I'm wondering how to make a request in a background thread, what I mean by that is the fetching in a background thread and then come back to the main thread
Currently I can go the main thread after the request by calling:
.observe(on: UIScheduler())
But I didn't succeeded to make the request in another thread, you think it's a good idea to make it in another thread ?
Assuming you have a SignalProducer, you can use the start(on:) operator so that it's start() method is run on a different scheduler. So you could do producer.start(on: QueueScheduler()) to ensure it is started asynchronously on a GCD queue.
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 am a bit confused between the concept of asynchronous loading and main thread. When something is loaded asynchronously, does it mean that it is not run on the main thread? As far as I know this is two different concept, something can be run on the main thread asynchronously and something can also be run on the background/secondary thread asynchronously. Correct me if I am wrong.
Not quite. Running asynchronously means that it doesn't stop execution from continuing on the current thread. This is an important difference because it's totally possible to do something asynchronously such that it ends up on the main thread (for example: dispatch_async(dispatch_get_main_queue(), ^{ some block });), which would mean that it's not stopping some other thread from continuing, but is blocking the main thread.
Because the main thread is so important to applications, the most common use of asynchronous code is to avoid blocking it, but it's not the only use.
(edited to add)
It's more useful, perhaps, to think of it in terms of "async with respect to x". If you do this, for example:
dispatch_async(aSerialQueue, ^{
work();
work();
});
Then the two invocations of work are synchronous with respect to each other, and synchronous with respect to all other work on aSerialQueue, but asynchronous with respect to everything else.
Running something asynchronously simply means that the call may return before the task is complete. This may be because it's running on a background thread. It may be because it will happen later on the current thread.
Running something synchronously doesn't mean it will happen on the main thread; it means that the call won't return until the task is complete. That may be because it happens on the same thread. It may be because it happens on another thread, but you want the current thread to wait for the other thread to complete.
Further, note that there is a difference between the main thread and the current thread. A synchronous task called on a background thread will keep that background thread from moving on until the task is complete, but won't block the main thread at all.
Of course, most of the time in iOS, your code will be running on the main thread. But you can push work onto background threads (e.g. with the dispatch APIs), so the distinction is important.
The main thread is where the bulk of your code gets executed, including all of the UI. There are other threads as well, like a web thread and a networking thread, and you can create your own threads.
Synchronous vs Asynchronous is a matter of when your code will be executed. You can perform a sync or async block of code on the main thread, or any other thread. If it's in sync, it will block the rest of the thread until it finishes. If it is async, it will run whenever the thread frees up from whatever it is currently performing.
I´m getting this error using NSLock which I tried to circumvent by using unlockWithCondition (using NSConditionLock), but regardless I get the same result:
* Break on _NSLockError() to debug.
* -[NSLock unlock]: lock ( '(null)') unlocked from thread which did not lock it.
I´m not sure if it´s bad, but what I´m doing is this:
new Thread:
[lockA lock];//waiting unlock
[lockB lock];//waiting unlock
..shared code..
[lockA unlock];
[lockB unlock];
in Main Thread:
//Do two HTTP request.
//when request respond, I unlock the locks in respective threads with [lockA unlock];
[lockB unlock];
So the section "..shared code.." can execute. I don´t understand why i´m getting this error.
Can anyone explain what I´m doing wrong? It´s look like it should work perfectly.
I think you're trying to use locks as semaphores here. Locks are meant to stop the background thread and the main thread from accessing something simultaneously. Hence, the thread holding the lock must release (unlock) it too.
If you want the background thread to wait for something to happen on the main thread, use semaphores.
Use GCD semaphores for nice and easy semaphores: https://developer.apple.com/library/ios/#documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html
If you are doing your HTTP requests on NSURLConnection or similar and trying to unlock in the delegate, you need to be careful where you create and initiate the NSURLConnection from, as it should return to that thread, unless you explicitly use scheduleInRunLoop:mode to put it on another thread or run loop.
If you are certain that you're locking on the main thread, you should be unlocking on that thread. To get back there, you can use either performSelectorOnMainThread:withObject:waitUntilDone: callback or GCD to call back on the main thread using:
dispatch_async(dispatch_get_main_queue(), ^(void) {
...
});
With your unlocks in the ... space. You could use dispatch_sync() instead if you need to know the unlocks have completed before moving on.
However, using NSConditionLock, as you'd indicated you'd already tried is the solution. However, you still need to do the lock on the retrieval thread, not on the main thread. The condition will be guarded by your -unlockWithCondition: using a specific condition, so it won't unlock prior to the retrieval thread marking it as ready.
So, in your main thread, launch the retrieval threads. In each retrieval thread, -lock and then proceed to retrieve the data, then -unlockWithContidition:. In the consumer thread, use -lockWhenCondition and you should be fine.
The key is, though, that you have to lock and unlock on the same thread.
I have two threads running one id UI and other is worker thread. Worker thread continuously reads on port for some data from server, when appears I need to update my webview in UI thread. Worker thread again continues to read and never ends. Plz suggest how to accomplish this??
it may be likely as 'signals in C++' which causes a method in called thread to be invoked..!!
I tried: 1)As running worker thread on UI (runOnUIThread)may degrade UI webview performance, and if I put the thread on sleep it may miss data appeared at port when sleeping..(i m not sure!)
2)using Handler, I hav to specify time before calling thread again n again which may cause to miss data if appeared on port as like sleep().
3)Having a separate thread in same class,it gives: any other thread cant update view of main UI thread.
Plz help.. :(
Maybe I am missing something, but it seems like creating a Handler object and then using the post() method is what you need: http://developer.android.com/reference/android/os/Handler.html#post%28java.lang.Runnable%29
I had a similar problem. I have a UI thread and a few threads that do backend serverices. I use the observer Pattern to have the services update the UI and to do so I have the update method in a new thread and call runOnUiThread() to have it update the UI. It seems to work well for something quick. If you want something better than a quick band-aid I would suggest reading this android threading guide from Google and trying to incorporate it with your design.
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.