NSURLConnection Download multiple images

Question

I am trying to download multiple images from a URL stored in an XML feed. Getting the image urls from the XML is working correctly. However, the NSURLConnection is creating

Answer 1

You have to wait until the connection tells you it has finished before you can write the data. The connection is handled on another thread; if you try to access the data immediately on the original thread as you're doing, there won't be anything in it.

You should move the writeToFile: call to the end of connectionDidFinishLoading:, or to another method that you call from there. That's the first point where you know that the data has all been collected.

I'd also suggest creating the NSMutableData instance in didRecieveResponse:, so that you know that it is available at the correct time. That will be more readable/understandable. You can think of the delegate methods as a collective "scope" -- the data is only used inside of them, so it should be created inside of one of them.

In reply to your comment:

One possibility, since you have so much that needs to be done around this one download, and don't seem to be touching the GUI, is to run the whole parsingComplete: method on a background thread, and using +[NSURLConnection sendSynchronousRequest:returningResponse:error:]. This way your code will just wait until the data comes back, in one piece, and you can write it immediately after the sendSynchronous... call returns.

NSError * err;
NSURLResponse * response;
NSData * receivedData = [NSURLConnection sendSynchronousRequest:imageRequest
                                              returningResponse:&response
                                                          error:&err];
if( !receivedData ){
    /* Handle error */
}
/* Check response */

BOOL result = [receivedData writeToFile:fileOut atomically:YES];
/* check result, etc. */

Answer 2

You can use a CustomURLConnection with Tag to name de images before they download.

With this code you can make a customURLConnection, name it when you make the request, and ask for the name of the image in the connectionDidFinishLoading:

CustomURLConnection.h

#import <Foundation/Foundation.h>

@interface CustomURLConnection : NSURLConnection 
{
NSString *tag;
}

@property (nonatomic, retain) NSString *tag;

- (id)initWithRequest:(NSURLRequest *)request delegate:(id)delegate startImmediately:(BOOL)startImmediately tag:(NSString*)aTag;

@end

CustomURLConnection.m

#import "CustomURLConnection.h"

@implementation CustomURLConnection

@synthesize tag;

- (id)initWithRequest:(NSURLRequest *)request delegate:(id)delegate startImmediately:(BOOL)startImmediately tag:(NSString*)aTag 
{
self = [super initWithRequest:request delegate:delegate startImmediately:startImmediately];

    if (self) {
        self.tag = aTag;
    }
    return self;
}

- (void)dealloc 
{
    [tag release];
    [super dealloc];
}

@end

Then make the connection, a custom url connection in your parsingComplete with:

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:yourURL];
[request setTimeoutInterval:3000.0];

CustomURLConnection *connection = [[CustomURLConnection alloc]     initWithRequest:request delegate:self startImmediately:YES tag:imageTag];

Now you can take the imageName with the CustomURLConnection tag, and save it in the connectionDidFinishLoading:

CustomURLConnection *urlConec = (CustomURLConnection*)connection;

NSMutableData *dataFromConnection = [self dataForConnection:urlConec];

and this is the code for the function dataForConnection:

- (NSMutableData*)dataForConnection:(CustomURLConnection*)connection 
{
    NSMutableData *data = [receivedData objectForKey:connection.tag];
    return data;
}

Hope that helps.