Downloading files - background thread or async?

[smithdale87]

I'd like to start a discussion on when to make your app multithreaded versus just using async methods or what have you.

Specifically, I ran across this situation using when I wanted to download images and put them in an imageView. I could use [NSURLConnection connectionWithRequest], or I could use [NSData dataWithContentsOfURL: ...].

The NSURLConnection approach is asynchronous, whereas datawithContentsOfURL is synchronous. However, if I do the synchronous approach in a background thread anyways, then it gives the same effect as the async request -- freeing up the UI thread.

#pragma mark --Download File with NSURLConnection--

-(void) downloadHTTPImageAsync:(NSURL*)imageURL
{
     self.downloadedData = [NSMutableData data];
     NSURLRequest* request = [NSURLRequest requestWithURL:imageURL];
    [NSURLConnection connectionWithRequest: request delegate:self];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
 [self.downloadedData appendData: data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {

      UIImage* image = [UIImage imageWithData: self.downloadedData];
}

#pragma mark -- Download File in background thread --
-(void) downloadHTTPImageInBackground:(NSURL*)imageURL
{
       [self performSelectorInBackground: @selector( _downloadHTTPImageInBackground ) withObject: imageURL waitUntilDone:NO];
}
-(void) _downloadHTTPImageInBackground:(NSURL*)imageURL
{
    NSAutoReleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSData* imageData = [NSData dataWithContentsOfURL: imageURL];
    UIImage* image = [UIImage imageWithData: imageData];

   [pool drain];
}

I didn't show it here, but it's trivial to retain the image for further use.


Thoughts? Biases of preferring one method over the other?

[Dutch]

Interesting question. I've never used the background thread technique up to this point. Maybe run a few time tests and see how they perform. That would be my only semi-intelligent advise. I would also be interested in these results for larger images....

[smithdale87]

I can think of specific cases where I would use the async over the threaded
1. If I needed to track download progress ( this is especially useful when user may be downloading large files. It helps to know how much time is left.)
2. If I needed to track download speed. You could use this information to determine whether the network connection could handle multiple downloads at the same time.

Cons
- dataWithContentsOfURL doesnt provided an NSError if anything goes wrong. It simply returns nil.


Thats all I can think of right now.

[kpdipen]

Originally Posted by smithdale87

I can think of specific cases where I would use the async over the threaded
1. If I needed to track download progress ( this is especially useful when user may be downloading large files. It helps to know how much time is left.)
2. If I needed to track download speed. You could use this information to determine whether the network connection could handle multiple downloads at the same time.

Cons
- dataWithContentsOfURL doesnt provided an NSError if anything goes wrong. It simply returns nil.


Thats all I can think of right now.

Hey did u manage to download image in backgroundd? Can u please share ur experience?

[Duncan C]

Originally Posted by smithdale87

I'd like to start a discussion on when to make your app multithreaded versus just using async methods or what have you.

Specifically, I ran across this situation using when I wanted to download images and put them in an imageView. I could use [NSURLConnection connectionWithRequest], or I could use [NSData dataWithContentsOfURL: ...].

The NSURLConnection approach is asynchronous, whereas datawithContentsOfURL is synchronous. However, if I do the synchronous approach in a background thread anyways, then it gives the same effect as the async request -- freeing up the UI thread.

#pragma mark --Download File with NSURLConnection--

-(void) downloadHTTPImageAsync:(NSURL*)imageURL
{
     self.downloadedData = [NSMutableData data];
     NSURLRequest* request = [NSURLRequest requestWithURL:imageURL];
    [NSURLConnection connectionWithRequest: request delegate:self];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
 [self.downloadedData appendData: data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {

      UIImage* image = [UIImage imageWithData: self.downloadedData];
}

#pragma mark -- Download File in background thread --
-(void) downloadHTTPImageInBackground:(NSURL*)imageURL
{
       [self performSelectorInBackground: @selector( _downloadHTTPImageInBackground ) withObject: imageURL waitUntilDone:NO];
}
-(void) _downloadHTTPImageInBackground:(NSURL*)imageURL
{
    NSAutoReleasePool *pool = [[NSAutoreleasePool alloc] init];
    NSData* imageData = [NSData dataWithContentsOfURL: imageURL];
    UIImage* image = [UIImage imageWithData: imageData];

   [pool drain];
}

I didn't show it here, but it's trivial to retain the image for further use.


Thoughts? Biases of preferring one method over the other?

If you read the Apple boards, there are several Apple engineers who rail loudly and passionately about the evils of threads. Threads are very expensive to set up and run. They tie up precious physical memory for the entire life of the thread, even if your app gets moved to the background. The only way the system can use that memory is to terminate your app.

NSOperationQueue and GCD are better than NSThreads, but still make your app create more threads, which is very costly.

Apple STRONGLY encourages you to use async operations rather than threads. The system can manage the download process with dedicated system resources, rather than having to create a new thread just for your app.

Using the async NSURLConnection is also safer. Multi-threaded code requires that you know what you're doing, and can cause both subtle and spectacular problems. Multi-threaded code is also fiendishly difficult to debug. The way the threads interact is non-deterministic.