How to download a file from a URL in OS X app - objective-c

Question

How do I download a file from a URL in an OS X app? I am a Unix C and Perl programmer new to Objective C. I've been reading the URL Loading System Programming Guide, which suggests NSURLSession for new OS X apps. So I'd prefer a solution with NSURLSession or NSURLSeesionDownloadTask, but am open to NSURLDownload or other solutions.
My first attempt was based an example found here
NSURLSessionDownloadTask *downloadTask =
[[NSURLSession sharedSession]
downloadTaskWithURL:[NSURL URLWithString:pdfFile]
completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
// move tmp file to permanent location
NSLog(#"Done...");
NSLog(#"Location: %#", location);
NSLog(#"Response: %#", response);
NSLog(#"Error %#", error);
NSFileManager *fileManager = [NSFileManager defaultManager];
BOOL fileCopied = [fileManager moveItemAtPath:[location path] toPath:[appDir stringByAppendingString:#"/demo.pdf"] error:&error];
NSLog(fileCopied ? #"File Copied OK" : #"ERROR Copying file.");
}];
[downloadTask resume];
NSLog(#"Now we are here");
The example code (and my version) don't have a separate delegate. The completionHandler is "in line". (Not sure if that's the correct terminology). I assume this code would be executed AFTER the download task completes. Is that correct?
My second attempt was based on the "Downloading Files" section of the URL Programming Guide:
// PDF file Download, try #2
NSError *error = nil;
NSString *appDir = NSHomeDirectory();
NSString *pdfFile = #"https://developer.apple.com/library/mac/documentation/IDEs/Conceptual/AppDistributionGuide/AppDistributionGuide.pdf";
// Configure Cache behavior for default session
NSURLSessionConfiguration
*defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
NSString *cachePath = [NSTemporaryDirectory()
stringByAppendingPathComponent:#"/pdfDownload.cache"];
NSLog(#"Cache path: %#\n", cachePath);
NSURLCache *myCache = [[NSURLCache alloc] initWithMemoryCapacity: 16384
diskCapacity: 268435456 diskPath: cachePath];
defaultConfigObject.URLCache = myCache;
defaultConfigObject.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
defaultConfigObject.timeoutIntervalForRequest = 100;
defaultConfigObject.timeoutIntervalForResource = 100;
// Create a delegate-Free Session
NSURLSession *delegateFreeSession =
[NSURLSession sessionWithConfiguration: defaultConfigObject
delegate: nil
delegateQueue: [NSOperationQueue mainQueue]];
[[delegateFreeSession downloadTaskWithURL:[NSURL URLWithString:pdfFile] completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
// move tmp file to permanent location
NSLog(#"Done...");
NSLog(#"Location: %#", location);
NSLog(#"Response: %#", response);
NSLog(#"Error %#", error);
NSFileManager *fileManager = [NSFileManager defaultManager];
BOOL fileCopied = [fileManager moveItemAtPath:[location path] toPath:[appDir stringByAppendingString:#"/demo.pdf"] error:&error];
NSLog(fileCopied ? #"File Copied OK" : #"ERROR Copying file.");
}] resume];
sleep(10);
NSLog(#"After sleep...");
In both cases, code execution never gets to the NSLog statements that are part of the completionHandler. I have breakpoints set in the simulator, and never get there. How do I create a simple "delegate free" file download method?
Since I am writing a "command line" OS X app, I don't have a UIViewController, or other existing structure for delegates.
Any suggestions on how to proceed? I don't need a download progress bar, etc., just a "pass/fail" status.
Also, my example here is for a single PDF file. My app has an array of URLs that I'd like to download. I don't need to download them in parallel. Could I reuse a single NSURLSession in a loop?
Any help would be appreciated!!

Answer 1

Forgot to close this once I got the solution. As Zev pointed out, the PDF file I chose was large, and needed more time to download. The 10 second 'sleep(10)' didn't provide enough time to download the 39 MB file.
Notifications should be used to indicate when the completionHandler finishes. But for now, I am using a simple polling loop. I changed sleep(10); to
int timeoutCounter = 60;
while ((timeoutCounter > 0) && !downloadDone) {
sleep(1);
NSLog(#"Timout: %d", timeoutCounter--);
}
if (!downloadDone) {
NSLog(#"ERROR: Timeout occurred before file completed downloading");
}
This requires the declaration of downloadDone to allow the completionHandler block to set it:
__block BOOL downloadDone = FALSE;
After the NSFileManger moves the file, this semaphore must be set:
downloadDone = TRUE;
Thanks again for the help Zev!
I'm still working on error checking/handling, but here is the full corrected method:
+ (NSError *) downloadFileAtURL:(NSString *)fileURL
toDir:(NSString *)toDir
timeout:(NSNumber *)timeout {
// completion handler semaphore to indicate download is done
__block BOOL downloadDone = FALSE;
// same PDF file name for later
NSString *downloadFileName = [fileURL lastPathComponent];
NSString *finalFilePath = [toDir stringByAppendingPathComponent:downloadFileName];
NSLog(#"PDF file name: '%#'", downloadFileName);
NSLog(#"Final file path: '%#'", finalFilePath);
// Configure Cache behavior for default session
NSURLSessionConfiguration
*defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
NSString *cachePath = [NSTemporaryDirectory()
stringByAppendingPathComponent:#"/downloadFileCache"];
NSLog(#"Cache path: %#\n", cachePath);
NSURLCache *myCache = [[NSURLCache alloc] initWithMemoryCapacity: 16384
diskCapacity: 268435456 diskPath: cachePath];
defaultConfigObject.URLCache = myCache;
defaultConfigObject.requestCachePolicy = NSURLRequestUseProtocolCachePolicy;
defaultConfigObject.timeoutIntervalForRequest = 100;
defaultConfigObject.timeoutIntervalForResource = 100;
// Create a delegate-Free Session
// delegateQueue: [NSOperationQueue mainQueue]];
NSURLSession *delegateFreeSession =
[NSURLSession sessionWithConfiguration: defaultConfigObject
delegate: nil
delegateQueue: nil];
[[delegateFreeSession downloadTaskWithURL:[NSURL URLWithString:fileURL] completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
// move tmp file to permanent location
NSLog(#"In the completion handler...");
NSLog(#"Location: %#", location);
NSLog(#"Response: %#", response);
NSLog(#"Error %#", error);
NSLog(#"Location path: %#", [location path]);
// Verify temp file exists in cache
NSFileManager *fileManager = [[NSFileManager alloc] init];
if ( [fileManager fileExistsAtPath:[location path]]) {
NSLog(#"File exists. Now move it!");
error = nil;
BOOL fileCopied = [fileManager moveItemAtPath:[location path]
toPath:finalFilePath
error:&error];
NSLog(#"Error: %#", error);
NSLog(fileCopied ? #"File Copied OK" : #"ERROR Copying file.");
}
downloadDone = TRUE;
}] resume];
// use timeout argument
int timeoutCounter = [timeout intValue];
while ((timeoutCounter > 0) && !downloadDone) {
sleep(1);
NSLog(#"Timout: %d", timeoutCounter--);
}
if (!downloadDone) {
NSLog(#"ERROR: Timeout occurred before file completed downloading");
}
NSError *error = nil;
return error;
}