Question

I’ve got a method that takes in the annotations (custom PostLocationAnnotation class) to be displayed on a map view and clusters close ones together, outputting an array of MKAnnotation of PostLocationAnnotations and LocationGroupAnnotations (the clusters, which each contain some PostLocationAnnotations). Here’s how I call the function (from within an ‘updateAnnotations’ method, called when the viewport of the map changes):

[annotationsToAdd addObjectsFromArray:[ffMapView annotations]];        
[ffMapView addAnnotations:[self clusterAnnotations:annotationsToAdd WithEpsilon:20.0f andMinPts:4]];

annotationsToAdd is initially populated by the annotations that have been retrieved from the server that have not already been added to the map. Therefore I am passing the full list of the annotations that should be put on the map into the clusterAnnotations method. Here is the body of the method:

- (NSArray *)clusterAnnotations:(NSArray *)annotations WithEpsilon:(float)eps andMinPts:(int)minPts
{
    NSMutableSet *D = [[NSMutableSet alloc] initWithCapacity:[annotations count]];
    NSMutableArray *C = [[NSMutableArray alloc] init];

    for (id <MKAnnotation> annotation in annotations)
    {
        if ([annotation isKindOfClass:[PostLocationAnnotation class]])
        {
             NSMutableDictionary *dictEntry = [NSMutableDictionary dictionaryWithObjectsAndKeys:
                                              annotation, @"point",
                                              [NSNumber numberWithBool:NO], @"visited",
                                              [NSNumber numberWithBool:NO], @"noise",
                                              [NSNumber numberWithBool:NO], @"clustered", nil];

            [D addObject:dictEntry];

            [dictEntry release];
        } else if ([annotation isKindOfClass:[LocationGroupAnnotation class]])
        {
            for (PostLocationAnnotation *location in [(LocationGroupAnnotation *)annotation locations])
            {
                NSMutableDictionary *dictEntry = [NSMutableDictionary dictionaryWithObjectsAndKeys:
                                                  location, @"point",
                                                  [NSNumber numberWithBool:NO], @"visited",
                                                  [NSNumber numberWithBool:NO], @"noise",
                                                  [NSNumber numberWithBool:NO], @"clustered", nil];

                [D addObject:dictEntry];

                [dictEntry release];
            }
        }

    }

    for (NSMutableDictionary *P in D)
    {
        if ([P objectForKey:@"visited"] == [NSNumber numberWithBool:NO])
        {
             [P setValue:[NSNumber numberWithBool:YES] forKey:@"visited"];

             NSMutableSet *N = [[NSMutableSet alloc] initWithSet:[self regionQueryForPoint:P andEpsilon:eps fromList:D]];

             if ([N count] < minPts)
             {
                 [P setValue:[NSNumber numberWithBool:YES] forKey:@"noise"];
             } else {
                 LocationGroupAnnotation *newCluster = [[LocationGroupAnnotation alloc] initWithLocations:nil];
                 [C addObject:newCluster];
                 [self expandDbscanClusterWithPoint:P andRegion:N andCluster:newCluster andEpsilon:eps andMinPts:minPts fromList:D];

                 [newCluster release];
            }

            [N release];

        }
    }


    NSMutableArray *annotationsToAdd = [[[NSMutableArray alloc] initWithCapacity:[annotations count]] autorelease];

    for (NSMutableDictionary *P in D)
    {
        if ([P objectForKey:@"clustered"] == [NSNumber numberWithBool:NO])
        {
            [annotationsToAdd addObject:[P objectForKey:@"point"]];
        }
    }

    for (LocationGroupAnnotation *cluster in C)
    {
        [cluster updateCenterCoordinate];
    }

    [annotationsToAdd addObjectsFromArray:(NSArray *)C];

    [D release];
    [C release];

    return (NSArray *)annotationsToAdd;
}

When I run this I get a zombie message, and I have found that removing [D release] fixes the zombie but causes a leak. Looking at Instruments I can see that the memory address is first Malloc’d in clusterAnnotations, then retained and released a couple of times, then retained a large number of times by regionQueryForPoint (reaching a peak of 47 references), then released twice by clusterAnnotations, then released by [NSAutoreleasePool drain] until the refcount reaches -1 and I get the zombie message error. Here is the code for regionQueryForPoint:

- (NSSet *)regionQueryForPoint:(NSMutableDictionary *)P andEpsilon:(float)eps fromList:(NSMutableSet *)D
{
    NSMutableSet *N = [[[NSMutableSet alloc] init] autorelease];

    for (NSMutableDictionary *dictEntry in D)
    {
        if ((dictEntry != P) &&
            ([[dictEntry objectForKey:@"point"] isKindOfClass:[PostLocationAnnotation class]]))
        {
            CGPoint p1 = [ffMapView convertCoordinate:[[P objectForKey:@"point"] coordinate] toPointToView:self.view];
            CGPoint p2 = [ffMapView convertCoordinate:[[dictEntry objectForKey:@"point"] coordinate] toPointToView:self.view];

            float dX = p1.x - p2.x;
            float dY = p1.y - p2.y;

            if (sqrt(pow(dX,2)+pow(dY,2)) < eps)
            {
                [N addObject:dictEntry];
            }
        }
    }
    return (NSSet *)N;
}

The large number of retains appear to happen when regionQueryForPoint is called from the expandDbScanClusterWithPoint method, so I’ve included that here for completeness:

- (void)expandDbscanClusterWithPoint:(NSMutableDictionary *)P andRegion:(NSMutableSet *)N
                      andCluster:(LocationGroupAnnotation *)cluster
                      andEpsilon:(float)eps
                       andMinPts:(int)minPts
                        fromList:(NSMutableSet *)D
{

    [cluster addAnnotation:(PostLocationAnnotation *)[P objectForKey:@"point"]];
    [P setValue:[NSNumber numberWithBool:YES] forKey:@"clustered"];

    BOOL finished = NO;

    while (!finished)
    {
        finished = YES;

        for (NSMutableDictionary *nextP in N)
        {
            if ([nextP objectForKey:@"visited"] == [NSNumber numberWithBool:NO])
            {
                [nextP setValue:[NSNumber numberWithBool:YES] forKey:@"visited"];

                NSSet *nextN = [self regionQueryForPoint:nextP andEpsilon:eps fromList:D];

                if ([nextN count] >= minPts)
                {
                    [N unionSet:nextN];
                    finished = NO;
                    break;
                }
            }

            if ([nextP objectForKey:@"clustered"] == [NSNumber numberWithBool:NO])
            {
                [cluster addAnnotation:[nextP objectForKey:@"point"]];
                [nextP setValue:[NSNumber numberWithBool:YES] forKey:@"clustered"];
            }

        }
    }

}

I’ve been dissecting this for ages, counting references, watching pointers and everything but I just can’t work out how to safely release this D set. Can anyone see anything I’m not seeing?

Answer 1

You seem to be over-releasing dictEntry with [dictEntry release];. When using dictionaryWithObjectsAndKeys, you’re getting an autoreleased object back. So releasing it again will decrease the retain count.

EDIT: If you’re unsure how it works and when you’re actually retaining objects, you might want to have a look at the memory management docs:

You create an object using a method whose name begins with “alloc”,
“new”, “copy”, or “mutableCopy” (for example, alloc, newObject, or
mutableCopy).