iOS multiple threads is not safety with nonatomic properties.

ThreadNotSafetyTests.m

#import <XCTest/XCTest.h>

@interface ThreadNotSafetyTests : XCTestCase

// no crash
//@property (atomic) NSObject *obj;

// crash
@property (nonatomic) NSObject *obj;
@end

@implementation ThreadNotSafetyTests

- (void)testExample {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        for (int i=0; i < 1000000; i++) {
            self.obj = NSObject.new;
        }
    });
    for (int i=0; i < 1000000; i++) {
        self.obj = NSObject.new;
    }
}

@end