Multitasking with download images in iOS sdk - ios

Question

I have a UItableView that is listing number of outlets respectively, each outlet has a logo image. I save those images locally on iPhone, so if any image is found on iPhone it will fetch from there if not then it will send a service call and fetch data. It is working accordingly but when I scroll down on tableview it gets hanged at certain point when the image is being downloaded, as it is downloaded it works fine again. Is there any solution where I can perform this multitasking of downloading images along with representing them either from service call or fetching it locally.
Here is my code..
// downloading images of outlets locally
NSLog(#"Downloading...");
NSString *imageLink = [NSString stringWithFormat:#"http://cf.abc.pk/outlets/l/%#",outs.logo];
UIImage *image = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:imageLink]]];
NSLog(#"%f,%f",image.size.width,image.size.height);
NSString *docDir =[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
NSLog(#"%#",docDir);
NSLog(#"saving png");
NSString *pngFilePath = [NSString stringWithFormat:#"%#/%d.png",docDir,[[arrOutletIds objectAtIndex:webserviceCall] intValue]];
BOOL fileExists = [[NSFileManager defaultManager] fileExistsAtPath:pngFilePath];
if(!fileExists)
{
NSData *data1 = [NSData dataWithData:UIImagePNGRepresentation(image)];
[data1 writeToFile:pngFilePath atomically:YES];
}

Answer 1

Let's try:
//When you download something, execute in background thread
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0),
^{
//download some thing
dispatch_async( dispatch_get_main_queue(), ^{
// when download finish execute in main thread to update data
// this function should be called in response of downloading
});
});
Hope this helps you.

Answer 2

Download image in background will solve your problem here
Try this.
if(!fileExists)
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSData *data1 = [NSData dataWithData:UIImagePNGRepresentation(image)];
[data1 writeToFile:pngFilePath atomically:YES];
});
}

Answer 3

You can use this code which integrates ImageCache to download async and cache images. This part of code shows downloading images in UITableView. Or you can check out this project on GitHub SDImageCache
if ([[ImageCache sharedImageCache] DoesExist:photoURL] == true)
{
image = [[ImageCache sharedImageCache] GetImage:photoURL];
cell.imageView.image = image;
}
else
{
cell.imageView.image = [UIImage imageNamed:#"PlaceHolder"];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^(void) {
NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:photoURL]];
UIImage* image = [[UIImage alloc] initWithData:imageData];
if (image) {
dispatch_async(dispatch_get_main_queue(), ^{
if (cell.tag == indexPath.section) {
cell.imageView.image = image;
[cell setNeedsLayout];
[[ImageCache sharedImageCache] AddImage:image imageURL:photoURL];
}
});
}
});
}
Import these files to your project
ImageCache.h
#import <Foundation/Foundation.h>
#interface ImageCache : NSObject
#property (nonatomic, retain) NSCache *imgCache;
#pragma mark - Methods
+ (ImageCache*)sharedImageCache;
- (void) AddImage:(UIImage *)image imageURL:(NSString *)imageURL;
- (UIImage*) GetImage:(NSString *)imageURL;
- (BOOL) DoesExist:(NSString *)imageURL;
#end
ImageCache.m
#import "ImageCache.h"
#implementation ImageCache
#synthesize imgCache;
#pragma mark - Methods
static ImageCache* sharedImageCache = nil;
+(ImageCache*)sharedImageCache
{
#synchronized([ImageCache class])
{
if (!sharedImageCache)
sharedImageCache= [[self alloc] init];
return sharedImageCache;
}
return nil;
}
+(id)alloc
{
#synchronized([ImageCache class])
{
NSAssert(sharedImageCache == nil, #"Attempted to allocate a second instance of a singleton.");
sharedImageCache = [super alloc];
return sharedImageCache;
}
return nil;
}
-(id)init
{
self = [super init];
if (self != nil)
{
imgCache = [[NSCache alloc] init];
}
return self;
}
- (void) AddImage:(UIImage *)image imageURL:(NSString *)imageURL
{
if (image==nil) {
}
else
{
[imgCache setObject:image forKey:imageURL];
}
}
- (NSString*) GetImage:(NSString *)imageURL
{
return [imgCache objectForKey:imageURL];
}
- (BOOL) DoesExist:(NSString *)imageURL
{
if ([imgCache objectForKey:imageURL] == nil)
{
return false;
}
return true;
}
#end