How can I make sure that my function is run only on the main thread? It updates UI elements.
Is a function like this considered 'bad'?
-(void)updateSomethingOnMainThread {
if ( ![[NSThread currentThread] isEqual:[NSThread mainThread]] )
[self performSelectorOnMainThread:_cmd withObject:nil waitUntilDone:NO];
else {
// Do stuff on main thread
}
}
I wrote it like this to avoid having a second function, initially I had it like this:
-(void)updateSomethingOnMainThread_real {
// Do stuff on main thread
}
-(void)updateSomethingOnMainThread {
[self performSelectorOnMainThread:#selector(updateSomethingOnMainThread_real) withObject:nil waitUntilDone:NO];
}
As an alternative to ayoy's method-based GCD implementation for guaranteeing execution on the main thread, I use the following GCD-based function in my code (drawn from another answer of mine):
void runOnMainThreadWithoutDeadlocking(void (^block)(void))
{
if ([NSThread isMainThread])
{
block();
}
else
{
dispatch_sync(dispatch_get_main_queue(), block);
}
}
You can then use this helper function anywhere in your code:
runOnMainThreadWithoutDeadlocking(^{
// Do stuff that needs to be on the main thread
});
This guarantees that the actions taken in the enclosed block will always run on the main thread, no matter which thread calls this. It adds little code and is fairly explicit as to which code needs to be run on the main thread.
This is fine. You can also use GCD to execute code on the main thread.
Checkout this SO post.
GCD to perform task in main thread
I wrote this simple #define which I've been using with great success:
#define ensureInMainThread(); if (!NSThread.isMainThread) { [self performSelectorOnMainThread:_cmd withObject:nil waitUntilDone:NO]; return; }
That way your method, assuming it's parameterless, looks like this
- (void) updateTheThings {
ensureInMainThread();
[self.dog setTailWag:YES];
// etc...
Alternatively, you can use Grand Central Dispatch API, but it's not very handy:
-(void)updateSomethingOnMainThread {
void (^doStuff)(void) = ^{
// stuff to be done
};
// this check avoids possible deadlock resulting from
// calling dispatch_sync() on the same queue as current one
dispatch_queue_t mainQueue = dispatch_get_main_queue();
if (mainQueue == dispatch_get_current_queue()) {
// execute code in place
doStuff();
} else {
// dispatch doStuff() to main queue
dispatch_sync(mainQueue, doStuff);
}
}
otherwise, if synchronous call isn't needed, you can call dispatch_async() which is much simpler:
-(void)updateSomethingOnMainThread {
dispatch_async(dispatch_get_main_queue(), ^{
// do stuff
});
}
Related
what's is more prefered way to write multi threaded apps. I see two ways.
Implement method with GCD inside and then just simple call (myMethodA), or just implement method and then call it with GCD? Thanks in advance.
My point:
ClassA / method implementation
- (void)myMethodA
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// doSomething1
// doSomething2
});
}
- (void)myMethodB
{
// doSomething1
// doSomething2
}
ClassB / method call
{
[myClassA methodA];
// or
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[myClassA methodB];
};
}
IMHO, neither.
The preferred way should be having an object which knowns where to execute its actions:
completion_block_t completionHandler = ^(id result) { ... };
AsyncOperation* op = [AsyncOperation alloc] initWithCompletion:completionHandler];
[op start]; // executes its actions on a private execution context
Then, one can wrap those AsyncOperation objects into a convenient method:
- (void) fetchUsersWithCompletion:(completion_block_t)completionHandler
{
NSDictionary* params = ...;
self.currentOperation = [[HTTPOperation alloc] initWithParams:params
completion:completionHandler];
[self.currentOperation start];
}
The client may only be interested in specifying where its completionHandler should be executed. The API may be enhanced as follows:
- (void) fetchUsersWithQueue:(NSOperationQueue*)handlerQueue
withCompletion:(completion_block_t)completionHandler
{
NSDictionary* params = ...;
self.currentOperation = [[HTTPOperation alloc] initWithParams:params
completion:^(id result){
// As per the documentation of HTTPOperation, the handler will be executed
// on an _unspecified_ execution context.
// Ensure to execute the client's handler on the specified operation queue:
[handlerQueue:addOperationWithBlock:^{
completionHandler(result);
}];
}];
[self.currentOperation start];
}
The latter API can be used as this:
[self fetchUsersWithQueue:[NSOperation mainQueue] completion:^(id result){
self.users = result;
[self.tableView reloadData];
}];
Personal preference. Choose whichever makes the code more readable / understandable / obvious. Also, consideration of whether the code should be possible to run on the 'current' thread or whether it should always be run on a background thread. You need to design your threading configuration, describe it and then implement with that in mind. If you're calling methods between classes like in your example then I'd generally say that any threading should be handled inside that class, not inside the calling class. But that's about distribution of knowledge.
It doesn't make much of a difference - it just depends on what you want to do.
If you want to execute the method on different queues each time, then the myMethodB system is more appropriate. If, however, you always want to run the method on the same queue, then myMethodA will save you time writing code (you only have to write the GCD code once).
I am trying to make screen shot of avplayer when video start playing so i need to run this code in fast in background so it will not block main thread and other controls run fast simultaneous,trying to run that code GCD format i am not able to run please help me to do that it stops at where i add into my array(in array i am adding UIImage Object)...
if (isCaptureScreenStart)
{
if (CMTimeGetSeconds([avPlayer currentTime])>0)
{
if (avFramesArray!=nil)
{
queue = dispatch_queue_create("array", NULL);
dispatch_sync(queue, ^{
[avFramesArray addObject:[self screenshotFromPlayer:avPlayer maximumSize:avPlayerLayer.frame.size :CMTimeGetSeconds([avPlayer currentTime])]];//stop at this line
NSLog(#"count:%d",[avFramesArray count]);
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"Frame are created:%d",[avFramesArray count]);
if ([avFramesArray count]==0)
{
NSLog(#"Frame are over");
}
});
});
}
}
}
dispatch_release(queue);
Edit:
I think i need to use dispatch_group_async this block now..please give some guideline that how to use:
if (isCaptureScreenStart)
{
if (CMTimeGetSeconds([avPlayer currentTime])>0)
{
if (avFramesArray!=nil) {
dispatch_group_async(serial_group1, serial_dispatch_queue1, ^{
[avFramesArray addObject:[self screenshotFromPlayer:avPlayer maximumSize:avPlayerLayer.frame.size :CMTimeGetSeconds([avPlayer currentTime])]];
});
}
}
dispatch_group_notify(serial_group1, serial_dispatch_queue1, ^{
NSLog(#"task competed");
});
}
Now I am using this block but above execution is contentious running and if i use dispatch_suspend(serial_dispatch_queue1); its stop but again i need to start block execution then what i need to use i have also try with dispatch_resume(serial_dispatch_queue1); again load but system show me crash
dispatch_release(queue); don't do it there, the dispatch queue that you are calling its going to a backThread, so wat is happening is :-
your queue is getting released before the block of code executes.
since your queue looks like an ivar, release it in dealloc. Rest, your code looks fine ..put a breakpoint inside and check if the block is executing.
EDIT
I dont understand, what u are trying to achieve by suspending the queue, there is no need to do it. You dont need to check whether the block has finished executing. The block will finish and then call the dispatch_async , get the main queue and update the UI from there.
Now, when you are creating the queue, create it lazily in your method. take the queue as an ivar in header file:
#interface YourFileController : UIViewController {
dispatch_queue_t queue;
}
Then in your method modify it as such:
if (isCaptureScreenStart)
{
if (CMTimeGetSeconds([avPlayer currentTime])>0)
{
if (avFramesArray!=nil)
{
if (!queue)
queue = dispatch_queue_create("array", DISPATCH_QUEUE_SERIAL);
dispatch_sync(queue, ^{
[avFramesArray addObject:[self screenshotFromPlayer:avPlayer maximumSize:avPlayerLayer.frame.size :CMTimeGetSeconds([avPlayer currentTime])]];//stop at this line
NSLog(#"count:%d",[avFramesArray count]);
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"Frame are created:%d",[avFramesArray count]);
if ([avFramesArray count]==0)
{
NSLog(#"Frame are over");
}
});
});
}
}
}
NOTE : DISPATCH_QUEUE_SERIAL creates a serial queue, meaning all the blocks submitted to it will execute serially in First in First Out order. Once all the blocks submitted get executed, the queue stays ;) ..submit another block to it and it executes the block :D
this represents one whole block:-
[avFramesArray addObject:[self screenshotFromPlayer:avPlayer maximumSize:avPlayerLayer.frame.size :CMTimeGetSeconds([avPlayer currentTime])]];//stop at this line
NSLog(#"count:%d",[avFramesArray count]);
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(#"Frame are created:%d",[avFramesArray count]);
if ([avFramesArray count]==0)
{
NSLog(#"Frame are over");
}
});
MySynchManager class is having a shared instance.
One of the function in MySynchManager class is
- (void)uploadSession:(NSString *)sessionId {
// run the upload process on a separate thread to not to block the main thread for user interaction
// process upload data in serial Queue
NSLog(#"Inside uploadSession");
if (!_serialQueue) {
NSLog(#"uploadSession, _serialQueue is NOT ACTIVE");
[self setRunLoopStarted:FALSE];
_serialQueue = dispatch_queue_create("sessionUploadQueue", NULL);
dispatch_async(_serialQueue, ^{
[[MySyncManager sharedInstance] dispatchSession:sessionId];
});
}
else {
//[self setRunLoopStarted:FALSE];
dispatch_async(_serialQueue, ^{
[self dispatchSession:sessionId];
});
NSLog(#"Adding block to the dispatch queue is complete");
}
}
uploadSession:#"session" is being called from view controllers.
The problem that I am facing is sometimes the code present in dispatchSession is called, but sometimes block is not called.
I only observe the log print statement after the block is printed.
Can any one of you explain the reason behind this?
This is weird code. Try this instead
-(void)uploadSession:(NSString *)sessionId
{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_serialQueue = dispatch_queue_create("sessionUploadQueue", NULL);
});
dispatch_async(_serialQueue, ^{
[self dispatchSession:sessionId];
});
}
how can i update GUI elements with values from a queue?
if i use async queue construct, textlable don't get updated.
Here is a code example i use:
- (IBAction)dbSizeButton:(id)sender {
dispatch_queue_t getDbSize = dispatch_queue_create("getDbSize", NULL);
dispatch_async(getDbSize, ^(void)
{
[_dbsizeLable setText:[dbmanager getDbSize]];
});
dispatch_release(getDbSize);
}
Thank you.
As #MarkGranoff said, all UI needs to be handled on the main thread. You could do it with performSelectorOnMainThread, but with GCD it would be something like this:
- (IBAction)dbSizeButton:(id)sender {
dispatch_queue_t getDbSize = dispatch_queue_create("getDbSize", NULL);
dispatch_queue_t main = dispatch_get_main_queue();
dispatch_async(getDbSize, ^(void)
{
dispatch_async(main, ^{
[_dbsizeLable setText:[dbmanager getDbSize]];
});
});
// release
}
Any UI update must be performed on the main thread. So your code would need to modified to use the main dispatch queue, not a queue of your own creation. Or, any of the performSelectorOnMainThread methods would work as well. (But GCD is the way to go, these days!)
How does one determine the thread a given function call is running on?
I want to make sure a method is called on the main thread in order to update some UI elements.
Can I for example do something like this?
- (void) myMethod {
if (<current thread is not main thread>) {
[self performSelectorOnMainThread: #selector(myMethod) withObject: nil waitUntilDone: NO];
} else {
// my code here
}
}
Check out [NSThread isMainThread].
http://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSThread_Class/Reference/Reference.html#//apple_ref/occ/clm/NSThread/isMainThread