NSURLSessionDownloadTask and retrying - ios

Question

I am using the following code to download a file from the web. How do i structure the code (using blocks) such that, if it fails, it will be retried with a max of retrycount. Here is the code that i am using to download the file.
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:configuration];
NSURL *myURL = [NSURL URLWithString:#"urlstring"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:myURL];
[request setValue:token forHTTPHeaderField:#"Authorization"];
NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request progress:nil destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
NSURL *directoryURL = [NSURL fileURLWithPath:NSTemporaryDirectory()];
return [directoryURL URLByAppendingPathComponent:fileName];
} completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
//check here if error then retry using block ?
if (!error)
{
//do something with the file
}
else
{
//retry download ??
}
}
}];
[downloadTask resume];

Answer 1

You could create a retryCount instance variable and set it to however many times you want to retry the network call. Then you could put all your networking code in a method with a custom completion handler. Your completion handler could be a block that takes a BOOL as a parameter and returns void. Then, in your networking code, create a local BOOL variable and set it to YES or NO based on whether the network call succeeded or failed. You'll run the completion handler argument within your networking code. Then the block (completion handler) that you send as a parameter to the method can "see" whether the local variable in your method was set to YES or NO. If it was YES, the network call succeeded, so do nothing. If it was NO, the network call failed, so check retryCount--if it's > 0, call the networking method again and decrement retryCount.
UPDATE
This should give you a rough idea...
typedef void (^CustomCompletionHandler)(BOOL); // Create your CustomCompletionHandler type
- (void)getDataAndCallCompletionHandler:(CustomCompletionHandler *)completionHandler {
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:configuration];
NSURL *myURL = [NSURL URLWithString:#"urlstring"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:myURL];
[request setValue:token forHTTPHeaderField:#"Authorization"];
NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request progress:nil destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
BOOL success = NO; // Create local variable to keep track of whether network call succeeded or failed
NSURL *directoryURL = [NSURL fileURLWithPath:NSTemporaryDirectory()];
return [directoryURL URLByAppendingPathComponent:fileName];
} completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
//check here if error then retry using block ?
if (!error)
{
success = YES;
//do something with the file
}
else
{
success = NO;
//retry download ??
}
}
// Get main queue and call completionHandler and pass success:
// completionHandler(success)
}];
[downloadTask resume];
}
You would call this method by doing something like...
[self getDataAndCallCompletionHandler:^(BOOL success) {
// If success == true {
// Do whatever you need to
// } else {
// Check retryCount
// If you need to, call getDataAndCallCompletionHandler: again
// }
}];