iPhone - a delay between operations in the queue

Question

I am adding operations to the queue using something like this

NSInvocationOperation *operation0 = [[NSInvocationOperation alloc] 
initWithTarget:self
selector:@s         


        

Answer 1

Using sleep is a big waste of a thread. This is bad in the Ghostbusters' sense.

If you need a queue that executes its operations non-concurrently i.e. sequentially you can simpy set its maxConcurrentOperationCount variable to 1.

queue.maxConcurrentOperationCount = 1;

To pause between operations you could do two different things:

a) Delay the queueing of each operation:

//loop over the operations to be queued with and index
      double delayInSeconds = 2.0 * index;
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
        dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
            queue.addOperation(operation at index); 
        });

This will queue all operations without blocking.

b) Every operation has to implement

- (BOOL)isFinished

as you probably already know.

If your operation is "done" just delay setting its "done" variable by your desired delay. Use something like:

// Operation is done with its work, but we tell the Queue only after 2 seconds, so the queue will start the next operation later
double delayInSeconds = 2.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
 __weak __typeof(self)weakSelf = self;
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
[weakSelf willChangeValueForKey:@"isExecuting"];
    [weakSelf willChangeValueForKey:@"isFinished"];
    weakSelf.executing = NO;
    weakSelf.finished  = YES;
    [weakSelf didChangeValueForKey:@"isFinished"];
    [weakSelf didChangeValueForKey:@"isExecuting"];
    });

Variant a) guarantees a pause between starting times of operations. Variant b) guarantees a pause between the end of an operation and the start of a next operation.