Copying Swift arrays from background to foreground - ios

Question

If we go from Swift background to foreground, what is the proper way to [nsObject copy] in Swift?
For example in Objective-C, we would loop through a long array of ALAssets (say like 10,000+) in the background by doing:
[alGroup enumerateAssetsUsingBlock:^(ALAsset *alPhoto, NSUInteger index, BOOL *stop)
{
// Here to make changes for speed up image loading from device library...
// =====================================================
// >>>>>>>>>>>>>>>>>>> IN BACKGROUND <<<<<<<<<<<<<<<<<<<
// =====================================================
if(alPhoto == nil)
{
c(#"number of assets to display: %d", (int)bgAssetMedia.count);
// c(#"All device library photos uploaded into memory...%#", bgAssetMedia);
dispatch_async(dispatch_get_main_queue(), ^(void)
{
// =====================================================
// >>>>>>>>>>>>>>>>>>> IN FOREGROUND <<<<<<<<<<<<<<<<<<<
// =====================================================
[ui hideSpinner];
if (_bReverse)
// Here we copying all the photos from device library into array (_assetPhotos)...
_assetPhotos = [[NSMutableArray alloc] initWithArray:[[[bgAssetMedia copy] reverseObjectEnumerator] allObjects]];
else
_assetPhotos = [[NSMutableArray alloc] initWithArray:[bgAssetMedia copy]];
// NSLog(#"%lu",(unsigned long)_assetPhotos.count);
if (_assetPhotos.count > 0)
{
result(_assetPhotos);
}
});
} else {
// if we have a Custom album, lets remove all shared videos from the Camera Roll
if (![self isPhotoInCustomAlbum:alPhoto])
{
// for some reason, shared glancy videos still show with 00:00 minutes and seconds, so remove them now
BOOL isVideo = [[alPhoto valueForProperty:ALAssetPropertyType] isEqual:ALAssetTypeVideo];
int duration = 0;
int minutes = 0;
int seconds = 0;
// NSString *bgVideoLabel = nil;
if (isVideo)
{
NSString *strduration = [alPhoto valueForProperty:ALAssetPropertyDuration];
duration = [strduration intValue];
minutes = duration / 60;
seconds = duration % 60;
// bgVideoLabel = [NSString stringWithFormat:#"%d:%02d", minutes, seconds];
if (minutes > 0 || seconds > 0)
{
[bgAssetMedia addObject:alPhoto];
}
} else {
[bgAssetMedia addObject:alPhoto];
}
}
}
// NSLog(#"%lu",(unsigned long)bgAssetMedia.count);
}];
Then, we would switch to the foreground to update the UIViewController, which are these lines in the above snippet:
_assetPhotos = [[NSMutableArray alloc] initWithArray:[bgAssetMedia copy]];
The "copy" function was the black magic that allowed us to quickly marshal the memory from background to foreground without having to loop through array again.
Is there a similar method in Swift? Perhaps something like this:
_assetPhotos = NSMutableArray(array: bgAssetMedia.copy())
Is Swift thread safe now for passing memory pointers from background to foreground? What's the new protocol? Thank you-

Answer 1

I found the answer. After running large queries on the Realm and CoreData database contexts. I found it easy to just make a basic copy of the memory pointer and downcast it to match the class.
let mediaIdFG = mediaId.copy() as! String
Full example in context below:
static func createOrUpdate(dictionary:NSDictionary) -> Promise<Media> {
// Query and update from any thread
return Promise { fulfill, reject in
executeInBackground {
// c("BG media \(dictionary)")
let realm:RLMRealm = RLMRealm.defaultRealm()
realm.beginWriteTransaction()
let media = Media.createOrUpdateInRealm(realm, withJSONDictionary:dictionary as [NSObject : AnyObject])
// media.type = type
c("BG media \(media)")
let mediaId = media.localIdentifier
do {
try realm.commitWriteTransaction()
executeInForeground({
let mediaIdFG = mediaId.copy() as! String
let newMedia = Media.findOneByLocalIdentifier(mediaIdFG)
c("FG \(mediaIdFG) newMedia \(newMedia)")
fulfill(newMedia)
})
} catch {
reject( Constants.createError("Realm Something went wrong!") )
}
}
} // return promise
} // func createOrUpdate
Posting my own answer to let you know my findings. I also found this helpful article about Swift's copy() aka objc's copyWithZone: https://www.hackingwithswift.com/example-code/system/how-to-copy-objects-in-swift-using-copy