Change priority of operations in NSOperationQueue iOS

Question

i am adding the operations to the NSOperationQueue in asynchronously. I have a requirement like I need to change the priority of the operations like I have A,B,C operations are added to the queue and I need to execute the C operation at first. If I get some error executing I have to that operation to the Queue with high priority. I googled about it, we can't do like this. By using GCD we can do like this using notify and wait. I have a query, if I added the A operation in asynchronously and B and C added in group notify. In that A operation will execute firstly after that B and C will execute. If I get the error while executing operation A then I added this to the group until and unless operation get success. Here in this process if after adding the Operation A which one will execute firstly Operation A or Operation B/C. In my requirement operation A has to execute firstly. I am new to blocks, Please suggest me if we can implement through Queues or GCD. Please provide some related code. I need to execute the one task at time .Also I am using maxConcurrentOperationCount=1.

Answer 1

Neither NSOperationQueue or GCD queues are meant to be priority queues. They are mostly FIFO queues with some additional smartness. Also they shine best if you need multiple concurrent operations.

In your scenario (maxConcurrentOperationCount=1 with custom scheduling) a plain old NSThread is just fine.

First you need to implement a priority queue of things or reuse a library that does that (one example - atljeremy PriorityQueue, but you'll probably have to adapt it to support modification of priorities after the item was added.

Then just run a loop (inside an NSThread) that takes a task from the queue and runs it:

while (!isStopped) {
    NSOperation *task = [queue poll];
    [task main];
}

You can reuse NSOperation or make your own task type, that's up to you. You can also use dispatch_block_t blocks as tasks.