Question

I want to use the results array after performing the block, my question is, should the results array instance be retained because it was originated from a block?

__block NSError *error = nil;    
__block NSArray *results;

[moc performBlockAndWait:^(void) {        
    results = [moc executeFetchRequest:fetchRequest error:&error]; 
    [results retain];
}];
if(results){
   //Do somehting
}

Answer 1

To be pedantically correct, you would do something like:

__block NSError *error = nil;    
__block NSArray *results;

[moc performBlockAndWait:^(void) {        
    results = [moc executeFetchRequest:fetchRequest error:&error]; 
    [results retain];
    if (!results) [error retain];
}];

if(results){
   //Do somehting
   [results release]; // or not -- assign it to something
} else {
   // Do something with error
   [error release]; // or autorelease
}

That is, you want a hard retain maintained from inside the block to outside the block always and regardless of synchronous execution. That retain must be balanced.

The over-arching rule is all about thread transfer; you are transferring objects from one thread of execution (wherever the block was executed) to the calling thread. There must be a hard retain across that transfer of ownership.