Executing method of a class multiple times parallel

Question

I have an array- say wordsArray where each object of wordArray is another array- say wordList. With for loop i am picking each object (wordList) and call a method -say wordScore of another class which will return NSDictionary with word and the score. Instead of calling wordScore for each object one by one, Is there a way I can run them parallel? wordScore for each wordList should execute in parallel and at the end of execution all the NSDictionaries of each wordScore should be merged into a single NSDictionary.

Answer 1

Let's say I have something silly like this class

@interface MyWordClass: NSObject
- (NSDictionary *)wordScore:(NSArray *)wordList;
@end

@implementation MyWordClass
- (NSDictionary *)wordScore:(NSArray *)wordList {
    NSMutableDictionary *scores = [NSMutableDictionary new];
    for (NSString *word in wordList) {
        scores[word] = @(word.hash %100);
    }
    return [scores copy];
}
@end

From here, I'll do a concurrent enumeration of wordsArray. I create a new instance of MyWordClass in each of the concurrent iterations. Finally, I use a @synchronized block to accumulate the results.

NSArray *wordsArray = @[@[@"a", @"b", @"c"], @[@"1", @"2", @"3"]];

NSMutableDictionary *result = [NSMutableDictionary new];
[wordsArray enumerateObjectsWithOptions:NSEnumerationConcurrent usingBlock:^(NSArray *wordList, NSUInteger idx, BOOL *stop) {
    // The scores are being calculated concurrently
    NSDictionary *scores = [[MyWordClass new] wordScore:wordList];

    @synchronized(result) {
        [result addEntriesFromDictionary:scores];
    }
}];

NSLog(@"%@", result);

Important take aways are 1) do a concurrent enumeration, 2) don't use shared instances in the concurrent block, and 3) use a @synchronized block to gather the result.