When I start a 2nd background thread and pause the main thread, will my First Responder still be in action? For example I have an overwriting method called -flagsChanged and was wondering if it would still be active if the main thread is offline.
Thanks,
Kevin
Don’t pause the main thread since the main thread is responsible for handling events and your application UI will become irresponsive. If the main thread is paused, it won’t handle events, hence it won’t dispatch key events to the first responder.
If you think you need to pause the main thread, you probably need to redesign your program so that the behaviour that requires sleeping (if it does require sleeping) is offset to a secondary thread. If you need to update the user interface from a secondary thread, you should use -performSelectorOnMainThread:withObject:waitUntilDone:.
Related
i noticed that when i hold UIPicker on selection, than all my timers pauses.
I have global timer like clock which is executed every second. However when i hold picker it's pauses all timers in app stops.
Maybe some one had this problem and have a solution ?
Thank you
If your timer should be running regardless of what happens in your UI, you should be running it on a background thread.
Of course, the timer creates its own background process to do the counting. But if you created it on the main thread then maybe the callback selector would be blocked if the UI is blocked, as is the case when holding a UIPickerView.
Just start your timer on a global queue and message the UI on the main queue.
I have a a few threads that needs to run constantly or be locked at my disposal. When I lock the phone or swap applications the thread seems to halt until the application is back in focus.
I have a class Worker that is a sub class of NSThread. There is a method called start which is called by the firstViewController that creates the Worker object.
//method start
[self performSelectorInBackground:#selector(run) withObject:self];
What do I need to do to make my thread run all the time, rather than only running while in focus?
Thanks :)
When your app enters the background, all threads as suspended - unless you've configured your application to use multi-tasking, and your work is being done using the multi-tasking methods. This is detailed at Apple developer.
In short, you basically can't have a thread running constantly in the background on iOS if you want to be accepted in the App store, unless you're a navigation or VOIP application. You can have a thread continue to run for around 10 minutes after you enter the background, but that's it.
I am not clear about what does waitUntilDone do, but I found this thread:
What is the significance of WaitUntilDOne in performSelectorOnMainThread?
which makes me a bit clear, however, if I perform some selector which makes NSURLConnection(which is asyncrhonous) and waitUntilDone set to YES, what will happen then? It will wait for the method to execute, but the method actually does some asynchronous operation(ie NSURLConnection), then what is the impact?
Thanks!
NSURLConnection is asynchronous. Your code runs on the main thread, and it makes delegate calls to you as the download progresses. You don't need to, and should not, run an NSURLConnection from a background thread.
If you DO have code that needs to run on a background thread, you can use the preformSelectorOnMainThread method to send messages from your worker thread to the main thread. One common reason to do this is that you can't update the UI from a background thread. You'd invoke a method to update the UI on the main thread.
The flag waitUntilDone controls what happens after the performSelectorOnMainThread call. If waitUntilDone is false, your background thread continues on with the next line without waiting for the code on the main thread to finish.
If waitUntilDone is true, your background thread will block until the main thread finishes performing the selector that you sent it.
I have a class containing a method with a loop. I need to be able to break the loop if a certain event (e.g. button press) occurs.
I am using the NSNotificationCenter to notify the class containing the loop when the button is pressed.
However, If I press the button while the loop is being executed, the notification occurs after the loop is complete instead of interrupting the loop.
I'm guessing this is because it is operating in the same thread.
So how do I get the NSNotificationCenter operating in a background / different thread? Is this possible? Or is there a better way to do it?
It's not just the notification center.
I have a class containing a method with a loop. I need to be able to break the loop if a certain event (e.g. button press) occurs.
The events for that button press come in on the main thread. If your loop is running on the main thread, then the button press itself does not get processed until your loop is finished. The notification is posted immediately, relative to the button press actually getting processed by your application.
Or, in list form:
The user presses the button.
Your loop runs out of things to do and returns.
The button press arrives in your application and is turned by the button into an action message.
You post the notification.
You receive the notification.
The delay that you're seeing is between steps 1 and 2; step 4 happens immediately after step 3.
Notifications on a local (not distributed) NSNotificationCenter are dispatched on the thread you post them from, so posting it from your action method means that it will be dispatched on the main thread. This is normal and OK.
Move the loop, not the notification, to a background thread, dispatch queue, or operation queue. If you use an operation queue, you may not need the notification at all, as you can tell an operation queue to cancel all pending operations. (Your operations will need to check at any appropriate time(s) whether they have been canceled; for reasons previously discussed, killing a thread/operation at a random time is a Bad Idea.)
Background threads, blocks, and operations can communicate back to the main thread when needed (e.g., to update the UI). To send a message through the main thread's run loop, use performSelectorOnMainThread:withObject:waitUntilDone:. To dispatch a block on the main thread, use dispatch_async and dispatch_get_main_queue. To schedule an operation on the main thread, add it to [NSOperationQueue mainQueue].
For more info, read the Concurrency Programming Guide and the Notification Programming Topics.
I would run your loop in a separate thread, and have an instance variable BOOL abort;, when your button press notification comes in, set abort = TRUE; then in the loop check this value and exit if it is true.
I would run the loop in a separate thread. Even better, make it an NSOperation so that you can call [.. cancel]. Just make sure to use performSelectorOnMainThread when updating the UI from the NSOperation object. It's not a good idea to have a long running loop on the main thread.
You can't put the notification center on another thread. That object is out of your control. The problem isn't so much that they are on the same thread as that you are not allowing the run loop, which is responsible for handling the button press, to do anything. (There's one and only one run loop per thread.) As has been stated by both edsko and Peter Hosey, the button press itself, and in fact your entire UI, is stopped while your loop is running. It is generally a good idea to put long-running operations onto a background thread, then call back to the main thread to update the UI, performSelectorOnMainThread:withObject:waitUntilDone: being an easy way to do such a call back.
That said, if you were to keep the loop on the main thread, you need to let control return to the run loop periodically so that the button press will be registered. There are two ways I can think of to do this. First, you can explicitly give the run loop control briefly during each iteration of your loop:
while( !buttonWasPressed ){
// Do work...
// Let the run loop do some processing before the next iteration.
[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.01]];
}
Or, you can make a single method that consists only of the code from your loop, and use performSelector:withObject:afterDelay: to have the method repeatedly called while still allowing the run loop to work:
- (void) loopTheLoop {
if( buttonWasPressed ) return;
// Do work...
// Run this method again as soon as possible.
[self performSelector:#selector(loopTheLoop)
withObject:nil
afterDelay:0.0];
}
I have a window that displays some data in an NSTableView. This data is loaded in the background. The data-loading-thread is started in the windowDidLoad: method. If the window is closed before loading has finished, the background thread should be cancelled. I do this by signalling the thread in the windowWillClose: delegate method and waiting for the background thread to finish.
Now this all works perfectly. But I have one problem: How can I update the data in the table view? I have tried calling reloadData via performSelectorOnMainThread: but this leads to a race condition: The reloadData call is sometimes queued on the main thread after the window close command, and will execute after the window has closed, and everything goes up in flames.
What's the best way to control and communicate with a background thread?
Well, you know, this is exactly what makes the use of threading complex: you always face synchronization issues.
What I suggest is, instead of calling [tableView reloadData] from your thread, simply signal your controller (by calling a method controllerShouldReloadTable) and let your controller do the check if windowWillClose has been called or not. There might be a chance that your controller has been also released by the time controllerShouldReloadTable, and to fix this you will definitely need to retain the controller from the secondary thread.
On a side note, I would cancel the thread in viewDidUnload (for symmetry).
Most important: I would use asynchronous calls and a delegate class so that the whole multithreading issue is solved at its root.
EDIT: Sending asynchronously a request will not block the sending thread waiting for the response. Instead, asynchronous send (for NSURLConnection is called start) immediately returns (so, no blocking) and when the response is received, a delegate method will be called (i.e., connectionDidFinishLoading:) so that you can updated the model and the UI. Take a look at NSURLConnection docs, but as usual, I strongly suggest using [ASIHTTPRequest][2], which has many advantages.