StackOverflow2013

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POiOS multithreading synchronization
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copied!<p>I am building an iOS app which does some heavy lifting on a background thread.</p> <p>I create my thread using</p> <pre><code> dispatch_queue_t backgroundQueue; backgroundQueue = dispatch_queue_create("MyQueue", NULL); </code></pre> <p>and then put it into GCD using:</p> <pre><code> dispatch_async(backgroundQueue, ^ { //some heavy operations here //each of them might run for >1 sec } </code></pre> <p>I just want a sequential execution of the queue, provided it doesn't block the main thread. If this block is called from method 3, method 2 and method 1 within 20ms.... they should be necessarily executed in the order 3 -> 2 -> 1, since the output of each method is used up by the next.</p> <p>I am new to GCD and naively imagined a dispatch queue would use a FIFO queue to execute sequential calls. However, in its current implementation, it is far from sequential.</p> <p>I tried using NSLock, but they didn't help either.</p> <p>I would like to know the proper mechanism for enforcing sequential execution in GCD.</p> <p><strong>EDIT 1:</strong></p> <p>I am declaring a global queue:</p> <pre><code> dispatch_queue_t backgroundQueue ; </code></pre> <p>and initiating it in viewDidLoad() : </p> <pre><code> backgroundQueue = dispatch_queue_create("MyQueue", DISPATCH_QUEUE_SERIAL); //using DISPATCH_QUEUE_SERIAL was a new idea, I also tried using NULL </code></pre> <p>I'm using GCD to basically call method in another class:</p> <pre><code> -(void)doAction() { dispatch_async(backgroundQueue, ^ { MyOtherClass *obj = [[MyOtherClass alloc] init]; [obj heavyMethod1: param1 : param2]; [obj release]; }); } -(void)doAnotherAction() { dispatch_async(backgroundQueue, ^ { MyOtherClass *obj = [[MyOtherClass alloc] init]; [obj heavyMethod2: param3 : param4]; [obj release]; }); } </code></pre> <p>Now, doAction and doAnotherAction method are being called from a number of other methods, depending upon the scenario. </p> <p>What I'm seeing here is - if I'm calling these methods in the sequence :: doAction -> doAction -> doAnotherAction -> doAction</p> <p>...I'm getting an output in a sequence like :: doAction -> doAnotherAction -> doAction -> doAction</p> <p>How do I maintain its sequence of invokation and keep it serial at the same time ?</p> <p>P.S: - If I remove GCD and allow things to carry on in the main thread, it is theoretically running fine - in simulator. (It crashes in iPhone.)</p>

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