NSOperation & Singleton: Correct concurency design

Question

I need an advice from you guys on the design of my app here, basically I would like to know if it will work as I expect ? As the multi-threading is quite tricky thing I would like to hear from you.

Basically my task is very simple -I've SomeBigSingletonClass - big singleton class, which has two methods someMethodOne and someMethodTwo These methods should be invoked periodically (timer based) and in separate threads. But there should be only one instance of each thread at the moment, e.g. there should be only one running someMethodOne at any time and the same for someMethodTwo.

What I've tried

GCD - Did implementation with GCD but it lacks very important feature, it does not provide means to check if there is any running task at the moment, i.e. I was not able to check if there is only one running instance of let say someMethodOne method.

NSThread - It does provide good functionality but I'm pretty sure that new high level technologies like NSOperation and GCD will make it more simple to maintain my code. So I decided to give-up with NSThread.

My Solution with NSOperation How I plan to implement the two thread invokation

@implementation SomeBigSingletonClass

- (id)init
{
    ...
    // queue is an iVar
    queue = [[NSOperationQueue alloc] init];

    // As I'll have maximum two running threads 
    [queue setMaxConcurrentOperationCount:2];
    ...
}

+ (SomeBigSingletonClass *)sharedInstance
{
    static SomeBigSingletonClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[SomeBigSingletonClass alloc] init];
    });
    return sharedInstance;
}

- (void)someMethodOne
{
    SomeMethodOneOperation *one = [[SomeMethodOneOperation alloc] init];
    [queue addOperation:one];
}

- (void)someMethodTwo
{
    SomeMethodTwoOperation *two = [[SomeMethodOneOperation alloc] init];
    [queue addOperation:two];
}
@end

And finally my NSOperation inherited class will look like this

@implementation SomeMethodOneOperation

- (id)init
{
    if (![super init]) return nil;
    return self;
}

- (void)main {
    // Check if the operation is not running
    if (![self isExecuting]) {
        [[SomeBigSingletonClass sharedInstance] doMethodOneStuff];
    }
}

@end

And the same for SomeMethodTwoOperation operation class.

nielsbot · Accepted Answer

If you are using NSOperation, you can achieve what you want be creating your own NSOperationQueue and setting numberOfConcurrentOperations to 1.

You could have also maybe used an @synchronized scope with your class as your lock object.

EDIT: clarification---

What I am proposing:

Queue A (1 concurrent operation--used to perform SomeMethodOneOperation SomeMethodTwoOperation once at a time)

Queue B (n concurrent operations--used for general background operation performing)

EDIT 2: Updated code illustrating approach to run maximum operation one and operation two, with max one each of operation one and operation two executing at any given time.

-(void)enqueueMethodOne
{
    static NSOperationQueue * methodOneQueue = nil ;
    static dispatch_once_t onceToken ;    
    dispatch_once(&onceToken, ^{
        queue = [ [ NSOperationQueue alloc ] init ] ;
        queue = 1 ;
    });

    [ queue addOperation:[ NSBlockOperation blockOperationWithBlock:^{
        ... do method one ...
    } ] ];
}

-(void)enqueueMethodTwo
{
    static NSOperationQueue * queue = nil ;
    static dispatch_once_t onceToken ;    
    dispatch_once(&onceToken, ^{
        queue = [ [ NSOperationQueue alloc ] init ] ;
        queue = 1 ;
    });

    [ queue addOperation:[ NSBlockOperation blockOperationWithBlock:^{
        ... do method two ...
    } ] ];
}

EDIT 3:

per our discussion:

I pointed out that isExecuting is a member variable and refers only to the state of the operation being queried, not if any instance of that class is executing

therefore Deimus' solution won't work to keep multiple instances of operation one running simultaneously for example

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