Question

I’m in the process of learning Objective-C, and am trying to get my head wrapped around the idea of using blocks.

In an example I’m seeing, there is a method meant to sort an NSArray named book in alphabetical order by name:

-(void) sort
{
    [book sortUsingComparator:
        ^(id obj1, id obj2) {
            return [[obj1 name] compare: [obj2 name]];
    } ];
}

How does the method pass the objects to the obj1 and obj2 parameters? Does the sortUsingComparator somehow automatically pass 2 objects to its comparator?

Answer 1

sortUsingComparator: iterates over the array and calls the block with various pairs of objects to determine how to sort them. There’s no magic here — it’s a normal sorting function, but it uses your block instead of > to compare the values.