Shift responsibility of IDFA collection to clients

[sjmadsen]

What does this PR do? This is a backport from segmentio/analytics-ios (https://github.com/segmentio/analytics-ios/pull/892).

Any references to Apple's AdSupport framework might trip the App Store's static analysis, making it a bit unclear if SDK users can confidently say that their apps do not use the IDFA. By moving the responsibility to collect the IDFA out of the PostHog SDK and into the client app, the AdSupport references are gone and the question is easily answered.

Where should the reviewer start? It's a pretty small PR, but note that the PHGIDFA() function is gone and the caller of that function now checks for and calls a block provided as part of the SDK initialization.

How should this be manually tested? With no changes to a client app, trigger an event, and verify that $device_advertisingId no longer has any value.

Then modify the app to provide a block for adSupportBlock in the PostHog configuration. Triggering an event should now include the return value from this block as the value for $device_advertisingId.

Any background context you want to provide?

What are the relevant tickets?

4 - Remove references to AdSupport framework

Screenshots or screencasts (if UI/UX change) n/a

Questions:

• Does the docs need an update? Yes; if apps want to collect the IDFA from the user's device, merely setting enableAdvertisingCapturing is no longer sufficient.
• Are there any security concerns? No.
• Do we need to update engineering / success? No?

[timgl]

@sjmadsen Awesome, thanks so much for contributing this.

@mariusandra would you mind reviewing this?

[mariusandra]

Thank you @sjmadsen , this is awesome!

Could you just also provide an example that we could put in the docs for users who want to use this AdSupport framework? Preferably for both Objective-C and Swift...

[sjmadsen]

I sure can. It's probably worth calling out separately from the "all configuration options" section now, since it's not enough to use the defaults and still get a value from clients for $device_advertiserId.

To capture the IDFA in Objective-C, SDK clients will want to do something like this:

// At the top of your .m file
@import AdSupport;  // or #import <AdSupport/ASIdentifierManager.h>

// During PostHog initialization
PHGPostHogConfiguration` *configuration = [PHGPostHogConfiguration configurationWithApiKey:@"YOUR_API_KEY"];
configuration.enableAdvertisingCapturing = YES;
configuration.adSupportBlock = ^{
    return [[ASIdentifierManager sharedManager].advertisingIdentifier uuidString];
};

Or in Swift:

// At the top of your .swift file
import AdSupport

// During PostHog initialization
let configuration = PHGPostHogConfiguration(apiKey: "YOUR_API_KEY")
configuration.enableAdvertisingCapturing = true
configuration.adSupportBlock = {
    return ASIdentifierManager.shared().advertisingIdentifier.uuidString
}