Run code in the background

Question

I'm trying to download an image from a website and save it as a UIImage but if the user has low connection this can take forever... how can I download it in the background so the user can keep using the app in the meantime?

here is the code:

theIcon.image = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://myWebsite.com/Icon.png"]]];

http://stackoverflow.com/questions/7502073/loading-images-from-a-background-thread-using-blocks — Vjy, Jul 17 '13 at 19:39
`dataWithContentsOfURL:` will not work reliable (if at all) for accessing remote resources. You should use either the asynchronous convenient method from class `NSURLConnection` or use it in asynchronous style and implement the delegates. — CouchDeveloper, Jul 17 '13 at 21:49
@CouchDeveloper What's wrong with dataWithContentsOfURL? It's worked fine on all my apps in the past, I'd hate to have to go update them all haha! /: — Albert Renshaw, Jul 17 '13 at 22:02
It's for use with _file_ URLs. That it works for remote URLs may be by accident, bad luck, or by mistake. It's unreliable and not appropriate for remote resources. Be careful! — CouchDeveloper, Jul 17 '13 at 22:15

score 1 · Accepted Answer · answered Jul 17 '13 at 19:45

1

Use GCD.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
    // do my background code
    dispatch_async(dispatch_get_main_queue(), ^{
        // do handling on main thread when done!
    });
});

answered Jul 17 '13 at 19:45

Nicholas Hart

1,734
13
22

This worked amazingly for the image, but when I tried to do it with text the app crashed... Here is the code I used (It works in the main thread, just not in your GCD code area) `advertisementTitle.text = [NSString stringWithContentsOfURL:[NSURL URLWithString:@"http://www.myWebsite.com/OneParagraph.html"] encoding:NSUTF8StringEncoding error:nil];` – Albert Renshaw Jul 17 '13 at 20:11
This was the error log: `Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...` – Albert Renshaw Jul 17 '13 at 20:12
(*Note: I'm using ARC) – Albert Renshaw Jul 17 '13 at 20:16
It worked when I stored *NSStrings in the "background code" region and then in the "main thread when done" region I set the IBOutlet UI stuff equal to those NSStrings! God bless :) – Albert Renshaw Jul 17 '13 at 20:23
@Nicholas Hart The question deserves more details. This answer is simply too vague, and naively applying this idiom to the OPs problem would produce incorrect, or at least suboptimal code. – CouchDeveloper Jul 17 '13 at 21:51
The original code was a vaguely worded "how do I run code in the background." It has since been edited to be specific to loading images. But thanks for the downvote. – Nicholas Hart Jul 17 '13 at 21:58
@Albert: you can only call UIKit methods from the main thread. – Nicholas Hart Jul 17 '13 at 22:12
@Nicholas Hart, I feel bad -- really. But you can edit your answer, too. I'm sure you get upvotes when you show how to use `NSURLConnection` in asynchronous mode and demonstrate the implementation of the principal delegates. – CouchDeveloper Jul 17 '13 at 22:26
LOL, it's ok. Cheers! – Nicholas Hart Jul 17 '13 at 23:46

score 1 · Answer 2 · answered Jul 17 '13 at 20:20

1

Use AFNetworking.

[imageView setImageWithURL:
                       [NSURL URLWithString:@"http://i.imgur.com/r4uwx.jpg"] 
                       placeholderImage:[UIImage imageNamed:@"placeholder-avatar"]];

answered Jul 17 '13 at 20:20

Samet DEDE

1,621
19
28

apollosoftware.org · Answer 3 · 2013-07-17T19:56:42.160

0

You can perform the selector in background thread while the main foreground thread runs.

 [self performSelectorInBackground:@selector(downloadFile) withObject:nil];

 - (void) downloadFile {
   //download file 
   //you can show UIAlertView when done
    }

In your - (void) downloadFile you can download this big file. and have an activity indicator show (or not). You can have the activity Indicator become not hidden or hidden and have it startAnimating and stopAnimating will make it spin and stop. This can be referenced from the foreground and background processes.

edited Jul 17 '13 at 19:56

answered Jul 17 '13 at 19:40

apollosoftware.org

12,161
4
48
69

Thanks! Testing now! (p.s. I did NOT downvote you!) – Albert Renshaw Jul 17 '13 at 19:44
Thanks for not downvoting me. This should work for your purposes. I've used it primarily for loading UIWebView content on pages, and downloading big files from UIWebView hyperlinks to the local documents folder in iOS. – apollosoftware.org Jul 17 '13 at 19:45
i changed the downloadFile a little bit, too. – apollosoftware.org Jul 17 '13 at 19:48
I'm getting an error: `*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSThread initWithTarget:selector:object:]: target does not implement selector (*** -[ViewController downloadFile:])'` – Albert Renshaw Jul 17 '13 at 19:50
I changed the downloadFile in the selector, remove the colon : – apollosoftware.org Jul 17 '13 at 19:51
Brilliant! Haha! Running now :) – Albert Renshaw Jul 17 '13 at 19:51
lol, yeah it should be downloadFile not downloadFile:, my bad – apollosoftware.org Jul 17 '13 at 19:51
You should throw up a UIAlertView or something when the file is done downloading or something that indicates to the user that the process has completed :) – apollosoftware.org Jul 17 '13 at 19:52
You can't call UIKit methods from a background thread. – Nicholas Hart Jul 17 '13 at 19:57
Got the error: `Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...` – Albert Renshaw Jul 17 '13 at 19:59
I'm sure that's because of what @NicholasHart just said. /: Going to attempt GCD now. – Albert Renshaw Jul 17 '13 at 19:59
it may be conflicting. For my download app I wrote in the appstore, it allows me to performSelector in background, and download files to the documents directory and then display messages. – apollosoftware.org Jul 17 '13 at 20:01
I suppose you can download the file to your NSDcouments directory, and load it when it's ready, but the background thread has to send a notification message to the foreground or main thread,. – apollosoftware.org Jul 17 '13 at 20:07
do something like this: http://stackoverflow.com/questions/7905192/iphone-grand-central-dispatch-main-thread – timpone Jul 18 '13 at 17:38

score 0 · Answer 4 · answered Jul 17 '13 at 22:11

The quick and dirty way:

NSMutableRequest* request = ... ;
[NSURLConnection sendAsynchronousRequest:request 
                                   queue:[NSOperationQueue mainQueue] 
                       completionHandler:^(NSURLResponse* response, NSData* data, NSError* error) {
    if (!error) {
        // do something with the response data.    
    }
}];

This approach is sufficient for "a prove of concept", toy programs with simplistic insecure connections, Apple samples, and for hobbyists learning iOS for fun, and for samples which demonstrate anti-patterns ("How you should do it, not!").

If you want a solid approach you need to use NSURLConnection in asynchronous mode and implement the delegates - or use a third party library. ;)

Run code in the background

4 Answers4