Sending Engagements on iOS



Integration of Promoted’s metrics logging library with your iOS app will involve four steps. The implementations and risks of each step are discussed below.

  1. Install build dependencies.
  2. Initialize at app startup.
  3. Add click, user, and view logging.
  4. Add impression logging to restaurants, promotions, and food items.

Promoted adheres to the following philosophies to mitigate risk:

  1. Smaller PRs whenever possible, to make review and rollback easier.
  2. Don’t merge PRs until they provide additional functionality.
  3. Adopt conventions of the surrounding code when making changes.
  4. Make no additional functionality changes to code when implementing logging, unless otherwise agreed upon.
  5. Requiring review and approval of PRs by your developers prior to merging, unless otherwise agreed upon.



Install the pod PromotedAIMetricsSDK in your Podfile.

target 'MyApp' do
  pod 'PromotedAIMetricsSDK'

Swift Package Manager

Add the following dependency to your Package.swift file.

.package(name: "PromotedAIMetricsSDK", url: "")

This will automatically install SwiftProtobuf and GTMSessionFetcher as transitive dependencies.

NPM (React Native)

Add a dependency on @promotedai/react-native-metrics in your package.json file. This will in turn add Cocoapod dependencies in your iOS project. These dependencies are the same as when installing directly via Cocoapods (see above).

In addition to the automated Cocoapod dependencies, we need to add the following line in your Podfile manually to work around a build issue.

pod 'GTMSessionFetcher/Core', '~> 1.5.0', :modular_headers => true

This pod is not a new dependency. It’s already included in PromotedAIMetricsSDK, but we need to add it to the Podfile to work around a build issue.


Although startup is inexpensive, it may involve network calls and disk IO, so you can choose when this startup occurs.


If you attempt to use the Promoted logging library without configuring it, a runtime error will occur.


Start the MetricsLoggerService during your app’s initialization, before you attempt to perform any logging. You can start the service at any time you wish. A common pattern would be to do this in your AppDelegate’s application(_:didFinishLaunchingWithOptions:) method, but you can control the exact time of initialization by deferring this call until after your critical startup path occurs.

func application(_ app: UIApplication, didFinishLaunchingWithOptions:...) {
  let config = ClientConfig()
  config.metricsLoggingURL = "https://..."
  config.metricsLoggingAPIKey = "..."
  self.promotedMetricsLoggerService = MetricsLoggerService(initialConfig: config)
  // The following line does most initialization. You decide when to call it.

React Native

Installing our NPM dependency will require you to instantiate and configure PromotedMetricsModule, our NativeModule, at app startup. (Explicit configuration is required because of the server URL and API key.) This can be done at app startup, in your AppDelegate’s application(_:didFinishLaunchingWithOptions:) method.

- (BOOL)application:(UIApplication *)application 
    didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  RCTBridge *bridge = [[RCTBridge alloc] initWithDelegate:self
  RCTRootView *rootView = [[RCTRootView alloc] initWithBridge:bridge

- (NSArray<id<RCTBridgeModule>> *)extraModulesForBridge:(RCTBridge *)bridge {
  PROClientConfig *config = [[PROClientConfig alloc] init];
  config.metricsLoggingURL = @"https://...";
  config.metricsLoggingAPIKey = @"...";
  config.devMetricsLoggingURL = @"https://...";
  config.devMetricsLoggingAPIKey = @"...";
  PROMetricsLoggerService *service =
      [[PROMetricsLoggerService alloc] initWithInitialConfig:config];
  [service startLoggingServices];
  PromotedMetricsModule *module =
      [[PromotedMetricsModule alloc] initWithMetricsLoggerService:service];
  return @[ module ];

Since React Native automatically initializes all NativeModules at startup, adding the Promoted dependency alone will cause PromotedMetricsModule to be created.


We’re exploring options for clients to configure logging directly from React Native. For now, this configuration must be done in native code.


Usage of these classes is primarily for UIKit-based apps. For React Native, much of the functionality of MetricsLogger and ScrollTracker are found in the PromotedMetrics module.


Entry point to the Promoted metrics library. Configures a logging session and its associated MetricsLogger.

Typically, instances of MetricsLoggers are tied to a MetricsLoggerService, which configures the logging environment and maintains a MetricLogger for the lifetime of the service.

The service also provides a facility to create ImpressionLoggers and ScrollTrackers.


Create and configure the service when your app starts, then retrieve the MetricsLogger instance from the service after it has been configured. You can also create ImpressionLogger or ScrollTracker instances using the service.

You may choose to make MetricsLoggerService a singleton, which makes its corresponding MetricsLogger a singleton. You may also choose to instantiate MetricsLoggingService and hold a reference to the instance. In either case, choose one way to create and access MetricsLoggerService and use it consistently in your app.

You can create multiple instances of the service with different backends if desired. However, you should not create multiple services that point at the same backend.


Use from main thread only.

Example (using instance)

let service = MetricsLoggerService(initialConfig: ...)
let logger = service.metricsLogger
let impressionLogger = service.impressionLogger()
let scrollTracker = service.scrollTracker(collectionView: ...)

Example (using shared service)

// Call this first before accessing the instance.
MetricsLoggerService.startServices(initialConfig: ...)
let service = MetricsLoggerService.shared
let logger = service.metricsLogger
let impressionLogger = service.impressionLogger()
let scrollTracker = service.scrollTracker(collectionView: ...)


Promoted event logging interface. Use instances of MetricsLogger to log events to Promoted's servers. Events are accumulated and sent in batches on a timer.

Typically, instances of MetricsLogger are tied to a MetricsLoggerService, which configures the logging environment and maintains a MetricsLogger for the lifetime of the service. See MetricsLoggerService for more information about the scope of the logger and the service.

Events are represented as protobuf messages internally. By default, these messages are serialized to binary format for transmission over the network.


To start a logging session, first call startSession(userID:) or startSessionSignedOut() to set up the user ID and log user ID for the session. You can call either startSession method more than once to begin a new session with the given user ID.

Use one of the log* methods to enqueue an event for logging. When the batching timer fires, all events are delivered to the server via the NetworkConnection.

If you want to deliver queued events immediately, say when your app enters the background, use flush(). It's not necessary for clients to call flush() to deliver queued events. Events are automatically delivered on a timer.


Use from main thread only.


let logger = service.metricsLogger
// Sets userID and anonUserID for subsequent log() calls.
logger.startSession(userID: myUserID)
let item = Item(name: "Stuffed Hippo", contentID: "ABCD-1234", insertionID: "XYZ")
logger.logPurchaseAction(item: purchasedItem)
// Resets userID and anonUserID for another session.
logger.startSession(userID: secondUserID)


Provides basic impression tracking across scrolling collection views, such as UICollectionView or UITableView. Works best with views that can provide fine-grained updates of visible cells, but can also be adapted to work with views that don't.


ImpressionLogger provides only basic impression tracking logic that considers a view as impressed as soon as it enters the screen. For more advanced functionality, see ScrollTracker, which offers visibility and time thresholds.

Used from React Native because RN's SectionList and FlatList provide this advanced functionality already.

Clients should create an instance of ImpressionLogger and reference it in their view controller, then provide updates to the impression logger as the collection view scrolls or updates.


class MyViewController: UIViewController {
  var collectionView: UICollectionView
  var logger: MetricsLogger
  var impressionLogger: ImpressionLogger
  private func content(atIndexPath path: IndexPath) -> Content? {
    let item = path.item
    if item >= self.items.count { return nil }
    let myItemProperties = self.items[item]
    return Item(properties: myItemProperties)

  func viewWillDisappear(_ animated: Bool) {

  func collectionView(_ collectionView: UICollectionView,
                      willDisplay cell: UICollectionViewCell,
                      forItemAt indexPath: IndexPath) {
    if let content = content(atIndexPath: indexPath) {
     impressionLogger.collectionViewWillDisplay(content: content)
  func collectionView(_ collectionView: UICollectionView,
                      didEndDisplaying cell: UICollectionViewCell,
                      forItemAt indexPath: IndexPath) {
    if let content = content(atIndexPath: indexPath) {
      impressionLogger.collectionViewDidHide(content: content)
  func reloadCollectionView() {
    let visibleContent = {
      path in content(atIndexPath: path)


Tracks scrolling behavior in client apps to deliver accurate impression events.

“Accurate” in this case means that the views scrolled on screen meet the following criteria.

  1. Ratio of view displayed on screen is at least ClientConfig.scrollTrackerVisibilityThreshold.
  2. (Coming later) Duration of impression is at least ClientConfig.scrollTrackerDurationThreshold.


See Impression Logging section for usage example.

Performance vs accuracy

ScrollTracker works by coalescing scroll events to achieve a balance between performance and accuracy. The more often ScrollTracker processes scroll events, the more accurately impressions are tracked, but this comes at a performance cost. Although this cost is usually negligible relative to the amount of computation in most UI updates, ScrollTracker minimizes this overhead by updating at a fixed frequency rather than in response to every scroll event. This update frequency is controlled via ClientConfig.scrollTrackerUpdateFrequency.


Configuration for Promoted metrics logging library internal behavior.

This class should only contain properties that apply to the Promoted logging library in general. Mechanisms that alter the way that client code calls the Promoted logging library should go in client code, external from this config.


Exposes internals of Promoted metrics logging library so that clients can inspect time profiles, network activity, and contents of log messages sent to the server.

Set xrayEnabled on the initial ClientConfig to enable this profiling for the session. Access the xray property on MetricsLoggerService.

Like all profiling mechanisms, Xray incurs performance and memory overhead, so use in production should be judicious. Xray makes best effort to exclude its own overhead from its reports.

Profile data from Xray is kept only in memory on the client, and not sent to Promoted's servers (as of 2021Q1).

For more info on iOS Performance Debugging Using OSLog and Xray


Performance and Energy


Only use the main thread to interact with the Promoted metrics logging library.

The library offloads many operations to background threads automatically.


Calls to the Promoted metrics logging library are designed to return as quickly as possible. You can call us in response to UI events, or add our listeners to your UIViews, without perceivable impact to the responsiveness of your UI.

React Native

Calls from Javascript/Typescript into the Promoted metrics logging library return asynchronously, so they should have no impact on your UI responsiveness. Behind the scenes, our library performs the majority of its work on the main thread, and offloads any intensive tasks to background threads.

We recommend constructing copies of any objects that you pass to Promoted. These copies should only include the minimum set of fields required for logging. Don’t pass Javascript data model objects directly, since React Native must construct native dictionaries for these objects, and this construction can be expensive and unnecessary.

User Logging

Call startSessionAndLogUser(userID:) or startSessionAndLogSignedOutUser() on MetricsLogger/PromotedMetrics to start the logging session. You can call these methods multiple times if the user changes their sign-in status.

The Promoted metrics logging library will cache the last-used anonUserID in your NSUserDefaults to provide a consistent anonUserID across multiple sessions. You can configure this persistence to use a different mechanism if desired.

Action Logging

Call the log*Action methods on MetricsLogger/PromotedMetrics to track actions. Promoted defines a series of preset action types. Each type has a corresponding method in MetricsLogger (eg. purchase → logPurchaseAction).

Action typeDescription
customActionTypeAction that doesn't correspond to any of the below.
navigateNavigating to details about content.
addToCartAdding an item to shopping cart.
removeFromCartRemove an item from shopping cart.
checkoutGoing to checkout.
purchasePurchasing an item.
shareSharing content.
likeLiking content.
unlikeUn-liking content.
commentCommenting on content.
makeOfferMaking an offer on content.
askQuestionAsking a question about content.
answerQuestionAnswering a question about content.
completeSignInComplete sign-in.
completeSignUpComplete sign-up.

Some actions require you to provide content for the action, and others do not take content. Prefer the provided methods in MetricsLogger/PromotedMetrics (instead of calling logAction(name:type:content:) directly) to ensure that these requirements are met at the call site.

Custom action can be logged via the logAction method, with customActionType.

Impression Logging

You can call logImpression on MetricsLogger/PromotedMetrics manually to track content impressions. For many popular usage patterns in UIKit and React Native, consider one of the more advanced methods below.


Impression logging for UICollectionView is handled automatically using ScrollTracker. Although ScrollTracker is designed for use with both UIKit and React Native, we provide UIKit specializations for tracking UIScrollViews and UICollectionViews.

There are two touch points to configure ScrollTracker.

// First, create a ScrollTracker with the collection view.
collectionView = UICollectionView()
scrollTracker = service.scrollTracker(collectionView: collectionView)

// Second, when the collection view’s data has loaded, create Content objects
// and load it into ScrollTracker.
var content = [Content]()
for object in myDataObjects {
  let content = Content(name:, contentID:, 
                        insertionID: object.insertionID)
scrollTracker.setFramesFrom(content: content)

Once this configuration is complete, ScrollTracker will automatically log impressions when the user interacts with the UICollectionView.

This functionality uses KVO on UIViews to eliminate the need to call scrollViewDidScroll() on ScrollTracker, and to wait for layout to update frames. To do the latter, call setFramesFrom* after the content view's content is loaded, and the frames will be automatically calculated when layout on the content view completes.

ScrollTracker initiates KVO on provided UIViews when used in this capacity. Releasing all references to ScrollTracker will stop KVO.

In addition to logging impressions for a single UICollectionView, ScrollTracker can handle collection views nested inside UIScrollViews. To accommodate this scenario, use the following pattern.

// First, the code that owns the CONTAINER UIScrollView should create a
// ScrollTracker with the scroll view.
// In StuffedAnimalsViewController:
stuffedAnimalsScrollView = UIScrollView(...)
hippoScrollTracker = service.scrollTracker(scrollView: stuffedAnimalsScrollView)

// Second, pass the ScrollTracker to code that owns the CONTENT UICollectionView.
// In StuffedAnimalsViewController:
hippoViewController = HippoViewController(scrollTracker: scrollTracker)

// Finally, when the collection view’s data has loaded, create Content objects
// and load it into ScrollTracker.
// In HippoViewController:
var content = [Content]()
for hippo myHippos {
  let content = Content(name:, contentID:, 
                        insertionID: hippo.insertionID)
scrollTracker.setFramesFrom(collectionView: hippoCollectionView, content: content)

If you wish to record impressions from multiple collection views in the same scroll view, create one instance of ScrollTracker for each collection view.


ScrollTracker performs a linear scan across all its content on a timer.

For this reason, passing an extremely large UICollectionView (more than a few hundred cells) will cause noticeable performance issues. If this is a concern for your app, let us know and we can explore solutions for you.

React Native

Use the useImpressionLogger() hook for accurate tracking in FlatLists and SectionLists. If using some other kind of scroll view, you can use ScrollTracker by setting the viewport manually through the viewport property when the scroll view updates.

Make sure that the identifier you provide is unique for every list on the same screen.

import { useImpressionLogger } from "@promotedai/react-native-metrics";

const {
  _viewabilityConfig, _onViewableItemsChanged 
} = useImpressionLogger("MyListIdentifier", (viewToken) => ({ 
  content_id: viewToken.item.contentId,
  insertion_id: viewToken.item.insertionId,

<SectionList onViewableItemsChanged={handleViewableItemsChanged} ... />

View Logging

Promoted requires accurate data about the current screen when your users view or interact with content. We recommend the following usage patterns with the metrics logging library.


Call logView or logPromotedViewForSelf in the viewDidAppear method of every UIViewController that you need to track. Promoted will only track screens for UIViewControllers that have been explicitly logged via this mechanism. If you don’t use logView to track a given UIViewController, Promoted won’t consider that UIViewController to be a screen that it should track.

Promoted needs to sync the state of the UIViewController stack before logging every batch of events. This is done by inspecting the rootViewController on the keyWindow of the UIApplication. (The viewWillAppear method on UIViewController is not always called when a view controller comes to the front.)

React Native

Use NavigationContainer to track the state of views. Promoted’s useViewTracker() hook provides handlers for onReady and onChange. These provide ideal entry points for view logging.

import { useImpressionLogger } from "@promotedai/react-native-metrics";

const navigationRef = useRef();
const { _onReady, _onStateChange } = useViewTracker(navigationRef);


What’s Next

Sending Engagements on Android