How to handle autoreleased objects in iOS with ARC

Question

I am creating an iPhone application with ARC enabled, in this I am having a situation.

In each page of the application, a web service call is happening. And in this method I am removing all the objects from the arrays before adding new value from the server. All is working fine, but sometimes app is crashing at [self.myArray removeAllObjects].

I have set @property for myArray @property (strong, nonatomic) NSMutableArray myArray;

What I am thinking is, as I am using ARC, the object, myArray, is got released at some point and I try to remove all the objects from the same array. That causing the crash, I am not sure but I don't see any other reason for this.

So, I was thinking about to check, whether the array is valid before I remove the objects in it. I wrote a sample code to check the different scenarios. Here it is:

NSMutableArray *testArray = [[NSMutableArray alloc]initWithObjects:@"1", @"2", @"3", @"4", nil];

if (testArray) {
    NSLog(@"i am alive");
}

[testArray release];

if (testArray) { //here how to check whether this array is valid or not?
    NSLog(@"i am alive: %@", testArray);
}else{
    NSLog(@"I am dead");
    testArray = [[NSMutableArray alloc]initWithObjects:@"5", @"6", @"7", @"8", nil];
}

[testArray release];
[testArray removeAllObjects];

I know it will cause crashes, but it is just for checking. Here, how can I check whether the array is valid? Is this is a right approach or something else?

Please guide me.
Thanks.

Actual Code:

- (void)getFriendsList{

    BOOL netIsAvailable = [self connected];

    if (netIsAvailable) {
        @try {
            NSString *accessToken = [self getAccessToken];
            NSString *tokenEncoded = [accessToken stringByReplacingOccurrencesOfString:@"\"" withString:@""];
            NSString *finalUrl = [NSString stringWithFormat:@"%@FriendsnSongs/%@",CommonWebServiceUrl,tokenEncoded];
            NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:finalUrl] cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:10];
            NSURLResponse *response = nil;
            NSError *error = nil;
            NSData *currentResult = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
//            NSString* responseString = [[NSString alloc] initWithData:currentResult encoding:NSUTF8StringEncoding];

            if (currentResult != nil) {
                NSDictionary* json = [NSJSONSerialization
                                      JSONObjectWithData:currentResult
                                      options:kNilOptions
                                      error:&error];
                NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
                int statusCode = [httpResponse statusCode];

                NSMutableArray *match = [json valueForKey:@"FriendsnSongsResult"];
                if (statusCode == 200) {
                    if (self.userGuidArray) {
                        [self.albumNameArray removeAllObjects];  //here got crash
                        [self.artistNameArray removeAllObjects];
                        [self.deviceNameArray removeAllObjects]; //here also got crash once
                        [self.nickNameArray removeAllObjects];
                        [self.userProfileImageArray removeAllObjects];
                        [self.songTitleArray removeAllObjects];
                        [self.songStatusArray removeAllObjects];
                        [self.userGuidArray removeAllObjects];
                    }

                    for(NSArray *player in match) {
                        [self.albumNameArray addObject:[(NSArray *)player valueForKey:@"AlbumName"]];
                        [self.artistNameArray addObject:[(NSArray *)player valueForKey:@"Artist"]];
                        [self.nickNameArray addObject:[(NSArray *)player valueForKey:@"NickName"]];
                        [self.userProfileImageArray addObject:[(NSArray *)player valueForKey:@"ProfileImage"]];
                        [self.songStatusArray addObject:[(NSArray *)player valueForKey:@"Status"]];
                        [self.songTitleArray addObject:[(NSArray *)player valueForKey:@"Title"]];
                        [self.userGuidArray addObject:[(NSArray *)player valueForKey:@"UserGuid"]];
                        [self.deviceNameArray addObject:[(NSArray *)player valueForKey:@"DeviceName"]];
                    }
                    //Start timer for updating the friends list
                    if (timerActivated)
                        [self performSelectorOnMainThread:@selector(backgroundFriendsListUpdate) withObject:nil waitUntilDone:NO];
                    //Update table if the table not in search mode
                    if (!isSearching)
                        [self performSelectorOnMainThread:@selector(updateTable) withObject:nil waitUntilDone:YES];
                }
            }
        }
        @catch (NSException *exception) {
            NSLog(@"Exception: %@",exception.name);
        }
    }
    [DejalActivityView removeView];
}

And the important point is, in this class a background thread is running in every 30 seconds. It will call this same method for refreshing the table.

Answer 1

As you suggested in the comments, using temporary array is a good idea. The following code will fill a temporary array and use it to replace your array in main thread.

NSMutableArray *tempAlbumArray = [NSMutableArray array];
//fill your array
[tempAlbumArray addObject:<#some object#>];
dispatch_async(dispatch_get_main_queue(), ^{
    //replace your array with the new one
    //this code will be executed in main thread
    self.albumNameArray = tempAlbumArray;
});

Answer 2

If you are using ARC then Remove [testArray release]; this line from your code because ARC handle it. Otherwise [testArray release]; write in -(void)dealloc method.

Give condition before remove Array Value

if(myArray.count > 0)// because sometime it is good logic for us.
  [myArray removeAllObjects];

And then insert data in myArray.