如何从块内部修改非局部（全局）变量？

Question

I am quite new to Objective-C & have to dynamically change the value of

@property (strong, nonatomic) NSMutableArray *allCategories

from inside of AFHTTPRequestOperationManager in success block.
[self.allCategories addObject:tempObject];
doesn't change the value of allCategories while iterating in a loop.

The variable has been initialized as
self.allCategories = [[NSMutableArray alloc]init];
in viewDidLoad.

I have also tried creating a temporary variable as __block NSMutableArray *tempCategories = [[NSMutableArray alloc]init]; before initiating AFHTTPRequestOperationManager object. tempCategories doesn't even retain its value.

Can't figure out what's happening.

Edit
Sorry for inconvenience
viewDidLoad has the following code
self.allCategories = [[NSMutableArray alloc]init];
[self loadData];

Here's the code

-(NSMutableArray *)loadData
{
    __block NSMutableArray *tempCategories = [[NSMutableArray alloc]init];

    manager = [AFHTTPRequestOperationManager manager];

    [manager GET:kAPICategoryList
      parameters:nil
         success:^(AFHTTPRequestOperation *operation, id responseObject) {

             // downcast id to NSMutableDictionary
             NSMutableDictionary *json = (NSMutableDictionary *)responseObject;

             // check if dictionary is non nil has at least 1 element
             if (json != nil && [json count] >= 1) {

                 // NSLog(@"json:\t%@", json);

                 // check json is non nil & has success message
                 if ([json objectForKey:kAPIKeyCategoryRoot] != nil) {

                     NSArray *arrCategoriesRoot = [json objectForKey:kAPIKeyCategoryRoot];

                     // check categories has some data
                     if (arrCategoriesRoot.count >= 1) {

                         for (int i = 0; i < arrCategoriesRoot.count; i++) {

                             SomeModel *pCategory;

                             NSDictionary *dctCategorySingle = [arrCategoriesRoot objectAtIndex:i];

                             // check category has sub category
                             if ([dctCategorySingle objectForKey:kAPIKeyCategorySubCategory] != nil) {
                                 // create category with sub category
                                 pCategory = [[SomeModel alloc]initWithSubCategorisedCategoryID:[dctCategorySingle objectForKey:kAPIKeyCategoryID]
                                                                                              name:[dctCategorySingle objectForKey:kAPIKeyCategoryName]
                                                                                             image:kIMGCategoryDefault
                                                                                       subCategory:[dctCategorySingle objectForKey:kAPIKeyCategorySubCategory]];
                             } else{
                                 // create just a category
                                 pCategory = [[SomeModel alloc]initWithCategoryID:[dctCategorySingle objectForKey:kAPIKeyCategoryID]
                                                                                name:[dctCategorySingle objectForKey:kAPIKeyCategoryName]
                                                                               image:kIMGCategoryDefault];
                             } // else just
                             [tempCategories addObject:pCategory];
                             [_allCategories addObject:pCategory];
                         } // for
                         NSLog(@"categories count %lu", [self.allCategories count]);
                     } // if count >= 1
                 }
                 else if ([json objectForKey:kAPIRespMsgCategoryFetchErrKey] != nil) {
                     [Utility showAlertWithTitle:kAPIRespMsgCategoryFetchErrKey
                                         message:[json objectForKey:kAPIRespMsgCategoryFetchErrVal]
                                          button:kMsgButtonOkayTtl];
                 }
             } else {
                 // error in login => enable login
                 NSLog(@"%@", kMsgNetworkEmptyJSON);
             }
         }
     // network error
         failure:^(AFHTTPRequestOperation *operation, NSError *error) {
            NSLog(@"error %@", [error localizedDescription]);
         }];
    NSLog(@"tempCategories count %lu", [tempCategories count]);
    return tempCategories;
}

Here's the output form NSLog:

2015-03-19 18:27:17.845 MyProject[4011:121268] viewDidLoad
2015-03-19 18:27:18.133 MyProject[4011:121268] tempCategories count 0
2015-03-19 18:27:18.136 MyProject[4011:121268] numberOfRowsInSection count 0
2015-03-19 18:27:18.137 MyProject[4011:121268] numberOfRowsInSection count 0
2015-03-19 18:27:19.019 MyProject[4011:121268] categories count 20

when loadData finishes allCategories has not data in it (nil).

Answer 1

As far as I know it should work that way.. are you sure your success block is being called before you check the content of allCategories ?

A success block work asynchronously, which means it will be executed only when the RequestOperation is completed (which can take a long time if you're downloading something big)

If you are trying to get the value of allCategories before the success block is executed you won't get what you're expecting. I would recommend using breakpoints or NSLog on your success block to see if it's been executed when you think it's doing it.

eg

 ...
 successBlock:^(AFHTTPRequestOperation *operation, id responseObject)
 {
     NSLog(@"Success");
     [self.allCategories addObject:tempObject]
  }]; //End of request

[operation start]; //Begin executing the AFHTTPOperation

NSLog("%@",self.allCategories.description); //probably nil or empty
//since the success block hasn't been called yet  

EDIT:

As I though, you are returning a value before is been set by the async operation, to return a value from an async operation I would suggest take a look to this answer and this one . Also you should read a bit of how async task work.

Basically what you want to do with async operations/tasks is make sure the value will be available when you want to use it. The main issue with that is that you don't know when the value will be set, but you can make sure what you want to do whenever it's set.

To do that you can create a simple method with a custom completion block

- (void)myCustomMethodWithCompletionBlock: (void (^)(NSArray *))completion {
   //Do your request
   //...
   successBlock:^(AFHTTPRequestOperation *operation, id responseObject)
  {
      NSLog(@"Success");
      completionBlock(allCategories);
   }]; //End of request
}

Meanwhile in your main method you call

 [self myCustomMethodWithCompletionBlock:^(NSArray *allCategories) {
    self.allCategories = allCategories;
    //Do other stuff you need to with that variable since now you are
    //sure the value will be set unless the operation failed
   }];

Answer 2

Try this:

dispatch_async(dispatch_get_main_queue(), ^{
 [self.allCategories addObject:tempObject]; 
});

Answer 3

I had the same problem a few days ago. My problem was my array seems nil, array allocations in viewdidload method may be your request run before viewDidLoad. Check it with debug if you see the array is nill then alloc array different place. PS: I m not expert but may be it's the same problem with me.

Answer 4

Define NSMutableArray with following line.

@property (nonatomic, strong) NSMutableArray * arrData;

initialize in viewDidLoad

- (void)viewDidLoad {

    [super viewDidLoad];

    self.arrData = [NSMutableArray array];
}

call following method with any UIButton action for see output OR working behavior

- (void) TestMethod {

    dispatch_queue_t queue = dispatch_queue_create("myQueue", 0);

    dispatch_async(queue, ^{
        AFHTTPClient *httpClient = [[AFHTTPClient alloc] initWithBaseURL:[NSURL urlWithEncoding:@"https://www.google.co.in/?gws_rd=ssl"]];
        [httpClient registerHTTPOperationClass:[AFJSONRequestOperation class]];
        [httpClient setDefaultHeader:@"Accept" value:@"application/json"];
        [httpClient setParameterEncoding:AFJSONParameterEncoding];
        NSMutableURLRequest *request = [httpClient requestWithMethod:@"GET" path:@"" parameters:nil];
        [request setTimeoutInterval:180];
        [AFJSONRequestOperation addAcceptableContentTypes:[NSSet setWithObject:@"text/html"]];
        dispatch_semaphore_t sema = dispatch_semaphore_create(0);

        AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON)
                                             {
                                                 [self.arrData addObject:[NSDictionary dictionaryWithObjectsAndKeys:@"test",@"t3da",@"adsf",@"afds", nil]];
                                                 dispatch_semaphore_signal(sema);
                                             } failure:^ (NSURLRequest *request, NSURLResponse *response, NSError *error, id json){

                                                 [self.arrData addObject:[NSDictionary dictionaryWithObjectsAndKeys:@"test",@"t3da",@"adsf",@"afds", nil]];
                                                 dispatch_semaphore_signal(sema);
                                             }];

        [operation start];

        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
        DLog(@"arrData = %@",self.arrData);
    });
}