Objective-C Check for downloaded file size - iphone

Question

I'm creating an app which downloads a .zip file from S3 server.
All works fine. Now I want to be able to interrupt the current download. If I could save the current size (bytes) of the file, I would be able to send a new request with a range header for the other part of the file.
Problem lies in the fact that I cannot determine the size of the 'already' downloaded content, because I can only see the file in my directory when the download is completed. So if I interrupt, there isn't a partial file saved.
At this time I use the following code for this:
-(void) downloadFile:(NSMutableArray*)paramArray withDict:(NSMutableDictionary*)options
{
NSLog(#"DOWNLOAD THREAD STARTED");
NSString * sourceUrl = [paramArray objectAtIndex:0];
NSString * fileName = [paramArray objectAtIndex:1];
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *newFilePath = [documentsDirectory stringByAppendingString:fileName];
NSError *error=[[[NSError alloc]init] autorelease];
NSURLConnection *fileURL = [NSData dataWithContentsOfURL: [NSURL URLWithString:sourceUrl]];
BOOL response = [fileURL writeToFile:newFilePath options:NSDataWritingFileProtectionNone error:&error];
if (response == TRUE)
{
NSLog(#"DOWNLOAD COMPLETED");
[self performSelectorOnMainThread:#selector(downloadComplete:withDict:) withObject:paramArray waitUntilDone:YES];
}
else
{
NSLog(#"Something went wrong while downloading file.");
NSString *callback = [NSString stringWithFormat:#"downloadInterrupted('%#');",fileName];
[webView stringByEvaluatingJavaScriptFromString:callback];
}
[pool drain];
}
AsiHTTP isn't an option because there are issues with the PhoneGap I'm using.

Answer 1

A better idea is to download the file asynchronously. This has several advantages: The most important one is that your user interface stays responsive. The user can go on using your application while it is downloading and waiting for the data. If the data you are downloading is absolutely essential for the application, display some sort of loading indicator.
You can easily start the asynchronous download via
NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:sourceUrl]];
NSURLConnection *connection = [NSURLConnection connectionWithRequest:request delegate:self];
Now, how do I get the downloades data in an NSData object? You implement the following delegate methods for self:
-connection:didReceiveData:
-connection:didFailWithError:
-connectionDidFinishLoading:
The idea is that you are notified whenever some data drops in through your connection or anything important else happens (success or failure for exmple). So you are going to declare a temporary NSMutableData object as an instance variable (say downloadData) and write to it until the download is complete. Do not forget to initialize the empty object and declare a property as well!
-connection:didReceiveData: is called whenever some sort of data (that is, a part of your downloaded file) arrives. So you are going to append it to your temporary object like this:
-(void) connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
[self.downloadData appendData:data];
}
Once the download has finished (successfully), the next delegate method is called:
-(void) connectionDidFinishLoading:(NSURLConnection *)connection {
//do whatever you need to do with the data in self.downloadData
}
If the downloads fails, -connection:didFailWithError: is called. You can then save the temporary object, get its size and resume the download later. [self.downloadData length]; gets you the size in bytes of the data in your object.

Answer 2

You are going to have to use a lower level api.
time to read up on unix socket programming. http://www.fortunecity.com/skyscraper/arpanet/6/cc.htm would be a good start.
It really won't be too hard. honest.

Answer 3

I recommend you to build a method that save data chunk every 1, 2 MB or maybe less in order to resume properly your download and avoid memory crash.
This because if you get an error in your transfer maybe your file could be result corrupted.
Anyway send a range HTML header is pretty simple
NSFileHandle *fileHandler = [NSFileHandle fileHandleForReadingAtPath:dataPreviouslySavedPath];
[fileHandler seekToEndOfFile];
unsigned long long int range = [fileHandler offsetInFile];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:downloadURL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:20.0];
[request setValue:[NSString stringWithFormat:#"bytes=%lli-", range] forHTTPHeaderField:#"Range"];
NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:request delegate:self];
Hope this help you.