Question

I am making an iPad app where you can download files (like PDF, doc, etc) and view them offline.

I already have the view part and you can download a file to the document directory.
As it is now you need to wait for the download to be finished to move on.
This can be solved by putting it in a thread, but what happens when the user downloads multiple files or even download the same file multiple times?

My idea is to make a download queue, with a view for the progress.

Workflow:

• The user opens a document and press download, the user gets a message that the download is started and can be viewed in the offline documents view.

• The user downloads 3 more documents.

• When the user goes to the offline document view the user sees a table view with 4 filled cells. 2 documents are done loading and 2 other are still downloading because there is a download/status bar shown in the table view cell.

• The downloaded documents can be viewed or deleted.

• The downloads in progress can not be watched (yet) but can be cancelled.

I want to make a threaded download class where you can add urls to be downloaded. the class has methods to cancel and delete document-downloads, but also has methods to return the progress.
If possible the class can handle simultaneous downloads.

The problem is, I don’t know where to start?

Answer 1

NSURLConnection is already asynchronous. All you need to do is to create NSURLConnection instances, associate them with your data structures, and have at it.

Here’s an example where I assume you have one UIView per item. If you use a table view you can’t count on view instances, but instead associate a download with an NSIndexPath, or something else.

@implementation MyDownloadView
- (void)startDownload {
    NSMutableURLRequest *req = [[NSMutableURLRequest alloc] initWithURL:myURL cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:10];
    [req setHTTPMethod:@"GET"];
    // Set headers etc. if you need
    [[[NSURLConnection alloc] initWithRequest:req delegate:self] autorelease];
    [req release];

    self.responseData = [[NSMutableData alloc] init];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    // Alternatively, store to a file so you don't run out of memory
    [self.responseData appendData:data];
}
@end

Then implement the other NSURLConnection delegate methods to do what you need.