I'm using NSURLConnection in an iPhone application and the interface seems to slow down after sending initWithRequest: to my NSURLConnection instance. I'm not sure if this is occurring because my processing code is taking a long time to handle the response, or if it's because NSURLConnection is blocking the main thread.
Can anyone confirm that NSURLConnection will create the connection and wait for data on a separate thread, and then call its delegate methods on the main thread?
Thanks!
NSURLConnection supports two modes of operation: asynchronous and synchronous. Neither uses separate threads at all. They both use just one thread, that being whatever thread you run them in.
In synchronous mode, NSURLConnection will block whatever thread you run it in. Asynchronous mode uses the run loop to behave (from the developer's perspective) similarly to a background thread but with lower overhead and without any thread-safety issues. If using asynchronous mode, you want to run it in the main thread. It won't block anything.
If your interface is slowing down, that is not consistent with using NSURLConnection synchronously, which would instead cause your interface to stop completely until the request is complete.
If you follow apples example on NSURLConnection the call will be handled in a different thread than the main thread.
Related
What is synchronous and asynchronous in ios ? I am new in objective c. Which one i should use in my code while i am getting data from server. So please help me.
Thanks in advance.
You should always use asynchronous loading of network requests.
Asynchronous never block the main thread waiting for a network response.
Asynchronous can be either synchronous on a separate thread, or scheduled in the run loop of any thread.
Synchronous blocks main thread until they complete request.
For Demo code or turorial have a look into this link Asynchronous web service client using NSURLConnection and SBJSON
The majority of the time you will go for asynchronous calls for that kind of operations, otherwise you're UI will block because you are using the main thread.
Synchronous, as the name suggests the action will happen in synchronous with the run loop of your application.
To understand it better, say you have to display some data in UITableview after fetching the data from server.Imagine that the request and response from server takes like 3 seconds. When you are fetching this data synchronously from the server, your app will freeze for like 3 seconds between loading tableview and loading the data contents into that tableview
Now if you are sending your request asynchronously, your app won't freeze but it will load the tableview and tableview contents before the server can respond. In other words, your app won't wait for the 3 second of server response time.You have to take necessary delegate actions or blocks actions to check the response and reload the tabledata so that the server response is displayed in tableview.
Which method is better is pure choice what the developer wants and their app should behave but Apple documentation recommends if you are using synchronous calls do not initiate the call from current run loop.
Using asynchronous all threads are execute the operations parallel. So, Never block the main thread waiting for a network response.
Using synchronous all threads are execute the operations one by one. so, should wait until the other thread task done.
Hope It will be suitable.
Quick note based on other answers: dispatch_sync will not block the main thread unless you dispatch to the main thread.
Example:
// Block main thread because the main queue is on it.
dispatch_sync(dispatch_get_main_queue(), ^{ /*do stuff*/ });
// Block background thread.
dispatch_sync(my_work_queue, ^{ /*do stuff*/ });
A Synchronous call(blocking) is one that has to be completed before subsequent calls can be run in the same queue. It is given all of the processor time for that queue until it is complete. This makes it block the queue.
Asynchronous calls can be started in a queue, and then left running on another thread(processor time schedule), owned by that queue, while other calls are started with other threads.
It is very important to use dispatch_async for web calls because it may take time to get a result back and you want other tasks to be able to start in the queue and use it's threads. A common practice is to do web work, like downloading a file, on a custom background queue and then dispatch to the main queue when it is complete, to update the user.
There is more to this and you can read about dispatch queues from Apple, here.
I have a UISearchBar and UISearchDisplayController. When the user writes text in it inside searchBar:textDidChange: I make a web-service call to filter my TableView. The problem is that the GUI get unresponsive until I get the result from the web-service. I've tried to solve it using [self performSelector:#selector(callWebService:) withObject:searchText];, but it's still unresponsive.
EDIT: Following Flink advice, I changed performSelector to performSelectorInBackground, but now the tableView doesn't filter correctly, it only show 'No Results'.
even tableView:numberOfRowsInSection: isn't get called.
EDIT Again: The reason I got 'No Results' was due to not calling reloadData on the correct tableView. UISearchDisplayController has a property named searchResultsTableView. So in the end what I used was [self.searchDisplayController.searchResultsTableView performSelectorOnMainThread:#selector(reloadData) withObject:nil waitUntilDone:false]; and now it works fine.
It should be noted that although I chose the performSelectorInBackground, I probably should have tried to use sendAsynchronousRequest method on NSURLConnection - See AliSoftware's answer.
You need to make your web call async.
http://www.raywenderlich.com/4295/multithreading-and-grand-central-dispatch-on-ios-for-beginners-tutorial
In your case, you can change performSelector to performSelectorInBackground
You should avoid creating a background queue or thread to perform your network request(which is what performSelectorInBackground: does) as this creates a worker thread just for this which is not as efficient as scheduling the request on the NSRunLoop.
Dedication a thread will make the processor activate the thread regularly to check if there are some data, and creating a thread for that is quite overkill. Scheduling the request on the run loop (as a run loop source) will use network interruptions to signal incoming data from the socket and thus will only be activated when there is actual data available.
To do this, simply use the asynchronous methods provided by NSURLConnection.
One solution is to use the delegate approach provided by NSURLConnection (this is the old way to do it, the only way that was available in the NSURLConnection API back in iOs3)
Another more modern solution is to use the block API provided by NSURLConnection which is easier to use and code.
[NSURLConnection sendAsynchronousRequest:request
queue:[NSOperationQueue mainQueue]
completionHandler:^(NSURLResponse* response, NSData* receivedData, NSError* error)
{
// Your code to execute when the network request has completed
// and returned a response (or timed out or encountered an error)
// This code will execute asynchronously only once the whole data is available
// (the rest of the code in the main thread won't be blocked waiting for it)
}];
// After this line of code, the request is executed in the background (scheduled on the run loop)
// and the rest of the code will continue: the main thread will not be frozen during the request.
Read more in the URL Loading System Programming Guide and in the NSURLConnection class reference.
I am new to iPhone development and I would like to ask a question concerning asynchronous events.
Supposing I have a NSURLConnection and the correspoding delegate methods ie. didReceiveResponse, didFailWithError etc. The methods are called asynchronously when events are fired. Are all methods executed on the main thread? Or does the iOS create separate threads that execute the corresponding method code?
I am facing some random crashes to my app, and I guess that it is a synchronization issue.
The delegate methods of NSURLConnection are executed in the main thread. That is the whole reason behind being asynchronous, no need to have a separate thread.
About the internals, I/O is an inherently asynchronous world, so I also do not think that internally NSURLConnection uses threading. I suppose that it is the OS that manages the communication in a async way, but I am not sure about it.
Anyway, you can be sure that your delegate methods are executed from within the main thread.
Suppose I have multiple NSOperation objects attached to a concurrent queue.
Within these NSOperations, I would call a synchronous method of NSURLConnectionClass, sendSynchronousRequest ... just to not mess up my code with tracing different connections from within a single delegate.
Apple says that sendSynchronousRequest ... is going to automatically create a separate thread with a run loop to trace NSURLConnection delegate messages.
But I already have several additional threads (running inside NSOperation)! So the question is: if I have, say, 10 NSOperation objects and each would call the synchronous method of NSURLConnection, will it produce 10 more additional ("automatically created") threads with run loops or there will be only one for all of them?
Don't worry about the thread that NSURLConnection creates. It is some internal detail. I'm pretty sure it is one global thread shared by all NSURLConnection instances.
For performance reasons, I instantiate a dedicated NSThread to process incoming messages that are streamed from a network server. I use an NSOperation for the purpose of instantiating the connection and receiving incoming data through the NSURLConnection delegates, but as soon as new data comes in and gets parsed, I offload the processing of the message to the dedicated NSThread. The idea is to let one thread focus on receiving incoming messages and let the other thread just do the processing.
What's the proper way to shut down the NSThread when the applicationDidEnterBackground comes in?
Also, how should I restart the NSThread when applicationWillEnterForeground comes in?
Other than the main thread, it seems the state of other background threads is not maintained between going to sleep and restarting.
By the way, I'm all for using NSOperations for most tasks that have a measurable amount of work -- ie, accessing a resource over the network, performing a calculation, etc. However, in this case, I need to process messages on the fly on a long-living dedicated thread that is always there by calling performSelector:onThread:withObject:waitUntilDone: and passing it the target thread. It seems NSOperation isn't a good fit for this.
I would appreciate your input.
"For performance reasons"?
If processing doesn't take much time, run everything (including NSURLConnection) on the main thread. Concurrency bugs are a major pain.
If you want things to run serially, you can emulate a "single thread" with an NSOperationQueue with maxConcurrentOperations = 1. I'm pretty sure that NSOperationQueue uses thread pools (and on 4.0, GCD, which probably uses thread pools), which means you don't need to keep a thread running all the time.
Apart from that, your process is automatically suspended and resumed by the system, so you don't need to kill off threads.
I'm not sure what you mean by "the state of other background threads".