EXC_BAD_ACCESS objective-c块

Question

I run this code inside my viewDidLoad method to fetch data from Firebase to put it in a UIPageViewController

@interface MapViewController () <RoutesPageDelegate>
@property (weak, nonatomic) IBOutlet MKMapView *mapView;
@property (weak, nonatomic) RoutesPageViewController *routesPageViewController;
@property (weak, nonatomic) FIRFirestore *db;
@end

@implementation MapViewController

- (void) viewDidLoad {
    [super viewDidLoad];

    self.db = [FIRFirestore firestore];

    for (UIViewController *obj in self.childViewControllers) {
        if ([obj isKindOfClass:[RoutesPageViewController class]]) {
            self.routesPageViewController = (RoutesPageViewController *)obj;
            self.routesPageViewController.routesPageDelegate = self;
        }
    }

    FIRCollectionReference *routesRef = [self.db collectionWithPath:@"routes"];
    [routesRef getDocumentsWithCompletion:^(FIRQuerySnapshot * _Nullable snapshot, NSError * _Nullable error) {
        if (error != nil) {
            // TODO: handle error
        } else {
            NSMutableArray<RouteModel*> *routes = [NSMutableArray array];

            // For each route
            for (FIRDocumentSnapshot *document in snapshot.documents) {
                RouteModel *route = [[RouteModel alloc] init];
                route.title = document.data[@"title"];
                route.color = document.data[@"color"];
                route.city = document.data[@"city"];

                [routes addObject:route];
            }

            [self.routesPageViewController setRoutes:routes];
        }
    }];


}

And this is the called setRoutes method:

- (void) setRoutes:(NSMutableArray<RouteModel *> *)routes {
    self.routes = routes;

    NSMutableArray<RoutePageViewController *> * routeViewControllers = [NSMutableArray array];
    for (RouteModel * route in routes) {
        [routeViewControllers addObject:[self viewControllerAtIndex:[routes indexOfObject:route]]];
    }

    [self setViewControllers:routeViewControllers direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:nil];
}

When the setRoutes method gets executed it throws the error in the image below, saying that it cannot dereference it:

The setRoutes methods gets executed inside a block.

And I get this weird thread stack:

How can I solve this?

Answer 1

Your problem is here:

- (void) setRoutes:(NSMutableArray<RouteModel *> *)routes {
    self.routes = routes;

invoking self.routes implicity calles the setter setRoutes which causes a recursive infinite calls as indicated by your stack.

Answer 2

At the time the block is passed onto getDocumentsWithCompletion method is executed, routes array has already been deallocated and set to nil because no one is retaining it anywhere outside the block.

You should either move it into the block or declare it as a class property so it won't be thrown out of memory while class instance is alive.

[routesRef getDocumentsWithCompletion:^(FIRQuerySnapshot *snapshot, NSError *error) {
    NSMutableArray<RouteModel*> *routes = [NSMutableArray array];
    ...
    [self.routesPageViewController setRoutes:routes];
}];

After the update:

Doing self.routes = routes invokes setRoutes: which in turn is causing a loop in your code. You should change it to:

- (void)setRoutes:(NSMutableArray<RouteModel *> *)routes {
  if (_routes != routes) {
    _routes = routes;
    ...
  }
}