So I'm using performSelectorInBackground:#selector(loadStuff) to load things in the background. It takes some time.
The user may want to re-load the items, while the method above is running in the background. If I performSelectorInBackground:#selector(loadStuff) again while the other method is already running, I get all kinds of errors.
Is there an easy way to handle this situation?
I'd like to stop the already-running-in-the-background method, then start the new one. (Or if there is a better way of accomplishing the end goal, that would be fine as well).
If you want to start over, you can cancel the connection and create a new one. Since you're going to run this method on a background thread you need to make sure that only one thread can access the related instance variables at a time:
- (void)loadStuff
{
#synchronized(self) {
if (currentConnection != nil)
[currentConnection cancel];
[currentConnection release];
}
currentConnection = [[NSURLConnection alloc] initWithRequest:request
delegate:self
startImmediately:YES];
}
}
Another way to do it is to use a flag to indicate a busy state. For example, if you wanted to just return if the long-running method were already in process on another thread, you could do this:
- (void)loadStuff
{
#synchronized(self) {
if (loadingStuff == YES)
return;
}
loadingStuff = YES;
}
NSURLRequest *request = ...
NSURLResponse *response;
NSError *error;
[NSURLConnection sendSynchronousRequest:request returningReseponse:&response error:&error];
#synchronized(self) {
loadingStuff = NO;
}
}
Related
I'm experimenting with Firebase's FDataSnapshot to pull in data and I would like it to write its data to my core data using MagicalRecord.
According to Firebases "best practice" blog I need to keep a reference to the "handle" so it can be cleaned up later on. Further, they mention to put the FDSnapshot code in viewWillAppear.
I am wanting a callback so that when its finished doing its thing to update core data.
But I'm really note sure how to do that; its doing two things and giving a return at the same time.
// In viewWillAppear:
__block NSManagedObjectContext *context = [NSManagedObjectContext MR_context];
self.handle = [self.ref observeEventType:FEventTypeValue withBlock:^(FDataSnapshot *snapshot) {
if (snapshot.value == [NSNull null])
{
NSLog(#"Cannot find any data");
}
else
{
NSArray *snapshotArray = [snapshot value];
// cleanup to prevent duplicates
[FCFighter MR_truncateAllInContext:context];
for (NSDictionary *dict in snapshotArray)
{
FCFighter *fighter = [FCFighter insertInManagedObjectContext:context];
fighter.name = dict[#"name"];
[context MR_saveToPersistentStoreWithCompletion:^(BOOL contextDidSave, NSError *error){
if (error)
{
NSLog(#"Error - %#", error.localizedDescription);
}
}];
}
}
}];
NSFetchRequest *fr = [[NSFetchRequest alloc] initWithEntityName:[FCFighter entityName]];
fr.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:#"name" ascending:YES]];
self.fighterList = (NSArray *) [context executeFetchRequest:fr error:nil];
[self.tableView reloadData];
In the above code, the core data reading does not wait for the firebase to complete.
Thus, my query -- how would I best combine a completion handler so that when it is complete to update core data, and reload the tableview.
Many thanks
This is a common issue when working with Asynchronous data.
The bottom line is that all processing of data returned from an async call (in this case, the snapshot) needs to be done inside the block.
Anything done outside the block may happen before the data is returned.
So some sudo code
observeEvent withBlock { snapshot
//it is here where snapshot is valid. Process it.
NSLog(#"%#", snapshot.value)
}
Oh, and a side note. You really only need to track the handle reference when you are going to do something else with it later. Other than that, you can ignore the handles.
So this is perfectly valid:
[self.ref observeEventType:FEventTypeValue withBlock:^(FDataSnapshot *snapshot) {
//load your array of tableView data from snapshot
// and/or store it in CoreData
//reload your tableview
}
I have an application that first loads some data into an UIManagedDocument, then executes saveToURL:forSaveOperation:completionHandler:. Inside the completionHandler block, it does an update of various elements of this database, and when it's done, it does another saving.
Besides that, the app has 3 buttons that reload the data, re-update the data, and delete one entity of the database, respectively. In every button method, the last instruction is a saving as well.
When I run all this in the simulator, all goes smoothly. But in the device doesn't. It constantly crashes. I have observed that, normally, it crashes when pressing the "delete" button, or when reloading or re-updating the database. And it's always in the saveToURL operation.
In my opinion, the problem comes when there are multiple threads saving the database. As the device executes the code slower, maybe multiple savings come at same time and the app can't handle them correctly. Also, sometimes the delete button doesn't delete the entity, and says that doesn't exist (when it does).
I'm totally puzzled with this, and all this saving operations must be done...In fact, if I remove them, the app behaves even more incoherently.
Any suggestions of what could I do to resolve this problem? Thank you very much!
[Edit] Here I post the problematic code. For first loading the data, I use a helper class, with this two methods in particular:
+ (void)loadDataIntoDatabase:(UIManagedDocument *)database
{
[database.managedObjectContext performBlock:^{
// Read from de plist file and fill the database
[database saveToURL:database.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:^(BOOL success) {
[DataHelper completeDataOfDatabase:database];
}];
}
+ (void)completeDataOfDatabase:(UIManagedDocument *)database
{
[database.managedObjectContext performBlock:^{
// Read from another plist file and update some parameters of the already existent data (uses NSFetchRequest and works well)
// [database saveToURL:database.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:nil];
[database updateChangeCount:UIDocumentChangeDone];
}];
}
And in the view, I have 3 action methods, like these:
- (IBAction)deleteButton {
[self.database.managedObjectContext performBlock:^{
NSManagedObject *results = ;// The item to delete
[self.database.managedObjectContext deleteObject:results];
// [self.database saveToURL:self.database.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:NULL];
[self.database updateChangeCount:UIDocumentChangeDone];
}];
}
- (IBAction)reloadExtraDataButton {
[DataHelper loadDataIntoDatabase:self.database];
// [self.database saveToURL:self.database.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:NULL];
[self.database updateChangeCount:UIDocumentChangeDone];
}
- (IBAction)refreshDataButton {
[DataHelper completeDataOfDatabase:self.database];
//[self.database saveToURL:self.database.fileURL forSaveOperation:UIDocumentSaveForOverwriting completionHandler:NULL];
[self.database updateChangeCount:UIDocumentChangeDone];
}
[Edit 2] More code: First of all, the initial view executes viewDidLoad this way:
- (void)viewDidLoad{
[super viewDidLoad];
self.database = [DataHelper openDatabaseAndUseBlock:^{
[self setupFetchedResultsController];
}];
}
This is what the setupFetchedResultsController method looks like:
- (void)setupFetchedResultsController
{
NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:#"Some entity name"];
request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:#"name" ascending:YES selector:#selector(localizedCaseInsensitiveCompare:)]];
self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
managedObjectContext:self.database.managedObjectContext
sectionNameKeyPath:nil
cacheName:nil];
}
Each view of the app (it has tabs) has a different setupFetchedResultsController in order to show the different entities the database contains.
Now, in the helper class, this is the first class method that gets executed, via the viewDidLoad of each view:
+ (UIManagedDocument *)openDatabaseAndUseBlock:(completion_block_t)completionBlock
{
NSURL *url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
url = [url URLByAppendingPathComponent:#"Database"];
UIManagedDocument *database = [[UIManagedDocument alloc] initWithFileURL:url];
if (![[NSFileManager defaultManager] fileExistsAtPath:[database.fileURL path]]) {
[database saveToURL:database.fileURL forSaveOperation:UIDocumentSaveForCreating completionHandler:^(BOOL success) {
[self loadDataIntoDatabase:database];
completionBlock();
}];
} else if (database.documentState == UIDocumentStateClosed) {
// Existe, pero cerrado -> Abrir
[database openWithCompletionHandler:^(BOOL success) {
[self loadDataIntoDatabase:database];
completionBlock();
}];
} else if (database.documentState == UIDocumentStateNormal) {
[self loadDataIntoDatabase:database];
completionBlock();
}
return database;
}
You didn't really provide much code. The only real clue you gave was that you are using multiple threads.
UIManagedDocument has two ManagedObjectContexts (one specified for the main queue, and the other for a private queue), but they still must each only be accessed from within their own thread.
Thus, you must only use managedDocument.managedObjectContext from within the main thread. If you want to use it from another thread, you have to use either performBlock or performBlockAndWait. Similarly, you can never know you are running on the private thread for the parent context, so if you want to do something specifically to the parent, you must use performBlock*.
Finally, you really should not be calling saveToURL, except when you initially create the database. UIManagedDocument will auto-save (in its own time).
If you want to encourage it to save earlier, you can send it updateChangeCount: UIDocumentChangeDone to tell it that it has changes that need to be saved.
EDIT
You should only call saveToURL when you create the file for the very first time. With UIManagedDocument, there is no need to call it again (and it can actually cause some unintended issues).
Basically, when you create the document DO NOT set your iVar until the completion handler executes. Otherwise, you could be using a document in a partial state. In this case, use a helper, like this, in the completion handler.
- (void)_document:(UIManagedDocument*)doc canBeUsed:(BOOL)canBeUsed
{
dispatch_async(dispatch_get_main_queue(), ^{
if (canBeUsed) {
_document = doc;
// Now, the document is ready.
// Fire off a notification, or notify a delegate, and do whatever you
// want... you really should not use the document until it's ready, but
// as long as you leave it nil until it is ready any access will
// just correctly do nothing.
} else {
_document = nil;
// Do whatever you want if the document can not be used.
// Unfortunately, there is no way to get the actual error unless
// you subclass UIManagedDocument and override handleError
}
}];
}
And to initialize your document, something like...
- (id)initializeDocumentWithFileURL:(NSURL *)url
{
if (!url) {
url = [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
url = [url URLByAppendingPathComponent:#"Default_Project_Database"];
}
UIManagedDocument *doc = [[UIManagedDocument alloc] initWithFileURL:url];
if (![[NSFileManager defaultManager] fileExistsAtPath:[doc.fileURL path]]) {
// The file does not exist, so we need to create it at the proper URL
[doc saveToURL:doc.fileURL forSaveOperation:UIDocumentSaveForCreating completionHandler:^(BOOL success) {
[self _document:doc canBeUsed:success];
}];
} else if (doc.documentState == UIDocumentStateClosed) {
[doc openWithCompletionHandler:^(BOOL success) {
[self _document:doc canBeUsed:success];
}];
} else {
// You only need this if you allow a UIManagedDocument to be passed
// in to this object -- in which case the code above that initializes
// the <doc> variable will be conditional on what was passed...
BOOL success = doc.documentState == UIDocumentStateNormal;
[self _document:doc canBeUsed:success];
}
}
The "pattern" above is necessary to make sure you do not use the document until it is fully ready for use. Now, that piece of code should be the only time you call saveToURL.
Note that by definition, the document.managedObjectContext is of type NSMainQueueConcurrencyType. Thus, if you know your code is running on the main thread (like all your UI callbacks), you do not have to use performBlock.
However, if you are actually doing loads in the background, consider..
- (void)backgroundLoadDataIntoDocument:(UIManagedDocument*)document
{
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
moc.parentContext = document.managedObjectContext;
[moc performBlock:^{
// Do your loading in here, and shove everything into the local MOC.
// If you are loading a lot of stuff from the 'net (or elsewhere),
// consider doing it in strides, so you deliver objects to the document
// a little at a time instead of all at the end.
// When ready to save, call save on this MOC. It will shove the data up
// into the MOC of the document.
NSrror *error = nil;
if ([moc save:&error]) {
// Probably don't have to synchronize calling updateChangeCount, but I do it anyway...
[document.managedObjectContext performBlockAndWait:^{
[document updateChangeCount:UIDocumentChangeDone];
}];
} else {
// Handle error
}
}];
}
Instead of parenting your background MOC to the mainMOC, you can parent it to the parentContext. Loading and then saving into it will put the changes "above" the main MOC. The main MOC will see those changes the next time it does a fetch operation (note the properties of NSFetchRequest).
NOTE: Some people have reported (and it also appears as a note in Erica Sadun's book), that after the very first saveToURL, you need to close, then open to get everything working right.
EDIT
This is getting really long. If you had more points, I'd suggest a chat. Actually, we can't do it through SO, but we could do it via another medium. I'll try to be brief, but please go back and reread what I posted, and pay careful attention because your code is still violating several tenants.
First, in viewDidLoad(), you are directly assigning your document to the result of calling openDatabaseAndUseBlock. The document is not in a usable state at that time. You do not want the document accessible until the completion handlers fire, which will not happen before openDatabaseAndUseBlock() returns.
Second, only call saveToURL the very first time you create your database (inside openDatabaseAndUseBlock()). Do not use it anywhere else.
Third. Register with the notification center to receive all events (just log them). This will greatly assist your debugging, because you can see what's happening.
Fourth, subclass UIManagedDocument, and override the handleError, and see if it is being called... it's the only way you will see the exact NSError if/when it happens.
3/4 are mainly to help you debug, not necessary for your production code.
I have an appointment, so have to stop now. However, address those issues, and here's on
I'm using an AFNetworking client object which makes an asynchronous request for an XML document and parses it.
Also using NSNotificationCenter to post a notification when the document has finished parsing.
Is there a way to wait for a notification to be posted without blocking the main thread?
E.g code:
-(void)saveConfiguration:(id)sender {
TLHTTPClient *RESTClient = [TLHTTPClient sharedClient];
// Performs the asynchronous fetching....this works.
[RESTClient fetchActiveUser:[usernameTextField stringValue] withPassword:[passwordTextField stringValue]];
/*
* What do I need here ? while (xxx) ?
*/
NSLog(#"Fetch Complete.");
}
Basically I'm wondering what sort of code I need in the above specified area to ensure that the function waits until the fetch has been completed ?
As it is right now I'll see "Fetch Complete." in the debug console before the fetch has been completed.
I tried adding a BOOL flag to the TLHTTPClient class:
BOOL fetchingFlag;
and then trying:
while([RESTClient fetchingFlag]) { NSLog(#"fetching...); }
When this class receives the notification it sets RESTClient.fetchingFlag = FALSE; which should technically kill the while loop right? Except my while loop runs infinitely ?!
Basically I'm wondering what sort of code I need in the above specified area to ensure that the function waits until the fetch has been completed ?
You need no code. Don't put anything in the method after you start the fetch, and nothing will happen. Your program will "wait" (it will actually be processing other input) until the notification is recieved.
In the notification handler method, put all the stuff that you need to do when the fetch is completed. This is (one of) the point(s) of notifications and other callback schemes -- your object won't do anything further until it gets the notification that it's time to act.
Is there a way to wait for a notification to be posted without blocking the main thread?
That's exactly how it works already.
If you don't need to inform multiple objects upon completion of the task, you could add a completion handler (objc block) to the -fetchActiveUser:withPassword: method (so it would become something like -fetchActiveUser:withPassword:completionHandler: and add the code to be executed in the completion handler.
An example, lets say your -fetchActiveUser:withPassword:completionHandler: method looks like the following:
- (void)fetchActiveUser:(NSString *)user
withPassword:(NSString *)pass
completionHandler:(void (^)(TLUser *user, NSError *error))handler
{
NSURL *URL = [NSURL URLWithString:#"http://www.website.com/page.html"];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL];
NSOperationQueue *queue = [NSOperationQueue currentQueue];
[NSURLConnection sendAsynchronousRequest:request
queue:queue
completionHandler:^ (NSURLResponse *response, NSData *data, NSError *error)
{
if (!handler)
{
return
};
if (data)
{
TLUser *user = [TLUser userWithData:data];
if (user)
{
handler(user, nil);
}
else
{
NSError *error = // 'failed to create user' error ...
handler(nil, error);
}
}
else
{
handler(nil, error);
}
}];
}
The completion handler will be called whenever the task is finished. It will either return a TLUser object or an Error if something went wrong (bad connection, data format changed while parsing, etc...).
You'll be able to call the method like this:
- (void)saveConfiguration:(id)sender
{
TLHTTPClient *RESTClient = [TLHTTPClient sharedClient];
// Performs the asynchronous fetching
[RESTClient fetchActiveUser:[usernameTextField stringValue]
withPassword:[passwordTextField stringValue]
completionHandler:^ (TLUser *user, NSError *error)
{
if (user)
{
NSLog(#"%#", user);
}
else
{
NSLog(#"%#", error);
}
}];
}
Of course, in this example I've used the build in asynchronous methods of NSURLConnection in stead of AFNetworking, but you should be able to get the general idea.
I am writing a network class for an iOS app. This class will take care of all logging and network traffic. I have a problem where I have to send possibly thousands of requests at one time, but NSURLConnections are timing out because the delegate methods will not be called until all the NSURLConnections are started, by which time the timeout period has expired. I am using a rest API for Drupal and, unfortunately, I do not know of a way to create multiple instances with one request. How can I receive responses while simultaneously sending them? If I use GCD to pass off the creation of the NSURLConnections, will that solve the problem? I think I would have to pass the entire operation of iterating over the objects to send and sending to GCD to free up the main thread to answer to responses.
-(BOOL)sendOperation:(NetworkOperation)op
NetworkDataType:(NetworkDataType)dataType
JsonToSend:(NSArray *)json
BackupData:(NSArray *)data
{
if(loggingMode)
{
return YES;
}
NSURLConnection *networkConnection;
NSData *send;
NSString *uuid = [self generateUUID];
NSMutableArray *connections = [[NSMutableArray alloc] init];
NSMutableURLRequest *networkRequest;
for (int i=0; i<[json count] && (data ? i<[data count] : YES); i++)
{
if(op == Login)
{
/*Grab all cookies from the server domain and delete them, this prevents login failure
because user was already logged in. Probably find a better solution like recovering
from the error*/
NSArray *cookies = [[NSHTTPCookieStorage sharedHTTPCookieStorage] cookiesForURL:
[[NSURL alloc] initWithString:networkServerAddress]];
for (NSHTTPCookie *cookie in cookies)
{
[[NSHTTPCookieStorage sharedHTTPCookieStorage] deleteCookie:cookie];
}
networkRequest = [[NSMutableURLRequest alloc] initWithURL:
[NSURL URLWithString:[networkServerAddress stringByAppendingString:#"/user/login"]]];
}
else if(op == StartExperiment)
{
networkRequest = [[NSMutableURLRequest alloc] initWithURL:
[NSURL URLWithString:[networkServerAddress stringByAppendingString:#"/node"]]];
}
else if(op == Event || op == EndExperiment || op == SendAll)
{
networkRequest = [[NSMutableURLRequest alloc] initWithURL:
[NSURL URLWithString:[networkServerAddress stringByAppendingString:#"/node"]]];
}
else if(op == Logout)
{
networkRequest = [[NSMutableURLRequest alloc] initWithURL:
[NSURL URLWithString:[networkServerAddress stringByAppendingString:#"/user/logout"]]];
}
send = [[json objectAtIndex:i] dataUsingEncoding:NSUTF8StringEncoding];
//Set the headers appropriately
[networkRequest setHTTPMethod:#"POST"];
[networkRequest setValue:#"application/json"
forHTTPHeaderField: #"Content-type"];
[networkRequest setValue:[NSString stringWithFormat:#"%d", [send length]]
forHTTPHeaderField:#"Content-length"];
[networkRequest setValue:#"application/json"
forHTTPHeaderField:#"Accept"];
//Set the body to the json encoded string
[networkRequest setHTTPBody:send];
//Starts async request
networkConnection = [[NSURLConnection alloc] initWithRequest:networkRequest delegate:self];
//Successfully created, we are off
if(networkConnection)
{
[networkConnectionsAndData setValue:[[NSMutableArray alloc] initWithObjects:uuid,
[[NSNumber alloc] initWithInt:op], [[NSNumber alloc] initWithInt:dataType], [[NSMutableData alloc] init], (data ? [data objectAtIndex:i] : [NSNull null]), nil]
forKey:[networkConnection description]];
}
else //Failed to conn ect
{
NSLog(#"Failed to create NSURLConnection");
return NO;
}
}
[[self networkOperationAndConnections] setObject:[[NSMutableDictionary alloc] initWithObjectsAndKeys:[[NSMutableArray alloc] initWithObjects:connections, nil], #"connections", [[NSMutableArray alloc] init], #"errors", nil]
forKey:uuid];
return YES;
}
The dictionaries are used to keep track of the correlating data with each NSURLConnection and also to group the NSURLConnections together into one group to determine ultimate success or failure of an entire operation.
Update
AFNetworking was key in finishing this project. It not only cleaned up the code substantially, but dealt with all the threading issues inherit in sending so many requests. Not to mention with AFNetworking I could batch all the requests together into a single operation. Using blocks, like AFNetworking uses, was a much cleaner and better solution than the standard delegates for NSURLConnections.
You definitely need to allow the NSURLRequest / Connection to be operating on another thread. (Not the main thread!)
Edited for clarity**:
I noticed your comment of "//Starts async request" and I wanted to be sure you realized that your call there is not what you would expect out of a typical "asynch" function. Really its just firing off the request synchronously, but since its a web request it inherently behaves asynchronously. You want to actually place these requests on a another thread for full asynch behavior.
Everything else aside, I really suggest digging into Apple's networking example project here: MVCNetworking
As for specifics on your question, there's a couple ways to do this.
One is to keep your connection from starting immediately using initWithRequest:<blah> delegate:<blah> startImmediately:FALSE and then schedule your NSURLConnection instances on another thread's run-loop using: scheduleInRunLoop:forMode:
(Note: You then have to kick off the connection by calling start-- it's best to do this via an NSOperation + NSOperationQueue.)
Or use this static method on NSURLConnection to create/launch the connection instead of doing an alloc/init: sendAsynchronousRequest:queue:completionHandler:
(Note: this approach accomplishes pretty much same as above but obfuscates the details and takes some of the control out of your hands.)
To be honest my quick answers above won't be sufficient to finish this kind of project, and you'll need to do a bit of research to fill in the blanks, especially for the NSOperationQueue, and that's where the MVCNetworking project will help you.
Network connections are a fickle beast -- You can time-out and kill your connections even if they're running on a background thread simply by trying to perform too much work simultaneously! I would seriously reconsider opening up several thousand NSURLConnections at once, and using an NSOperationQueue would help work around this.
ADDITIONAL RESOURCES:
Here's a 3rd party library that may make your networking adventures less painful:
https://github.com/AFNetworking/AFNetworking
http://engineering.gowalla.com/2011/10/24/afnetworking/
I have a Cocoa Mac application set up to download files to a specific folder using NSURLDownload. This works great with a single download at a time. However, if I attempt to start multiple downloads, all but the last will fail immediately.
Is there any way to use NSURLDownload for multiple simultaneous downloads? Or what would be a good way to queue up multiple URLs to be downloaded in order? Or is there a more appropriate way to accomplish this (NSURLConnection seemed possible but I was unsure if I could set the download location and filename as I can with NSURLDownload)?
Each NSURLDownload represents a single downloading instance. You're probably trying to reuse the same one multiple times. It's an inherently asynchronous system that already used background threads. Here's an example based on Apple's sample code:
- (void)startDownloadingURL:sender
{
// Create a couple requests.
NSURLRequest *requestOne = [NSURLRequest requestWithURL:[NSURL URLWithString:#"http://www.apple.com"]
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:60.0];
NSURLRequest *requestTwo = [NSURLRequest requestWithURL:[NSURL URLWithString:#"http://stackoverflow.com"]
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:60.0];
// Create two download instances
NSURLDownload *downloadOne = [[NSURLDownload alloc] initWithRequest:requestOne delegate:self];
NSURLDownload *downloadTwo = [[NSURLDownload alloc] initWithRequest:requestTwo delegate:self];
if (downloadOne) {
// Set the destination file.
[downloadOne setDestination:#"/tmp" allowOverwrite:YES];
} else {
// inform the user that the download failed.
}
if (downloadTwo) {
// Set the destination file.
[downloadTwo setDestination:#"/tmp" allowOverwrite:YES];
} else {
// inform the user that the download failed.
}
}
- (void)download:(NSURLDownload *)download didFailWithError:(NSError *)error
{
// Release the connection.
[download release];
// Inform the user.
NSLog(#"Download failed! Error - %# %#",
[error localizedDescription],
[[error userInfo] objectForKey:NSErrorFailingURLStringErrorKey]);
}
- (void)downloadDidFinish:(NSURLDownload *)download
{
NSLog(#"The download %# has finished.", download)
// Release the download connection.
[download release];
}
If you attempt to use the same NSURLDownload for both NSURLRequests, then it will kill the previous connection.
I'd second using NSOperation if your on 10.5+ or greater. You could just throw 1 operation onto a queue for each download. Or you could even just use sendSynchronous request and use it with NSOperationQUeue's addOperationWithBlock (10.6+) method and then in your block you are throwing onto the queue you can just use [[NSOperationQueue mainQueue] addOperationWithBlock:^{ when you are doe with the code you need to execute or just periodically need to refresh the UI on the main thread, like so...
[myQueue addOperationWithBlock:^{
//download stuff here...
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
//update main thread UI
}];
}];
you would just need to do this for each download.
If you're targeting 10.5+, you should look at NSOperation. It should allow you to build a generic solution for a single download, and then use the built-in queue facilities to manage dependencies if you require certain operations finish downloading before others begin.
Keep in mind that these APIs often expect delegate methods involved to be run on the main thread, so you'll need ensure that occurs if you're working with asynchronous APIs that function via delegate methods. (You can do this pretty simply by using performSelectorOnMainThread: and friends)