问题
I am using the following code in a loop:
-(void)getUpdatedComments
{
if(checkComments)
{
objParseOperation=[[ParseOperation alloc] initWithUDID:[[NSUserDefaults standardUserDefaults] valueForKey:kDeviceUDID]:self];
[operationQueue addOperation:objParseOperation3];
}
}
where operationQueue is an object of type NSOperationQueue. I am calling this method every few seconds.
If I call this method after the first time, do I need to cancel the previous operation or just do [objParseOperation release]?
回答1:
If you are going to use the same queue for adding more new operations then there isn't much use in releasing it and creating a new one. Simply canceling all operations and then adding more operations is enough. You can do so by calling cancelAllOperations on the queue. Note that operations that are already running will continue to run unless they check for the cancellation.
This method sends a cancel message to all operations currently in the queue. Queued operations are cancelled before they begin executing. If an operation is already executing, it is up to that operation to recognize the cancellation and stop what it is doing.
Wether or not releasing the queue will immediately release the enqueued operations or not is not documented and hence should be considered as undefined behavior. Therefor you shouldn't assume that it will work either way and you should not rely on it (see dangerous to autorelease NSOperationQueue).
There are however evidence that suggests that operations will retain their queue such as that GCD queues are retained by the system while they have asynchronous blocks running/pending. You can read in the Grand Central Dispatch documentation that "the queue is retaind by the system until the block has run to completion". Still, the documentation doesn't specify the behavior for NSOperationQueues like mentioned above.
dispatch_async
Submits a block for asynchronous execution on a dispatch queue and returns immediately.
void dispatch_async( dispatch_queue_t queue, dispatch_block_t block);Parameters
queue
The queue on which to submit the block. The queue is retained by the system until the block has run to completion. This parameter cannot be NULL.
block
The block to submit to the target dispatch queue. This function performs Block_copy and Block_release on behalf of callers. This parameter cannot be NULL.
Discussion
This function is the fundamental mechanism for submitting blocks to a dispatch queue. Calls to this function always return immediately after the block has been submitted and never wait for the block to be invoked. The target queue determines whether the block is invoked serially or concurrently with respect to other blocks submitted to that same queue. Independent serial queues are processed concurrently with respect to each other.
来源：https://stackoverflow.com/questions/9206401/cancel-nsoperation