had some confusion when it came to saving the image in documents directory using SDWebImage, so i decided not to use it. I used dispatch_sync(dispatch_get_main_queue() and my method now looks like :
- (void)saveImageFromURL:(UIImage*)image:(NSURL*)imageURL {
NSString *url = [imageURL absoluteString];
NSArray *parts = [url componentsSeparatedByString:#"/"];
NSString *filename = [parts lastObject];
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *fullPath = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:#"%#.png", filename]];
BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:fullPath];
if (fileExists) {
NSLog(#"File already exists");
_myUIImage.image = [UIImage imageNamed:fullPath];
return;
}
else {
dispatch_async(dispatch_get_main_queue(), ^{
[self.myUIImage sd_setImageWithURL:imageURL placeholderImage:image];
UIImage *imageFromURL = [UIImage imageWithData:[NSData dataWithContentsOfURL:imageURL]];
NSData *imageDataNew = UIImagePNGRepresentation(imageFromURL);
NSFileManager *fileManager = [NSFileManager defaultManager];
[fileManager createFileAtPath:fullPath contents:imageDataNew attributes:nil];
});
}
}
I have a couple of questions, is this implementation good enough since i am working on a app that will be on app store ? Will the downloading of the image from URL be done asynchronously ? (i know i am using dispatch_async but just need to confirm). If yes, then this wont block my UI, right ?

Answer 1

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
// *** Load Image from URL Asynchronously without blocking Main thread ***
UIImage *imageFromURL = [UIImage imageWithData:[NSData dataWithContentsOfURL:imageURL]];
dispatch_async(dispatch_get_main_queue(), ^(void){
// *** Create file path to DocumentDirectory ***
NSString * docDirPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
NSString *filePath = [docDirPath stringByAppendingPathComponent:#"myImage.jpg"]
// *** Write Image to disk ***
BOOL isSaved = [UIImageJPEGRepresentation(image, 1.0f) writeToFile:dirPath atomically:YES];
if(isSaved)
{
NSLog(#"Image write to disc successfully.");
}
});
});