I'm trying to implement downloading timeline data in a widget and so I created a background URLSession with a corresponding data task to download the JSON:
let session = URLSession(
configuration: .background(withIdentifier: identifier),
delegate: self,
delegateQueue: nil
)
let request = URLRequest(url: ...)
session.dataTask(with: request).resume()
On my widget I then added the onBackgroundURLSessionEvents to store the completion handler, as per the Apple docs:
.onBackgroundURLSessionEvents { identifier in
return SessionCache.shared.isValid(for: identifier)
} _: { identifier, completion in
let data = SessionCache.shared.sessionData(for: identifier)
data.sessionCompletion = completion
}
However, neither the urlSessionDidFinishEvents(forBackgroundURLSession:) nor the onBackgroundURLSessionEvents methods are called. When the network download completes, it just calls the normal urlSession(_:task:didCompleteWithError:) method.
I'm clearly missing something here, but I'm just not seeing what.
Apple's documentation isn't 100% clear on this, but you need to use URLSession.downloadTask instead of dataTask for background sessions. URLSessionDataTasks deliver bytes to those specific delegate methods in your in-memory process. Background download & upload tasks are handed off to nsurlsessiond and only delivered back to your app when they are fully resolved.
Related
This is my first time posting on stackoverflow, and I'm aware of the strict posting requirements. Please let me know if I'm not following any of the guidelines.
I'm currently writing an IOS (8.4) application in Xcode, using Objective-C. The goal is to use MCSessions in order to stream data between IOS devices. I'm currently struggling with the concept of sessions, despite reading numerous posts here and elsewhere that attempt to clarify the topic. Here are the resources I'm already aware of:
https://developer.apple.com/videos/play/wwdc2013-708/
https://medium.com/#phoenixy/using-multipeer-connectivity-120abacb9db
Here's my current understanding: At the most basic level, you have an advertiser, and a browser. The advertiser has a local session, which allows them to "advertise". When the browser sees an advertiser, the browser sends an invite to the advertiser to his (the browser's) local MCSession. Assuming this is all correct, here's where I'm getting confused. The advertiser can accept the invite, and in the process, passes his local session to the invitationHandler.
I have implemented the following logic in code, as shown below. However, in tracing MCSession state changes for both the advertiser and browser, a connection is attempted, but the final state is always didNotNonnect.
Code for sending invitation (Browser):
[self.broadcasterBrowser invitePeer:[broadcasterPeerIDs objectAtIndex:indexPath.row]
toSession: self.appDelegate.mpcHandler.session withContext:nil timeout:30.0 ];
Code for accepting invitation (Advertiser):
- (void)advertiser:(MCNearbyServiceAdvertiser *)advertiser
didReceiveInvitationFromPeer:(MCPeerID *)peerID withContext:(NSData *)context invitationHandler:(void(^)(BOOL accept, MCSession *session))invitationHandler
{
ArrayInvitationHandler = [NSArray arrayWithObject:[invitationHandler copy]];
// ask the user
UIAlertView *alertView = [[UIAlertView alloc]
initWithTitle:peerID.displayName
message:#"Would like to create a session with you"
delegate:self
cancelButtonTitle:#"Decline"
otherButtonTitles:#"Accept", nil];
[alertView show];
if (alertViewResult)
{
void (^invitationHandler)(BOOL, MCSession *) = [ArrayInvitationHandler objectAtIndex:0];
invitationHandler(YES, self.appDelegate.mpcHandler.session);
}
}
Any help is greatly appreciated!
Austin
I ran into a similar problem trying to use MPC. I created a custom class to handle all of the MPC connectivity details. While testing though, every time my advertiser would accept the invite, it would complain about wrong connection data and fail. I discovered that the problem was that I was vending out the MCPeerID object for my device from a class variable I created as below:
static var peerObject : MCPeerID {
return MCPeerID(displayName: deviceNameString)
}
lazy var sessionIVar = MCSession (peer: MyConnectivityClass.peerObject)
func startAdvertisingForConnectivity () {
advertiserService = MCNearbyServiceAdvertiser (peer: MyConnectivityClass.peerObject, discoveryInfo: nil, serviceType: "my-multipeer-connectivity-service-identifier")
}
Then when I got an invitation I would initialize a MCSession object using the "peerObject" computed property and return it in the invitation handler, like this:
func advertiser(_ advertiser: MCNearbyServiceAdvertiser, didReceiveInvitationFromPeer peerID: MCPeerID, withContext context: Data?, invitationHandler: #escaping (Bool, MCSession?) -> Swift.Void) {
invitationHandler(true, sessionIVar)
}
I assumed that each time I called for "MyConnectivityClass.peerObject" it would give back an identical peerID because I was always initializing it with the same display name. It turns out that's not true. So when I was advertising I was using one peerID object and then when I was responding to the invitation, I was responding with a MCSession object that contained an entirely different peerID.
So the solution was to change the "MyConnectivityClass.peerObject" computed class property to a constant, or an Ivar, in my connection handler class. Like this:
let peerObject : MCPeerID = MCPeerID(displayName: deviceNameString)
Then the rest of the code just worked because no matter how many times I called for the MCPeerID object, it was always the same. Looking back I don't know why I started out with it the way I did. :-)
Then in my connectivity class I archived and stored the MCPeerID objects for both the browser and the advertiser so that I could have the advertiser automatically accept the invitation for trusted MCPeerIDs. That's not possible if you create the MCPeerID object each time you use it, even if you always initialize it with the same DisplayName.
How to receive data from the server(the server echo's it as JSON) using NSURLSession in the most efficient way?
Example:
I'm trying to use Instagram's news feed as a save template. They show 10 images and then it will load 10 more(from the server) when you pull down and will not block the user (They receive/download it on the background - while the users are still using the app).
I want to do the same thing in the most efficient way. In my example the screen is frozen until data is fetched. How can I avoid blocking the user?
var imgurl = "http://www.joomlaworks.net/images/demos/galleries/abstract/7.jpg"
var sessionConfog = NSURLSessionConfiguration.defaultSessionConfiguration()
var sessions : NSURLSession = NSURLSession(configuration: sessionConfog, delegate: nil, delegateQueue: nil)
var getImageTask = sessions.downloadTaskWithURL(NSURL(string: imgurl)!, completionHandler: { (location : NSURL!, response : NSURLResponse!, error : NSError?) -> Void in
var image : UIImage = UIImage(data: NSData(contentsOfURL: location)!)!
dispatch_async(dispatch_get_main_queue(), { () -> Void in
self.iamgeview.image = image
let m = self.saveImage(image, path: self.documentsDirectory())
println(m)
})
})
getImageTask.resume()
EDIT:
I just noticed, you are NOT using NSURLSession correctly! In your completion block I see this:
var image : UIImage = UIImage(data: NSData(contentsOfURL: location)!)!
DON'T do this. The NSURLSession download task already downloaded the data for you, you don't have to download it again with the method 'contentsOfURL'. Also that method is synchronous, and blocks the calling thread until the contents of the URL are loaded. You should never use that method to make network URLs.
What you need to do is implement the delegate methods for the download task so you get the data when it completes. An easier way for you is to use dataTaskWithURL:completionHandler - for that message, the completion handler receives the NSData from the response.
END EDIT
You are using NSURLSession correctly, but if you profile your app, most probably you will see that the most expensive operation here, and probably the one that blocks the UI thread, is:
let m = self.saveImage(image, path: self.documentsDirectory())
If you can save your image in a separate serial queue, then you'll see a big improvement.
The client I'm building is using Reactive Cocoa with Octokit and so far it has been going very well. However now I'm at a point where I want to fetch a collection of repositories and am having trouble wrapping my head around doing this the "RAC way"
// fire this when an authenticated client is set
[[RACAbleWithStart([GHDataStore sharedStore], client)
filter:^BOOL (OCTClient *client) {
return client != nil && client.authenticated;
}]
subscribeNext:^(OCTClient *client) {
[[[client fetchUserRepositories] deliverOn:RACScheduler.mainThreadScheduler]
subscribeNext:^(OCTRepository *fetchedRepo) {
NSLog(#" Received new repo: %#",fetchedRepo.name);
}
error:^(NSError *error) {
NSLog(#"Error fetching repos: %#",error.localizedDescription);
}];
} completed:^{
NSLog(#"Completed fetching repos");
}];
I originally assumed that -subscribeNext: would pass an NSArray, but now understand that it sends the message every "next" object returned, which in this case is an OCTRepository.
Now I could do something like this:
NSMutableArray *repos = [NSMutableArray array];
// most of that code above
subscribeNext:^(OCTRepository *fetchedRepo) {
[repos addObject:fetchedRepo];
}
// the rest of the code above
Sure, this works, but it doesn't seem to follow the functional principles that RAC enables. I'm really trying to stick to conventions here. Any light on capabilities of RAC/Octokit are greatly appreciated!
It largely depends on what you want to do with the repositories afterward. It seems like you want to do something once you have all the repositories, so I'll set up an example that does that.
// Watch for the client to change
RAC(self.repositories) = [[[[[RACAbleWithStart([GHDataStore sharedStore], client)
// Ignore clients that aren't authenticated
filter:^ BOOL (OCTClient *client) {
return client != nil && client.authenticated;
}]
// For each client, execute the block. Returns a signal that sends a signal
// to fetch the user repositories whenever a new client comes in. A signal of
// of signals is often used to do some work in response to some other work.
// Often times, you'd want to use `-flattenMap:`, but we're using `-map:` with
// `-switchToLatest` so the resultant signal will only send repositories for
// the most recent client.
map:^(OCTClient *client) {
// -collect will send a single value--an NSArray with all of the values
// that were send on the original signal.
return [[client fetchUserRepositories] collect];
}]
// Switch to the latest signal that was returned from the map block.
switchToLatest]
// Execute a block when an error occurs, but don't alter the values sent on
// the original signal.
doError:^(NSError *error) {
NSLog(#"Error fetching repos: %#",error.localizedDescription);
}]
deliverOn:RACScheduler.mainThreadScheduler];
Now self.repositories will change (and fire a KVO notification) whenever the repositories are updated from the client.
A couple things to note about this:
It's best to avoid subscribeNext: whenever possible. Using it steps outside of the functional paradigm (as do doNext: and doError:, but they're also helpful tools at times). In general, you want to think about how you can transform the signal into something that does what you want.
If you want to chain one or more pieces of work together, you often want to use flattenMap:. More generally, you want to start thinking about signals of signals--signals that send other signals that represent the other work.
You often want to wait as long as possible to move work back to the main thread.
When thinking through a problem, it's sometimes valuable to start by writing out each individual signal to think about a) what you have, b) what you want, and c) how to get from one to the other.
EDIT: Updated to address #JustinSpahrSummers' comment below.
There is a -collect operator that should do exactly what you're looking for.
// Collect all receiver's `next`s into a NSArray. nil values will be converted
// to NSNull.
//
// This corresponds to the `ToArray` method in Rx.
//
// Returns a signal which sends a single NSArray when the receiver completes
// successfully.
- (RACSignal *)collect;
The problem: I'm crashing when I want to render my incoming data which was retrieved asynchronously.
The app starts and displays some dialog boxes using XAML. Once the user fills in their data and clicks the login button, the XAML class has in instance of a worker class that does the HTTP stuff for me (asynchronously using IXMLHTTPRequest2). When the app has successfully logged in to the web server, my .then() block fires and I make a callback to my main xaml class to do some rendering of the assets.
I am always getting crashes in the delegate though (the main XAML class), which leads me to believe that I cannot use this approach (pure virtual class and callbacks) to update my UI. I think I am inadvertently trying to do something illegal from an incorrect thread which is a byproduct of the async calls.
Is there a better or different way that I should be notifying the main XAML class that it is time for it to update it's UI? I am coming from an iOS world where I could use NotificationCenter.
Now, I saw that Microsoft has it's own Delegate type of thing here: http://msdn.microsoft.com/en-us/library/windows/apps/hh755798.aspx
Do you think that if I used this approach instead of my own callbacks that it would no longer crash?
Let me know if you need more clarification or what not.
Here is the jist of the code:
public interface class ISmileServiceEvents
{
public: // required methods
virtual void UpdateUI(bool isValid) abstract;
};
// In main XAML.cpp which inherits from an ISmileServiceEvents
void buttonClick(...){
_myUser->LoginAndGetAssets(txtEmail->Text, txtPass->Password);
}
void UpdateUI(String^ data) // implements ISmileServiceEvents
{
// This is where I would render my assets if I could.
// Cannot legally do much here. Always crashes.
// Follow the rest of the code to get here.
}
// In MyUser.cpp
void LoginAndGetAssets(String^ email, String^ password){
Uri^ uri = ref new URI(MY_SERVER + "login.json");
String^ inJSON = "some json input data here"; // serialized email and password with other data
// make the HTTP request to login, then notify XAML that it has data to render.
_myService->HTTPPostAsync(uri, json).then([](String^ outputJson){
String^ assets = MyParser::Parse(outputJSON);
// The Login has returned and we have our json output data
if(_delegate)
{
_delegate->UpdateUI(assets);
}
});
}
// In MyService.cpp
task<String^> MyService::HTTPPostAsync(Uri^ uri, String^ json)
{
return _httpRequest.PostAsync(uri,
json->Data(),
_cancellationTokenSource.get_token()).then([this](task<std::wstring> response)
{
try
{
if(_httpRequest.GetStatusCode() != 200) SM_LOG_WARNING("Status code=", _httpRequest.GetStatusCode());
String^ j = ref new String(response.get().c_str());
return j;
}
catch (Exception^ ex) .......;
return ref new String(L"");
}, task_continuation_context::use_current());
}
Edit: BTW, the error I get when I go to update the UI is:
"An invalid parameter was passed to a function that considers invalid parameters fatal."
In this case I am just trying to execute in my callback is
txtBox->Text = data;
It appears you are updating the UI thread from the wrong context. You can use task_continuation_context::use_arbitrary() to allow you to update the UI. See the "Controlling the Execution Thread" example in this document (the discussion of marshaling is at the bottom).
So, it turns out that when you have a continuation, if you don't specify a context after the lambda function, that it defaults to use_arbitrary(). This is in contradiction to what I learned in an MS video.
However by adding use_currrent() to all of the .then blocks that have anything to do with the GUI, my error goes away and everything is able to render properly.
My GUI calls a service which generates some tasks and then calls to an HTTP class that does asynchronous stuff too. Way back in the HTTP classes I use use_arbitrary() so that it can run on secondary threads. This works fine. Just be sure to use use_current() on anything that has to do with the GUI.
Now that you have my answer, if you look at the original code you will see that it already contains use_current(). This is true, but I left out a wrapping function for simplicity of the example. That is where I needed to add use_current().
everybody.
I want to understand, how i shoud procceed situations when an asynchronous method has "didFinish:#selector(SEL)" parameter.
My code example is:
//
// Authentication check
- ( void )authenticationSuccess: ( GDataServiceTicket* ) ticket
authenticatedWithError: ( NSError* ) error {
if ( error == nil )
{
NSLog( #"authentication success" );
}
else
{
NSLog( #"authentication error" );
}
}
//
- ( void ) fetchFeedOfSpreadsheets {
//create and authenticate to a google spreadsheet service
if ( !(mService) )
{
GDataServiceGoogleSpreadsheet *service = [self spreadsheetService];
[mService autorelease];
mService = [service retain];
}
// check autentication success ( invoke "authenticationSuccess" method for debug success & error )
[mService authenticateWithDelegate: self
didAuthenticateSelector:#selector(authenticationSuccess:
authenticatedWithError:) ];
// HERE I WANT TO MAKE A PAUSE AND WHAIT THE RESULT, EITHER I AUTHENTICATED OR NOT
// AND MAKE AN "IF" STATEMENT TO CONTINTUE WORKING ON SERVER, OR RETURN ERROR
//fetch retrieves the feed of spreadsheets entries
NSURL *feedURL = [ NSURL URLWithString: kGDataGoogleSpreadsheetsPrivateFullFeed ];
GDataServiceTicket *ticket;
ticket = [mService fetchFeedWithURL: feedURL
delegate: self
didFinishSelector: #selector(spreadsheetsTicket:finishedWithFeed:
error: ) ];
// HERE I WANT TO WAIT SECOND TIME. I WANT "spreadsheetsTicket:
// finishedWithFeed:error:" TO PROCCEED ERROR AND PUT A FEED IN SOME NSARRAY OBJECT
// AND AFTER THAT I WANT TO WORK WITH THAT NSARRAY RIGHT HERE
}
I's clear, that i can push the code i want into the end of "authenticationSuccess" method section, but it's also clear, that it's a wrong a way to solve the proble. There a number of situations like this, where i call an asynchronous method with a selector parameter, and i want to find a solution providing me a flexible code writing.
Thanks in advance.
It's a standard practice in Objective-C to put the code to be executed after the authentication in the authenticationSucess: method. You might not like it, but that is life.
Many people had the same complaint as you, so
on iOS 4 and later, there's something called blocks which allow you to write the code to be executed after the authentication in the method which initiates the authentication, as in
[mService authenticateAndExecute:^{
code to be executed when successfully authenticated ;
} whenError:^{
code to be executed when authentication failed;
} ];
But in this case you need to modify the API, which is possible by using categories. See this blog post by Mike Ash. He has many other posts on blocks on the same blog, which are also very instructive.
If you're going to use a library that works asynchronously (and therefore doesn't block your UI), you should have a good reason for trying to force it to work synchronously.
You should be checking for an authentication error at the end of your authenticationSuccess:authenticatedWithError: method, and calling the next request from there if there's a success. Similarly, in your spreadsheetsTicket:finishedWithFeed:error: check for an error, and continuing processing if there isn't one. It might be a better design to do that continued work in a separate method, but that's up to you.
Is there a specific reason you want to use the GData API in a synchronous fashion?