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

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;