How to retrieve iPhone IDFA from API?

First of all:

#import <AdSupport/ASIdentifierManager.h> 

If you would like to get it as an NSString, use:

[[[ASIdentifierManager sharedManager] advertisingIdentifier] UUIDString]

So your code might look like this:

NSString *idfaString = [[[ASIdentifierManager sharedManager] advertisingIdentifier] UUIDString];

You first have to check if user user has decided to opt out from ad tracking. Only if he allowed it you can use the IDFA.

You can check it by calling isAdvertisingTrackingEnabled method of ASIdentifierManager.

isAdvertisingTrackingEnabled

Check the value of this property before performing any advertising tracking. If the value is NO, use the advertising identifier only for the following purposes: frequency capping, conversion events, estimating the number of unique users, security and fraud detection, and debugging.

The following code snippet shows how to obtain a string value of IDFA.

ObjC

@import AdSupport;- (NSString *)identifierForAdvertising {    // Check whether advertising tracking is enabled    if([[ASIdentifierManager sharedManager] isAdvertisingTrackingEnabled]) {        NSUUID *identifier = [[ASIdentifierManager sharedManager] advertisingIdentifier];        return [identifier UUIDString];    }    // Get and return IDFA    return nil;}

Swift

import AdSupportfunc identifierForAdvertising() -> String? {    // Check whether advertising tracking is enabled    guard ASIdentifierManager.shared().isAdvertisingTrackingEnabled else {        return nil    }    // Get and return IDFA    return ASIdentifierManager.shared().advertisingIdentifier.uuidString}

ASIdentifierManager is the official way to garner the Advertising Identification Number from a device running iOS 6+. You can use -[[ASIdentifierManager sharedManager] advertisingIdentifier]; to get it.