NSData writeToFile make synchronous or find out when it completes - objective-c

Question

I know there are a few questions asked on the behavior of NSData writeToFile atomically: and how that is supposed to be a SYNC method.
But in my experience it is not behaving like that. I've included pseudo code of my method that will accept and array of file names and will save them from Camera Roll to Documents folder of the app.
The For loop runs through each item in array and calls 'writeToFile'. My NSLogs show that the loop runs through and the NSLog message after the loop completion is displayed somewhere in the middle of the files being saved. for eg: I pass 4 images to this array to save, the NSLogs show that first file was saved and then it displays log that says it reached end of For loop and then the rest of the files being saved is displayed.
My question is - how do I make it synchronous or setup a completion handler of some sort that will only get called when the file write has completely finished. I need to eventually upload the files once they've been saved in the apps documents folder. Rest of the logic is in place. I just am having trouble finding the moment when saving of all files has completed.
I do not want to do a hack and set a delay because my file sizes and number of files can vary widely. Any guidance on this is much appreciated. Thanks
//'each item in 'arrayOfSelectedMedia' holds a dictionary of keys 'imageData' ,absolutePathToFileData' and 'filename'
-(void) saveArrayOfFiles: (NSMutableArray*) arrayOfSelectedMedia
toLocation: (NSString*) subFolder
whenCompletePostNotificationTo:(NSString*)backToCaller{
NSError *error;
if (![[NSFileManager defaultManager] fileExistsAtPath:globalChatsMediaFolder]){
[[NSFileManager defaultManager] createDirectoryAtPath:mediaFolder withIntermediateDirectories:NO attributes:nil error:&error]; //Create folder
}
else{
NSLog(#"folder %# exists already",mediaFolder);
}
NSString *completePath = [globalChatsMediaFolder stringByAppendingPathComponent:subFolder];
if (![[NSFileManager defaultManager] completePath]){
[[NSFileManager defaultManager] completePath withIntermediateDirectories:NO attributes:nil error:&error]; //Create folder
NSLog(#"creating folder %# ", completePath);
}
else{
NSLog(#"folder %# exists already", completePath);
}
/******* go through all files in array and save it to their specific folder ***********/
for(int i=0; i < [arrayOfSelectedMedia count]; i++){
NSString *saveFileAs = [NSString stringWithFormat:#"%#",[arrayOfSelectedMedia[i] objectForKey:#"filename"]];
NSURL *absolutePathToSelectedFile = [arrayOfSelectedMedia[i] objectForKey:#"absolutePathToFileInCameraRoll"];
// Create assets library
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init] ;
// Try to load asset at mediaURL
[library assetForURL:absolutePathToSelectedFile resultBlock:^(ALAsset *asset) {
// If asset exists
if (asset) {
// Type your code here for successful
ALAssetRepresentation *rep = [asset defaultRepresentation];
Byte *buffer = (Byte*)malloc((NSUInteger)rep.size);
NSUInteger buffered = [rep getBytes:buffer fromOffset:0.0 length:(NSUInteger)rep.size error:nil];
NSData *dataToUpload = [NSData dataWithBytesNoCopy:buffer length:buffered freeWhenDone:YES];
NSLog(#"%s , A direct BYTE size of file %d bytes", __PRETTY_FUNCTION__, [dataToUpload length]);
NSLog(#"absolute path to asset is %# ", asset.defaultRepresentation.url);
NSString *absolutePathToFile = [completePath stringByAppendingPathComponent:saveFileAs];
BOOL writeStatus = [dataToUpload writeToFile:absolutePathToFile options:NSDataWritingAtomic error:&error];
if(writeStatus){
NSLog(#"file %# with %lu bytes was successfully saved locally", saveFileAs, (unsigned long)[dataToUpload length]);
}
else{
NSLog(#"write to local file failed: %#",error);
}
}
else {
// Type your code here for not existing asset
NSLog(#"%s, File at path '%#' could not be found.",__PRETTY_FUNCTION__,absolutePathToSelectedFile);
}
} failureBlock:^(NSError *error) {
// Type your code here for failure (when user doesn't allow location in your app)
NSLog(#"%s, failure to read file at path: %#. Check if user has allowed access to file location.",__PRETTY_FUNCTION__,absolutePathToSelectedFile);
}];
} // end of For loop
NSLog(#"Finished saving array of Files to local Disk. back to caller provided as '%#'",backToCaller);
if(backToCaller.length > 0){ // i.e. it was not set to nil and I need to post notification
NSLog(#"posting notification to go back to Caller %#", backToCaller);
[[NSNotificationCenter defaultCenter] postNotificationName:backToCaller object:nil userInfo:nil];
}
}