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I wnt to call to my webservice once every 45sec. Actually what I want to do is, user start to play some music within my application if user playing a song, every 45sec app should send a call to web service saying "User playing a song" likewise. I dont have any idea about iOS threads. So can anybody tell me how can I do this? Do I need to do threads or are there any other way to do this?
Thankyou
Here's some code to help you.
Set a NSTimer somewhere, like in viewDidLoad, to run every 45secs, performing your web service call:
[NSTimer scheduledTimerWithTimeInterval:45.0
target:self
selector:#selector(callService)
userInfo:nil
repeats:YES];
And then implement the selector used in the timer, running some code in background, as you want:
-(void)callService
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//Code in this part is run on a background thread, call your service here
});
}
You need to perform this action on background thread. So it wont affect the UI. User can perform any other action. For that you may want to use an NSTimer and performSelectorInBackground: withObject:. Using an NSTimer You can repeatedly call a method. From that call the web service method in background. GCD also a better option.
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Can we create multiple main thread in single application?
Why all UI changes made in main thread?
I have faced these question in interviews. I am unable to find any solution. Can any one help me to find the answers of this question? That will help me in future interviews and implementations in iOS app.
Thanks in advance!
There is only ever one main thread in your application, however tasks from multiple dispatch queues may execute on this thread. Tasks that are dispatched on the main queue are guaranteed to run on the main thread.
Generally with GCD you do not think about threads, but rather queues which are abstracted from the underlying threads. They provide a level of abstraction that makes it easier to manage tasks without being concerned about the number or state of the threads that are being used.
THe autolayout engine performs a number of calculations and applies rules in order to arrive at a layout solution. If the properties of a UI element change then the autolayout solution is invalidated and needs to be recalculated.
As the main queue is a serial dispatch queue, dispatching UI updates onto the main queue ensures that either the autolayout calculations are being performed or a UI property is being updated but not both at the same time (of course there are more than just these two activities happening on the main queue, but we are just addressing autolayout here)
No, there is only one main thread for the application that is created by system for application. Main thread is the thread that directly gives changes to user. Thread which is directly connect with user you can said.
Check the Apple Documentation, It states,
The main queue is automatically created by the system and associated with your application’s main thread. Your application uses one (and only one) of the following three approaches to invoke blocks submitted to the main queue:
Calling dispatch_main
Calling UIApplicationMain (iOS) or NSApplicationMain (OS X)
Using a CFRunLoopRef on the main thread
As with the global concurrent queues, calls to dispatch_suspend,
dispatch_resume, dispatch_set_context, and the like have no effect
when used with queues returned by this function.
Now, let's take an example, If you are downloading an image and want to display it to user. Now if you will download it on main thread then it will block other UI till it completed download. So, you should not download it on main thread or synchronously . you should download it asynchronously and when download is completed, you should display it to user on main thread, so it will quick displayed to user after download is completed.
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Is it ok to wait for an asynchronous request (say an NSURLSessionDataTask,) using a semaphore for example.
There is a lot of advice out there claiming that synchronous networking is bad, however I don't see that as grounds for refusing synchronous requests, especially when they're done in a background thread.
Synchronous requests (in a background thread/queue) have the benefit of not needing to be callback-nested.
The answer in https://stackoverflow.com/a/31563134/466604 shows only a method to achieve this, but it comes with implied discouragement. (Along with other discouragement from https://devforums.apple.com/thread/9606?tstart=0)
My question is whether discouraging this discouragement is warranted, given those synchronous requests happen in a background thread, primarily as a way to avoid nesting callbacks.
No, it's not ok. Don't do this.
iOS might even kill your app in the process. Besides, look at callbacks as they were the next step in your serial execution.
The processor needs those cycles to do other things, don't be a bully!
INSIGHT:
As #EricD said, if your execution fails for some reason, that thread is lost until you exit your app and the O.S. claims it back.
That thread is a valuable resource that must not be wasted and much
more valuable in Mobile Operating Systems as iOS.
iPhone's processors are not as powerful as nowadays Desktop' processors or GPU's processors that can build up to hundreds of thousands threads.
If you use a semaphore, you are waking up to ask that thread after some time, and if the conditions has not being met, then you go to sleep again. This is usually in the ms time, but still, is some time that the processor dedicate to something useless.
MORE CLEAR
When you define a callback, is as if you were to sleep and you say to somebody.
Don't call me, I will call you when I'm ready to go.
Instead, using a semaphore is like:
Is ok if you call me 1000..0 times to ask me if I'm ready to go
The overheading in this case is obvious...you usually needs a time to wake up and to sleep back again. That's wasted processor time
Keep in mind that in the best case you are using a semaphore that put the Thread into sleep for a while, and then ask again if it is OK to proceed.
If this is not the case, then this thread is using ALL the processor cycles assigned to it to ask you if it's ok to proceed, that is 100% CPU Usage in that Thread evaluating just a condition.
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Tell me if this make sense. This is an iOS question.
I'm seeing code where it is already in the main thread, but the code would dispatch_async all sorts of UI code to the main thread's queue. Layouts, animations etc etc.
I was told this some how speeds up responsiveness (example, when pushing a view controller, you would dispatch other UI ops there so it won't block the push transition.
This doesn't make sense because first it is dangerous, second, it doesn't guarantee when the UI code gets run(even though it will probably run with milliseconds). The only good reason I can see is for it to guarantee that the UI code is not accidentally run in a different thread.
What do you guys think?
There are definitely times you employ this pattern of dispatching back to the main queue, so I might not be too quick to dismiss it or label it as "dangerous" (though the way you characterize it, it does sound suspicious). You should share some code samples of how you're seeing this pattern being used, and we can comment further.
When would you dispatch to the main queue? The prototypical example is when you're doing something on a background queue, but then want to dispatch the UI updates back to the main queue, e.g.:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// do something time consuming here, like network request or the like
// when done, update the UI on the main queue:
dispatch_async(dispatch_get_main_queue(), ^{
// update the UI here
});
});
But I'm assuming this is not the sort of dispatch to the main queue you're talking about. I'm assuming from your comment where you have something on the main queue dispatching asynchronously right back to the main queue itself.
The reason you would do that is if you don't want the dispatched code to run immediately, but rather to be queued for the next iteration of the run loop. This lets the autorelease pool drain, lets the current method complete (and release any resources it might have been using), lets other dispatched tasks run first, gives the UI a chance to reflect any changes you may have initiated, etc.
A few examples of when some developers might do this include:
You might use this pattern if you want a method call itself recursively, but you want to yield back to the run loop, to let resources be freed, let the UI reflect any changes, etc. You're basically saying "ok, let this method finish, but in the next run loop, run this method again."
A little more suspect, I've seen this pattern in viewDidLoad where you want to give auto layout a chance to "catch up" and update the frames. E.g. there is a common third-party progress indicator that won't work if you just invoke it from viewDidLoad, but it works if you dispatch that update back to the main queue.
Having articulated reasons why I've seen developers dispatch back to the main queue from the main queue, itself, I must confess that many of these patterns suffer from code smell and are often better accomplished via different patterns. But these are a few examples that I've seen.
But, again, if you want assistance on your particular code sample, you have to share it with us. We can't tell what the developer intended without seeing the code sample.
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is it possible to call an action that contain heavy operations several times? like save button (I save the first object) and want to add others, but the app becomes so slow and I can't navigate to other views
You can always play around with heavy data saving related tasks in the background. You can distribute the content related stuff to various threads (queues), you need to separate the stuff which you are saving and if it is blocking your UI and making your app slow then you need to perform these heavy operations in the background.
Make sure, you do NOT perform any UI updation operation in the background. Try reading about GCD (how it works), how you can create a background Queue etc and how you can play around with it. I assume you might be using core data in order to save the contents on save button. Try reading about the Parent/Child Manage Context Objects. Play around with it a bit and move your heavy task to background, updation of UI will always be on Main or UI thread (which you can always do it by calling get_main_queue() if you are working with dispatch queues). Happy Coding. If you have specific code, which is doing this, then let us know. We will be glad to help. :)
You need to make sure that you aren't tying up the main UI thread of your application. A potential easy fix is to do your save operation on a different thread. Here is an example of how to do just that using Grand Central Dispatch: iPhone - Grand Central Dispatch main thread
Example:
//notice this saveQueue is a new dispatch queue that's been created.
dispatch_queue_t saveQueue = dispatch_queue_create("com.mycompany.myqueue", 0);
dispatch_async(saveQueue, ^{
//this command represents your long running operation
doSaveOperation();
dispatch_async(dispatch_get_main_queue(), ^{
//always update your UI on the main thread!
[self showCompleteMessage];
});
});
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Closed 10 years ago.
Possible Duplicate:
Understanding NSRunLoop
Till now I know that every thread has its own runloop I don't know whether I'm thinking right.
I don't have a brief idea about nsrunloop but I'd like to know what is nsrunloop and what it's purpose and why all the thread has its own runloop(if I'm right). Please help me to let me know about nsrunloop.
a runloop is basically an extended while loop that works like C select call.
it is responsible for getting events from its sources and dispatching those.
mouse clicks, window moves, timers, stream events, ...... anything can be a runloop source.
thats the gist. read more about it in the countless dupes this has on SO or in the apple docs :)