dispatch_async doesn't go into loop - objective-c

Question

I need to fill in array. I have two methods. First of this create object that need to be added to array. I do it like this:
__block NSMutableArray *array = [[NSMutableArray alloc] initWithCapacity:inputArray.count];
dispatch_queue_t queue =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_sync(queue, ^{
dispatch_apply(inputArray.count,queue , ^(size_t index) {
[array insertObject:[self getObject:[inputArray objectAtIndex:index]] atIndex:index];
});
});
Finally I receive array with 2 objects instead of 100. I need to get array with objects that processed in getObject function.
If i code this:
dispatch_apply(inputArray.count, queue, ^(size_t index){
[array insertObject:[self getObject:[inputArray objectAtIndex:index]] atIndex:index];
});
I receive different numbers of array count. Can anybody tell me how to do it correctly?

Answer 1

The method initWithCapacity does not create an array with that many elements, it is just a hint to NSMutableArray on what the number of elements is likely to be. When you then try to insert an object at a specific instances things go wrong. From the docs for insertObject:atIndex::
Note that NSArray objects are not like C arrays. That is, even though you specify a size when you create an array, the specified size is regarded as a “hint”; the actual size of the array is still 0. This means that you cannot insert an object at an index greater than the current count of an array. For example, if an array contains two objects, its size is 2, so you can add objects at indices 0, 1, or 2. Index 3 is illegal and out of bounds; if you try to add an object at index 3 (when the size of the array is 2), NSMutableArray raises an exception.
You'll get different behaviour depending on the order the blocks queued by dispatch_apply are executed.
I'm assuming you are doing this as your getObject: is a time consuming process but multiple of them may be performed in parallel. If that is the case then:
a) After allocating the array pre-fill it up with cheap objects, e.g. [NSNull null] will do.
b) Use replaceObjectAtIndex:withObject: to add your objects to the array, this will replace your pre-filled object with your real ones.
Note: You are also misusing __block. This attribute is for when you wish to change the value of a local variable in the enclosing method from within a block. You never change the value of array either in the method or the block after you assign a reference to an NSMutableArray instance to it when it is declared. Your code then modifies the contents of the referenced object, not the value of the reference. Remove the __block.
HTH