According to the AWS SDK documentation, the following will not work:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
S3PutObjectRequest *request = [[S3PutObjectRequest alloc] initWithKey:objectKey inBucket:bucketName];
request.data = data;
request.delegate = self;
[s3Client putObject:request];
});
meaning, the delegate methods will not be called, due to the following reason:
The block in this GCD queue will be executed in the background thread,
which has its own run loop, separate from the one on the main thread.
The delegate will be registered with the background thread's run loop
that is responsible for calling the AmazonServiceRequestDelegate
method. However, putObject: immediately returns and the entire block
also returns. When the block finishes its execution, the background
thread will be collected by GCD, and nothing that is responsible for
calling the delegate methods will be left behind. That's why the above
code sample doesn't work.
And it's true, the delegate methods are never called for that block of code. However, I expected this to work:
self.queue = dispatch_queue_create("MyQueue", NULL);
dispatch_async(self.queue, ^{
S3PutObjectRequest *request = [[S3PutObjectRequest alloc] initWithKey:objectKey inBucket:bucketName];
request.data = data;
request.delegate = self;
[s3Client putObject:request];
});
Notice that I'm retaining the queue, so after the block returns, the queue sticks around. However, this still doesn't work - the delegate callbacks are never called. Why is this so?
You have created a Serial Dispatch Queue, but it doesn't mean that there should be a thread attached to the queue, the system manager the background thread and it will provide a thread to some queue to execute the code you push to the queue.
The Serial Dispatch Queue guarantee the works you push to it is executed one by one but it doesn't guarantee the thread which do the work is the same.
So you face the same problem when using Global Concurrent Dispatch Queues.
Related
Recently I came to a point where I needed some block of code to execute always on the main thread synchronously. This block can be called from any thread. I solved this problem with the code that was already suggested in this SO answer by #Brad Larson
As the comments to this answer it is evident that the deadlock can occur, but I got into the deadlock very very easily. Please have a look at this code.
-(IBAction) buttonClicked
{
// Dispatch on the global concurrent queue async.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSString* data = [self getTheString];
NSLog(#"From Background Thread: %#", data);
};
// Dispatch on the main queue async.
dispatch_async(dispatch_get_main_queue(), ^{
NSString* data = [self getTheString];
NSLog(#"From Main Thread: %#", data);
};
}
// This method can be called from any thread so synchronize it.
// Also the code that sets the string variable based on some logic need to execute on main thread.
-(NSString*) getTheString
{
__block NSString* data = nil;
#synchronized(self)
{
// Have some code here that need to be synchronized between threads.
// .......
//
// Create a block to be executed on the main thread.
void (^blockToBeRunOnMainThread)(void) = ^{
// This is just a sample.
// Determining the actual string value can be more complex.
data = #"Tarun";
};
[self dispatchOnMainThreadSynchronously:blockToBeRunOnMainThread];
}
}
- (void) dispatchOnMainThreadSynchronously:(void(^)(void))block
{
if([NSThread isMainThread])
{
if (block)
{
block();
}
}
else
{
dispatch_sync(dispatch_get_main_queue(), ^{
if (block)
{
block();
}
});
}
}
In this piece of code there are two simultaneous asynchronous requests to function getTheString (Assume you have no control over the buttonClicked method and how it calls getTheString api) . Suppose the request from global queue comes first and it is trying to run the block on the main thread synchronously, till that time background thread in waiting for main thread to execute the block synchronously, at the same time request from main queue comes and trying the acquire the lock from background thread, but as background thread in not complete main thread waiting for background thread to complete. Here we have a deadlock on main thread as main thread waiting for background thread to finish, and background thread is waiting for main thread to execute block.
If I remove the #synchronize statement everything works fine as expected. May be I don't need a #synchronize statement here but in same case you may need to have this. Or it can even happen from some other parts of the code.
I tried to search the whole net for the solution and also tried dispatch_semaphore but couldn't solve the issue. May be I am just not doing things the right way.
I assume this is classic problem of deadlock and faced by developers again and again, and probably have solved it to some extent. Can anyone help with this, or point me to right direction?
I would create a synchronous queue (NSOperationQueue would be simplest) and submit the block to be run on the main thread to that queue. The queue would dispatch the blocks in the order received, maintaining the ordering you desire. At the same time, it disassociates the synchronicity between calling the getTheString method and the dispatch to the main thread.
I have the following situation where I create a GCD dispatch queue and in it I schedule an NSStream to the current NSRunLoop, as is required in its specification for it to emit delegate events, and then I make the run loop for that thread run using [[NSRunLoop currentRunLoop run].
This generates three possible scenarios:
Create a serial queue in which an initial write message is sent through the stream and other write messages are only sent when there's a delegate callback from the NSStream object, as attempting to write new messages without respecting this pattern (this would be desirable) will fail as the queue is locked by the run loop running.
Create a concurrent queue in which messages can be written to the stream freely, as blocks sent to the queue will be executed concurrently with the block that's running the run loop. However, while it is desirable to make writing messages and the run loop running concurrent, it certainly is not desirable to have to blocks in the queue running concurrently attempting to write at the same time to the stream.
Create two queues -- one responsible for keeping the run loop alive and receive read-from-stream callbacks and another one for sending asynchronous write messages to the stream. This would seem ideal, however it seems that the NSStream documentation specifically states that one should not attempt to read/write to a stream outside the thread it is scheduled in.
Given these scenarios none of which are ideal, how to solve these problems?
Late to the party, but instead of using runloops you can set the desired dispatch queue for your streams directly using
void CFReadStreamSetDispatchQueue(CFReadStreamRef stream, dispatch_queue_t q);
void CFWriteStreamSetDispatchQueue(CFWriteStreamRef stream, dispatch_queue_t q);
Where CFReadStreamRef can take a bridged NSInputStream and CFWriteStreamRef a bridged NSOutputStream. This way you don't have to schedule or unschedule runloops at all and your streams will run in the background.
Snippet from this Apple sample code:
CFReadStreamSetDispatchQueue((__bridge CFReadStreamRef) self.inputStream, self.queue);
CFWriteStreamSetDispatchQueue((__bridge CFWriteStreamRef) self.outputStream, self.queue);
In Swift, you can just directly call the functions:
CFReadStreamSetDispatchQueue(inputStream, streamQueue)
CFWriteStreamSetDispatchQueue(outputStream, streamQueue)
As you noted from the docs, when you have a run-loop-based API like NSStream, the general expectation is that all interaction with that object will occur on the thread that owns the run loop on which it's scheduled. I'm not sure there's really any benefit to mixing these two idioms (GCD and run loops) when it comes to working with NSStream.
Other than the main queue, GCD has no concept of thread-affinity, so unless the run loop you schedule the NSStream on happens to be the main thread run loop, there's no good way to use dispatch_async to schedule blocks for execution on that thread.
At the risk of stating the obvious, you should probably just use the standard methods for scheduling methods on other threads. -performSelector:onThread:withObject:waitUntilDone:modes: is the most obvious. If your confusion is that you want to work with blocks, it helps to know that heap-allocated blocks can be treated like Objective-C objects and implement the -invoke selector just like NSInvocations do. A trivial example relevant to your question might look like this:
#interface AppDelegate ()
{
NSThread* bgthread;
}
#end
#implementation AppDelegate
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
// Basic loop to get the background thread to run until you call -cancel on it
dispatch_block_t threadMain = [^{
NSThread* thread = [NSThread currentThread];
NSParameterAssert(![thread isMainThread]);
NSRunLoop* currentRunLoop = [NSRunLoop currentRunLoop];
NSPort* port = [NSPort port];
// If we dont register a mach port with the run loop, it will just exit immediately
[currentRunLoop addPort: port forMode: NSRunLoopCommonModes];
// Loop until the thread is cancelled.
while (!thread.cancelled)
{
[currentRunLoop runMode: NSDefaultRunLoopMode beforeDate: [NSDate distantFuture]];
}
[currentRunLoop removePort: port forMode: NSRunLoopCommonModes];
[port invalidate];
port = nil;
} copy];
// Start the thread
bgthread = [[NSThread alloc] initWithTarget: threadMain selector: #selector(invoke) object: nil];
[bgthread start];
// Fetch the runloop, so you can schedule an NSStream on it...
__block NSRunLoop* runloopForStream = nil;
dispatch_block_t getrunloop = [^{
runloopForStream = [NSRunLoop currentRunLoop];
} copy];
// Dispatch synchronously, so that runloopForStream is populated before we continue...
[getrunloop performSelector: #selector(invoke) onThread: bgthread withObject: nil waitUntilDone: YES];
// Schedule your stream, etc.
NSOutputStream* mystream = ...; // Your code here...
[mystream scheduleInRunLoop: runloopForStream forMode: NSDefaultRunLoopMode];
// Then later, when you want to write some data...
NSData* dataToWrite = [NSMutableData dataWithLength: 100];
dispatch_block_t doWrite = [^{
[mystream write: dataToWrite.bytes maxLength: dataToWrite.length];
} copy];
// Dispatch asynchronously to thread
[doWrite performSelector: #selector(invoke) onThread: bgthread withObject: nil waitUntilDone: NO];
}
#end
Note that the -copy of the blocks is necessary to get them copied to the heap, otherwise they'll be deallocated when the declaring method goes out of scope.
Is there any difference between if dispatch_sync is called in 3 different queue like
1.
dispatch_sync(dispatch_get_main_queue(),^(void){
NSLog(#"this execute in main thread") // via [NSThread isMainThread]
});
2.
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^(void){
NSLog(#"this also execute in main thread") // via [NSThread isMainThread]
}
3.
dispatch_queue_t queue;
queue = dispatch_queue_create("com.example.MyQueue", NULL);
dispatch_sync(queue, ^(void){
NSLog(#"this also execute in main thread") // via [NSThread isMainThread]
}
Whenever i call dispatch_sync, block executed in main thread, without considering in which queue it is dispatched. So why this function take queue as a argument as it doesn't use it. Can someone please clarify this?
dispatch_sync is a blocking operation. That is, the function will not return until the work represented in the block is completed.
When dispatched to an asynchronous queue -- like one of the global queues or a concurrent queue of your own making -- there is no reason to do anything but invoke the block on the thread that called dispatch_sync(). Even in the case of invoking the block on a synchronous queue, the dispatch_sync() is going to wait until completion anyway so, internally, it might as well stop until the rest of the work is done in the queue and then execute the block directly.
As it turns out, passing data from thread A to thread B is expensive. If the queue is in the state where execution can happen immediately, then dispatch_sync will fast path the execution by simply calling the block on the thread that dispatch_sync was called on.
And, by definition, you shouldn't care. The calling thread is blocked -- can't do a thing -- until dispatch_sync() returns.
So, really, all of this is an implementation detail. GCD is free to execute the blocks on whatever threads it deems most appropriate. It just so happens that don't context switch is often the most important rule of figuring that out.
See dispatch_sync documentation, which notes
As an optimization, this function invokes the block on the current thread when possible.
If you dispatch something synchronously, since the thread must wait for the dispatched code to complete, anyway, it will frequently run that code on the current thread. So if dispatched synchronously from the main thread, it will run on main thread. If dispatched synchronously from a background thread, it will run on that background thread.
As noted by ipmcc, a well-known exception is when a background thread dispatches something synchronously to the main thread. As the libdispatch source says:
It's preferred to execute synchronous blocks on the current thread
due to thread-local side effects, garbage collection, etc. However,
blocks submitted to the main thread MUST be run on the main thread.
For your problem: you call the dispatch_sync always in main queue, and if you wanna know why, see the following:
At first you need to pay attention to the description on of "dispatch_sync"
Submits a block to a dispatch queue for synchronous execution. Unlike dispatch_async,
this function does not return until the block has finished.
Calling this function and targeting the current QUEUE(NOT THREAD) results in deadlock.
#define logStep(step,queue) NSLog(#"step: %d at thread: %# in -- queue: %s",step,[NSThread currentThread],dispatch_queue_get_label(queue));
// call the method in main thread within viewDidLoad or viewWillAppear ...
- (void)testDispatchSync{
//let's distinctly tell the 4 queues we often use at first
self.concurrentQ = dispatch_queue_create("com.shared.concurrent", DISPATCH_QUEUE_CONCURRENT);
self.serialQ = dispatch_queue_create("com.shared.serial", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t mainQ = dispatch_get_main_queue();
dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
logStep(1,mainQ) //we're in main thread and main queue, current queue IS main queue
// do a sync in main thread & concurrent queue
dispatch_sync(_concurrentQ, ^{
logStep(2,_concurrentQ)
});
// do a sync in main thread & serial queue
dispatch_sync(_serialQ, ^{
logStep(3,_serialQ)
});
//uncommenting the following code that you'wll see a crash will occur, because current queue is main queue
// dispatch_sync(mainQ, ^{
// logStep(4, mainQ)
// });
dispatch_async(_concurrentQ, ^{
// inside of the this scope, current queue is "_concurrentQ"
logStep(11,_concurrentQ)
// using sync in any queue here will be safe!
dispatch_sync(_concurrentQ, ^{
logStep(12,_concurrentQ)
});
dispatch_sync(_serialQ, ^{
logStep(13,_concurrentQ)
});
dispatch_sync(mainQ, ^{
logStep(14,mainQ)
});
dispatch_sync(globalQ, ^{
logStep(15,globalQ)
});
// using async in any queue here will be safe!
dispatch_async(_concurrentQ, ^{
logStep(111,_concurrentQ)
});
dispatch_async(_serialQ, ^{
logStep(112,_concurrentQ)
});
dispatch_async(mainQ, ^{
logStep(113,mainQ)
});
dispatch_async(globalQ, ^{
logStep(114,globalQ)
});
});
dispatch_async(_serialQ, ^{
// inside of the this scope, current queue is "_serialQ"
logStep(21,_serialQ)
// using async in any queue except current queue here will be safe!
dispatch_sync(_concurrentQ, ^{
logStep(22,_concurrentQ)
});
dispatch_sync(mainQ, ^{
logStep(23,mainQ)
});
dispatch_sync(globalQ, ^{
logStep(24,globalQ)
});
//uncommenting the following code that you'wll see a crash will occur, because current queue is "_serialQ"
// dispatch_sync(_serialQ, ^{ //app will die at here
// logStep(25,_serialQ)
// });
});
}
So we got the conclusion:
the key problem is that thread will be blocked while "dispatch_sync" running on current queue which is a serial queue at the same time.
main queue is also a serial queue so that it explained why you cann't call dispatch_sync in main thread
I am trying to wait for asynchronous threads in objective-c without blocking the UI thread (it is an app).
I have the following code:
-(void)MainUIThread
{
[exporter exportAsynchronouslyWithCompletionHandler:^{
dispatch_async(dispatch_get_main_queue(), ^{
[self exportDidFinish:exporter];
//wait here without blocking
});
}];
}
In C# I would use async and await, can I easily achieve this in objective-c?
[exporter exportAsynchronouslyWithCompletionHandler:^{
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
[self exportDidFinish:exporter]; // Its non blocking process
dispatch_async(dispatch_get_main_queue(), ^(void){
//Update your UI.
});
}];
Try above snippet.
You calling [self exportDidFinish:exporter] method on main thread that why it block your UI. I suppose exportDidFinish method have just a business logic then post that logic calculation in background thread using dispatch_get_global_queue then get it back on main thread using dispatch_get_main_queue().
What makes you think that you should wait for an asynchronous operation to finish? That's the whole point of asynchronous operation, you don't wait for it!
You called exportAsynchronouslyWithCompletionHandler and gave it a block. I don't know that method, but I assume it will do things asynchronously, and when it is finished it will call the block that you give it. The call to exportAsynchronouslyWithCompletionHandler will return very quickly, as soon as the method has set up its asynchronous operation.
That block dispatches another block calling "exportDidFinish:exporter" on the main thread, then it returns (with no reason to wait), then all the asynchronous code in exportAsynchronouslyWithCompletionHandler is finished.
All we have now is one block dispatched to the main queue. As soon as the run loop is idle, that block is executed on the main thread. The commented line that you added "// Wait here" is pointless. exportDidFinish:exporter will be called on the main thread when it's time to be called, and then we are done.
I wanted a quick and easy way to get data from a URL without having to mess with delegates.
Is there anything wrong with the following?
// Use gcd
dispatch_queue_t queue = dispatch_queue_create("com.dowork", 0);
dispatch_queue_t main = dispatch_get_main_queue();
// do the long running work in bg async queue
// within that, call to update UI on main thread.
dispatch_async(queue, ^{
// Do work in the background
NSData *response = [NSURLConnection sendSynchronousRequest:serviceRequest returningResponse:&serviceResponse error:&serviceError];
dispatch_async(main, ^{
// Update UI
self.data = response;
[self.tableView reloadData];
});//end
});//end
I thought I read somewhere long ago that using the NSURLConnection synchronous method in a background thread would cause memory leaks. Is this true?
Are there any issues with the codes that is posted there? Any issues with assigning the data to self.data within the block?
If you are targeting ios5 and later, there's NSURLConnection's sendAsynchronousRequest:queue:completionHandler:
To answer your specific question, it looks to me like response might leak: I don't know if there is an implicit autorelease pool on GCD threads.
Done some research now: GCD threads have their own autorelease pools but you don't know when they will be drained. You probably want to bracket the first two statements with an explicit autorelease pool.
See also Do you need to create an NSAutoreleasePool within a block in GCD?