ArrayUpdate.m

//
//  ViewController.m
//  ArrayCrash
//
//  Created by Paul Wilkinson on 24/5/17.
//  Copyright © 2017 Paul Wilkinson. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()
 @property(nonatomic, strong) NSMutableArray *arr;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
     self.arr = [NSMutableArray new];
}


- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

-(void)viewDidAppear:(BOOL)animated{
    
    [super viewDidAppear:animated];
    
    __weak typeof(self) weakSelf = self;
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
        for (int i = 0; i < 20000; i++) {
           NSUInteger randomIndex = arc4random_uniform([weakSelf.arr count]);
            [weakSelf.arr insertObject:[NSNumber numberWithInt:i] atIndex:randomIndex];
       //     [weakSelf.arr addObject:[NSNumber numberWithInt:i]];
            NSLog(@"Added %@", weakSelf.arr[randomIndex]);
        }
        
        NSLog(@"Final count %ld", [self.arr count]);
    });
    
    [self performSelector:@selector(removeObjects) withObject:nil afterDelay:0.1];
}

-(void)removeObjects{
    __weak typeof(self) weakSelf = self;
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        for (int i = 0; i < 1000; i++) {
            if (weakSelf.arr.count > 1) {
                 NSUInteger randomIndex = arc4random_uniform([weakSelf.arr count]);
                [weakSelf.arr removeObjectAtIndex:randomIndex];
            }
            NSLog(@"Remove object");
        }
    });
}


@end