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Source code: statsig-io/dart-sdk

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Set Up the SDK

1

Install the SDK

With Dart:
dart pub add statsig
With Flutter:
flutter pub add statsig
If you are using Flutter, be sure to add Statsig as part of the app lifecycle to avoid losing events.
2

Initialize the SDK

Next, initialize the SDK with a client SDK key from the “API Keys” tab on the Statsig console. These keys are safe to embed in a client application.Along with the key, pass in a User Object with the attributes you’d like to target later on in a gate or experiment.
import 'package:statsig/statsig.dart';

await Statsig.initialize('client-sdk-key', StatsigUser(userId: "a-user-id"));

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Use the SDK

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Checking a Feature Flag/Gate

Now that your SDK is initialized, let’s check a Feature Gate. Feature Gates can be used to create logic branches in code that can be rolled out to different users from the Statsig Console. Gates are always CLOSED or OFF (think return false;) by default. 
if (Statsig.checkGate("new_homepage_design")) {
  // Gate is on, show new home page
} else {
  // Gate is off, show old home page
}

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Reading a Dynamic Config

Feature Gates can be very useful for simple on/off switches, with optional but advanced user targeting. However, if you want to be able send a different set of values (strings, numbers, and etc.) to your clients based on specific user attributes, e.g. country, Dynamic Configs can help you with that. The API is very similar to Feature Gates, but you get an entire json object you can configure on the server and you can fetch typed parameters from it. For example: 
var config = Statsig.getConfig("awesome_product_details");

// The 2nd parameter is the default value to be used in case the given parameter name does not exist on
// the Dynamic Config object. This can happen when there is a typo, or when the user is offline and the
// value has not been cached on the client.
var itemName = config.get("product_name", "Awesome Product v1");
var price = config.get("price", 10.0);
var shouldDiscount = config.get("discount", false);

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Getting a Layer/Experiment

Then we have Layers/Experiments, which you can use to run A/B/n experiments. We offer two APIs, but we recommend the use of layers to enable quicker iterations with parameter reuse. 
// Values via getLayer

var layer = Statsig.getLayer("user_promo_experiments");
var promoTitle = layer.getString("title", "Welcome to Statsig!");
var discount = layer.getDouble("discount", 0.1);

// or, via getExperiment

var titleExperiment = Statsig.getExperiment("new_user_promo_title");
var priceExperiment = Statsig.getExperiment("new_user_promo_price");

var promoTitle = titleExperiment.get("title", "Welcome to Statsig!");
var discount = priceExperiment.get("discount", 0.1);

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Logging an Event

Now that you have a Feature Gate or an Experiment set up, you may want to track some custom events and see how your new features or different experiment groups affect these events. This is super easy with Statsig - simply call the Log Event API for the event, and you can additionally provide some value and/or an object of metadata to be logged together with the event: 
// Provide a doubleValue argument for number values
Statsig.logEvent("purchase", doubleValue: 2.99, metadata: {"item_name": "remove_ads"});

// or provide a stringValue argument for string values
Statsig.logEvent("login", stringValue: "a.user@mail.com");

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Parameter Stores

Parameter Stores hold a set of parameters for your mobile app. These parameters can be remapped on-the-fly from a static value to a Statsig entity (Feature Gates, Experiments, and Layers), so you can decouple your code from the configuration in Statsig. Read more about Param Stores here. 
var homepageStore = Statsig.getParameterStore("homepage");

var title = homepageStore.get("title", "Welcome");
var shouldShowUpsell = homepageStore.get("upsell_upgrade_now", false);

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Statsig User

You need to provide a StatsigUser object to check/get your configurations. You should pass as much information as possible in order to take advantage of advanced gate and config conditions. Most of the time, the userID field is needed in order to provide a consistent experience for a given user (see logged-out experiments to understand how to correctly run experiments for logged-out users). Besides userID, we also have email, ip, userAgent, country, locale and appVersion as top-level fields on StatsigUser. In addition, you can pass any key-value pairs in an object/dictionary to the custom field and be able to create targeting based on them. Once the user logs in or has an update/changed, make sure to call updateUser with the updated userID and/or any other updated user attributes: 
await Statsig.updateUser(StatsigUser("a_new_user"));

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Private Attributes

Have sensitive user PII data that should not be logged? No problem, we have a solution for it! On the StatsigUser object we also have a field called privateAttributes, which is a simple object/dictionary that you can use to set private user attributes. Any attribute set in privateAttributes will only be used for evaluation/targeting, and removed from any logs before they are sent to Statsig server. For example, if you have feature gates that should only pass for users with emails ending in “@statsig.com”, but do not want to log your users’ email addresses to Statsig, you can simply add the key-value pair { email: "my_user@statsig.com" } to privateAttributes on the user and that’s it!

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Statsig Options

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initTimeout
int
default:"3"
Used to decide how long (in seconds) the Statsig client waits for the initial network request to respond before calling the completion block. The Statsig client will return either cached values (if any) or default values if checkGate/getConfig/getExperiment is called before the initial network request completes.If you always want to wait for the latest values fetched from Statsig server, you should set this to 0 so we do not timeout the network request.
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api
string
default:"https://statsigapi.net/v1/"
The endpoint to use for all SDK network requests. You should not override this (unless you have another API that implements the Statsig API endpoints).
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environment
string
default:""
The environment tier to evaluate rules for the current user. Default empty, which is the same as “production.” On non-production tiers, events will not make it in to downstream pulse results.

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Lifecycle & Advanced Usage

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Shutting Statsig Down

In order to save users’ data and battery usage, as well as prevent logged events from being dropped, we keep event logs in client cache and flush periodically. Because of this, some events may not have been sent when your app shuts down. To make sure all logged events are properly flushed or saved locally, you should tell Statsig to shutdown when your app is closing: 
await Statsig.shutdown();

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Flutter Lifecycle Hooks

Due to the nature of mobile development, apps can be closed by the operating system when they are no longer in the foreground. To be sure that all events are logged before an app is closed by the operating system, we recommend adding Statsig to the app lifecycle events. This way we can flush all pending events when an app state change is detected. Something like the following: 
// An example App Lifecycle Observer
class StatsigLifecycleObserver extends WidgetsBindingObserver {
  @override
  void didChangeAppLifecycleState(AppLifecycleState state) async {
    switch (state) {
      case AppLifecycleState.resumed:
        await Statsig.initialize('client-sdk-key');
        break;
      case AppLifecycleState.paused:
        await Statsig.shutdown();
        break;
    }
  }
}
Then in your app code, add this observer to the WidgetsBinding instance. 
@override
void initState() {
  super.initState();
  WidgetsBinding.instance.addObserver(StatsigLifecycleObserver());
}

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FAQs

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How do I run experiments for logged out users?

See the guide on device level experiments.
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