Question

I would like to download files directly from an URL to the disk using objective-c on the iPhone os.

Currently I am using NSURLConnection to send a synchronousRequest, writing the returned NSData into a file.

How can I change the download handling (still having the request beeing synchronous, it is already in a background thread) to write the data directly to disk, not using memory variables to store the complete content (only small parts)?

A sample code would be appreciated.

Thank you all in advance for your responses!

Answer 1

You can do this, but it’s a bit complicated to set up. Here’s how I’d do it:

warning: the following code was typed in a browser and compiled in my head. Also, there’s not a lot of error handling. Caveat Implementor.

//NSURLConnection+DirectDownload.h
@interface NSURLConnection (DirectDownload)

+ (BOOL) downloadItemAtURL:(NSURL *)url toFile:(NSString *)localPath error:(NSError **)error;

@end

//NSURLConnection+DirectDownload.m
@implementation NSURLConnection (DirectDownload)

+ (BOOL) downloadItemAtURL:(NSURL *)url toFile:(NSString *)localPath error:(NSError **)error {
  NSMutableURLRequest * request = [[NSMutableURLRequest alloc] initWithURL:url];
  //configure the request, or leave it as-is

  DirectDownloadDelegate * delegate = [[DirectDownloadDelegate alloc] initWithFilePath:localPath];
  NSURLConnection * connection = [[NSURLConnection alloc] initWithRequest:request delegate:delegate];
  [delegate autorelease];
  [request release];

  while ([delegate isDone] == NO) {
    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1.0]];
  }

  [connection release];

  NSError * downloadError = [delegate error];
  if (downloadError != nil) {
    if (error != nil) { *error = [[downloadError retain] autorelease]; }
    return NO;
  }

  return YES;
}

//DirectDownloadDelegate.h
@interface DirectDownloadDelegate : NSObject {
  NSError *error;
  NSURLResponse * response;
  BOOL done;
  NSFileHandle * outputHandle;
}
@property (readonly, getter=isDone) BOOL done;
@property (readonly) NSError *error;
@property (readonly) NSURLResponse * response;

@end

//DirectDownloadDelegate.m
@implementation DirectDownloadDelegate
@synthesize error, request, done;

- (id) initWithFilePath:(NSString *)path {
  if (self = [super init]) {
    if ([[NSFileManager defaultManager] fileExistsAtPath:path]) {
      [[NSFileManager defaultManager] removeItemAtPath:path error:nil];
    }
    [[NSFileManager defaultManager] createFileAtPath:path contents:nil attributes:nil];
    outputHandle = [[NSFileHandle fileHandleForWritingAtPath:path] retain];
  }
  return self;
}

- (void) dealloc {
  [error release];
  [response release];
  [outputHandle closeFile];
  [outputHandle release];
  [super dealloc];
}

- (void) connection:(NSURLConnection *)connection didFailWithError:(NSError *)anError {
  error = [anError retain];
  [self connectionDidFinishLoading:connection];
}

- (void) connection:(NSURLConnection *)connection didReceiveData:(NSData *)someData {
  [outputHandle writeData:someData];
}

- (void) connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)aResponse {
  response = [aResponse retain];
}

- (void) connectionDidFinishLoading:(NSURLConnection *)connection {
  done = YES;
}

The basic idea is that you create a standard NSURLConnection, which is normally asynchronous, but just block the thread by spinning the runloop yourself until the connection is done. You also use a custom url connection delegate to just pipe any data the connection receives directly to a file.

You can now do:

NSError * downloadError = nil;
BOOL ok = [NSURLConnection downloadItemAtURL:someURL toFile:someFile error:&downloadError];
if (!ok) {
  NSLog(@"ack there was an error: %@", error);
} else {
  NSLog(@"file downloaded to: %@", someFile);
}