app crashes when retrieving the ALAssets from ALAssetsGroupSavedPhotos group up on receiving ALAssetsLibraryChangedNotification

Question

Part of my app has a photo browser similar to Apple's Photos app (Grid like view). To refresh my photos whenever there is any change in original photo app, i registered for ALAssetsLibraryChangedNotification

self.assetsLibrary = [[ALAssetsLibrary alloc] init];

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveLibraryChangedNotification:) name:ALAssetsLibraryChangedNotification object:self.assetsLibrary];

In "receiveLibraryChangedNotification" method - i check if the userInfo has ALAssetLibraryUpdatedAssetsKey & then call the refresh photos method.

 - (void) receiveLibraryChangedNotification:(NSNotification *) notif{
        NSDictionary *userInfo = [notif userInfo];
        if(userInfo){
            id updatedAssets = [userInfo objectForKey:ALAssetLibraryUpdatedAssetsKey];

            if(updatedAssets){

                   [self refreshPhotos];
                }
    }




- (void) refreshPhotos {

    self.assetArray = nil;

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^(void){
                [self.assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {

                    if(group){
                        [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, [group numberOfAssets])] options:0 usingBlock:^(ALAsset *result, NSUInteger index, BOOL *shouldStop) {
                            if(result){

                                if([[result valueForProperty:@"ALAssetPropertyType"] isEqualToString:@"ALAssetTypePhoto"]){
                                    [self.assetArray addObject:result];
                                }
                            }
                        }];
                    }
                } failureBlock:^(NSError *error) {

                    DebugLog(@"error >>> %@",[error description]);

                }];
            });
}

Problem i am facing is that some times the notification is being triggered multiple times & the app crashes at

[self.assetArray addObject:result];

with error -

malloc: *** error for object 0x4aa9000: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug

or

malloc: *** error for object 0x16fd3e74: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug

also some times i am not receiving the ALAssetLibraryUpdatedAssetsKey in the userInfo of notification, so the photos are never refreshed.

can some one guide me in correct direction.

Thanks in advance.

Answer 1

Allocate self.assetArray after the nil assignment or write [self.assetArray removeAllObjects] instead of assigning it to nil.

- (void) refreshPhotos {

    self.assetArray = nil;
    self.assetArray = [[NSMutableArray alloc] init];

    //or remove the top 2 lines and try `[self.assetArray removeAllObjects]`

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^(void){
            [self.assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop) {

                if(group){
                    [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, [group numberOfAssets])] options:0 usingBlock:^(ALAsset *result, NSUInteger index, BOOL *shouldStop) {
                        if(result){

                            if([[result valueForProperty:@"ALAssetPropertyType"] isEqualToString:@"ALAssetTypePhoto"]){
                                [self.assetArray addObject:result];
                            }
                        }
                    }];
                }
            } failureBlock:^(NSError *error) {

                DebugLog(@"error >>> %@",[error description]);

            }];
        });
}

also some times i am not receiving the ALAssetLibraryUpdatedAssetsKey in the userInfo of notification,

there are four keys used to get values from the user information dictionary of the ALAssetsLibraryChangedNotification notification.

NSString * const ALAssetLibraryUpdatedAssetsKey;
NSString * const ALAssetLibraryInsertedAssetGroupsKey;
NSString * const ALAssetLibraryUpdatedAssetGroupsKey;
NSString * const ALAssetLibraryDeletedAssetGroupsKey;