Adding a Flutter screen to an iOS app

This guide describes how to add a single Flutter screen to an existing iOS app.

Start a FlutterEngine and FlutterViewController

To launch a Flutter screen from an existing iOS, you start a FlutterEngine and a FlutterViewController.

The FlutterEngine may have the same lifespan as your FlutterViewController or outlive your FlutterViewController.

See Loading sequence and performance for more analysis on the latency and memory trade-offs of pre-warming an engine.

Create a FlutterEngine

The proper place to create a FlutterEngine is specific to your host app. As an example, we demonstrate creating a FlutterEngine, exposed as a property, on app startup in the app delegate.

In AppDelegate.h:

@import UIKit;
@import Flutter;

@interface AppDelegate : FlutterAppDelegate // More on the FlutterAppDelegate below.
@property (nonatomic,strong) FlutterEngine *flutterEngine;
@end

In AppDelegate.m:

#import <FlutterPluginRegistrant/GeneratedPluginRegistrant.h> // Used to connect plugins.

#import "AppDelegate.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application
    didFinishLaunchingWithOptions:(NSDictionary<UIApplicationLaunchOptionsKey, id> *)launchOptions {
  self.flutterEngine = [[FlutterEngine alloc] initWithName:@"my flutter engine"];
  // Runs the default Dart entrypoint with a default Flutter route.
  [self.flutterEngine run];
  [GeneratedPluginRegistrant registerWithRegistry:self.flutterEngine];
  return [super application:application didFinishLaunchingWithOptions:launchOptions];
}

@end

In AppDelegate.swift:

import UIKit
import Flutter
import FlutterPluginRegistrant // Used to connect plugins.

@UIApplicationMain
class AppDelegate: FlutterAppDelegate { // More on the FlutterAppDelegate.
  lazy var flutterEngine = FlutterEngine(name: "my flutter engine")

  override func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    // Runs the default Dart entrypoint with a default Flutter route.
    flutterEngine.run();
    GeneratedPluginRegistrant.register(with: self.flutterEngine);
    return super.application(application, didFinishLaunchingWithOptions: launchOptions);
  }
}

Show a FlutterViewController with your FlutterEngine

The following example shows a generic ViewController with a UIButton hooked to present a FlutterViewController. The FlutterViewController uses the FlutterEngine instance created in the AppDelegate.

@import Flutter;
#import "AppDelegate.h"
#import "ViewController.h"

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];

    // Make a button to call the showFlutter function when pressed.
    UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
    [button addTarget:self
               action:@selector(showFlutter)
     forControlEvents:UIControlEventTouchUpInside];
    [button setTitle:@"Show Flutter!" forState:UIControlStateNormal];
    button.backgroundColor = UIColor.blueColor;
    button.frame = CGRectMake(80.0, 210.0, 160.0, 40.0);
    [self.view addSubview:button];
}

- (void)showFlutter {
    FlutterEngine *flutterEngine =
        ((AppDelegate *)UIApplication.sharedApplication.delegate).flutterEngine;
    FlutterViewController *flutterViewController =
        [[FlutterViewController alloc] initWithEngine:flutterEngine nibName:nil bundle:nil];
    [self presentViewController:flutterViewController animated:YES completion:nil];
}
@end
import UIKit
import Flutter

class ViewController: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()

    // Make a button to call the showFlutter function when pressed.
    let button = UIButton(type:UIButton.ButtonType.custom)
    button.addTarget(self, action: #selector(showFlutter), for: .touchUpInside)
    button.setTitle("Show Flutter!", for: UIControl.State.normal)
    button.frame = CGRect(x: 80.0, y: 210.0, width: 160.0, height: 40.0)
    button.backgroundColor = UIColor.blue
    self.view.addSubview(button)
  }

  @objc func showFlutter() {
    let flutterEngine = (UIApplication.shared.delegate as! AppDelegate).flutterEngine
    let flutterViewController =
        FlutterViewController(engine: flutterEngine, nibName: nil, bundle: nil)
    present(flutterViewController, animated: true, completion: nil)
  }
}

Now, you have a Flutter screen embedded in your iOS app.

Alternatively - Create a FlutterViewController with an implicit FlutterEngine

As an alternative to the previous example, you can let the FlutterViewController implicitly create its own FlutterEngine without pre-warming one ahead of time.

This is not usually recommended because creating a FlutterEngine on-demand could introduce a noticeable latency between when the FlutterViewController is presented and when it renders its first frame. This could, however, be useful if the Flutter screen is rarely shown, when there are no good heuristics to determine when the Dart VM should be started, and when Flutter doesn’t need to persist state between view controllers.

To let the FlutterViewController present without an existing FlutterEngine, omit the FlutterEngine construction, and create the FlutterViewController without an engine reference.

// Existing code omitted.
- (void)showFlutter {
  FlutterViewController *flutterViewController =
      [[FlutterViewController alloc] initWithProject:nil nibName:nil bundle:nil];
  [self presentViewController:flutterViewController animated:YES completion:nil];
}
@end
// Existing code omitted.
func showFlutter() {
  let flutterViewController = FlutterViewController(project: nil, nibName: nil, bundle: nil)
  present(flutterViewController, animated: true, completion: nil)
}

See Loading sequence and performance for more explorations on latency and memory usage.

Using the FlutterAppDelegate

Letting your application’s UIApplicationDelegate subclass FlutterAppDelegate is recommended but not required.

The FlutterAppDelegate performs functions such as:

  • Forwarding application callbacks such as openURL to plugins such as local_auth.
  • Forwarding status bar taps (which can only be detected in the AppDelegate) to Flutter for scroll-to-top behavior.

If your app delegate can’t directly make FlutterAppDelegate a subclass, make your app delegate implement the FlutterAppLifeCycleProvider protocol in order to make sure your plugins receive the necessary callbacks. Otherwise, plugins that depend on these events may have undefined behavior.

For instance:

@import Flutter;
@import UIKit;
@import FlutterPluginRegistrant;

@interface AppDelegate : UIResponder <UIApplicationDelegate, FlutterAppLifeCycleProvider>
@property (strong, nonatomic) UIWindow *window;
@property (nonatomic,strong) FlutterEngine *flutterEngine;
@end

The implementation should delegate mostly to a FlutterPluginAppLifeCycleDelegate:

@interface AppDelegate ()
@property (nonatomic, strong) FlutterPluginAppLifeCycleDelegate* lifeCycleDelegate;
@end

@implementation AppDelegate

- (instancetype)init {
    if (self = [super init]) {
        _lifeCycleDelegate = [[FlutterPluginAppLifeCycleDelegate alloc] init];
    }
    return self;
}

- (BOOL)application:(UIApplication*)application
didFinishLaunchingWithOptions:(NSDictionary<UIApplicationLaunchOptionsKey, id>*))launchOptions {
    self.flutterEngine = [[FlutterEngine alloc] initWithName:@"io.flutter" project:nil];
    [self.flutterEngine runWithEntrypoint:nil];
    [GeneratedPluginRegistrant registerWithRegistry:self.flutterEngine];
    return [_lifeCycleDelegate application:application didFinishLaunchingWithOptions:launchOptions];
}

// Returns the key window's rootViewController, if it's a FlutterViewController.
// Otherwise, returns nil.
- (FlutterViewController*)rootFlutterViewController {
    UIViewController* viewController = [UIApplication sharedApplication].keyWindow.rootViewController;
    if ([viewController isKindOfClass:[FlutterViewController class]]) {
        return (FlutterViewController*)viewController;
    }
    return nil;
}

- (void)touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event {
    [super touchesBegan:touches withEvent:event];

    // Pass status bar taps to key window Flutter rootViewController.
    if (self.rootFlutterViewController != nil) {
        [self.rootFlutterViewController handleStatusBarTouches:event];
    }
}

- (void)application:(UIApplication*)application
didRegisterUserNotificationSettings:(UIUserNotificationSettings*)notificationSettings {
    [_lifeCycleDelegate application:application
didRegisterUserNotificationSettings:notificationSettings];
}

- (void)application:(UIApplication*)application
didRegisterForRemoteNotificationsWithDeviceToken:(NSData*)deviceToken {
    [_lifeCycleDelegate application:application
didRegisterForRemoteNotificationsWithDeviceToken:deviceToken];
}

- (void)application:(UIApplication*)application
didReceiveRemoteNotification:(NSDictionary*)userInfo
fetchCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler {
    [_lifeCycleDelegate application:application
       didReceiveRemoteNotification:userInfo
             fetchCompletionHandler:completionHandler];
}

- (BOOL)application:(UIApplication*)application
            openURL:(NSURL*)url
            options:(NSDictionary<UIApplicationOpenURLOptionsKey, id>*)options {
    return [_lifeCycleDelegate application:application openURL:url options:options];
}

- (BOOL)application:(UIApplication*)application handleOpenURL:(NSURL*)url {
    return [_lifeCycleDelegate application:application handleOpenURL:url];
}

- (BOOL)application:(UIApplication*)application
            openURL:(NSURL*)url
  sourceApplication:(NSString*)sourceApplication
         annotation:(id)annotation {
    return [_lifeCycleDelegate application:application
                                   openURL:url
                         sourceApplication:sourceApplication
                                annotation:annotation];
}

- (void)application:(UIApplication*)application
performActionForShortcutItem:(UIApplicationShortcutItem*)shortcutItem
  completionHandler:(void (^)(BOOL succeeded))completionHandler NS_AVAILABLE_IOS(9_0) {
    [_lifeCycleDelegate application:application
       performActionForShortcutItem:shortcutItem
                  completionHandler:completionHandler];
}

- (void)application:(UIApplication*)application
handleEventsForBackgroundURLSession:(nonnull NSString*)identifier
  completionHandler:(nonnull void (^)(void))completionHandler {
    [_lifeCycleDelegate application:application
handleEventsForBackgroundURLSession:identifier
                  completionHandler:completionHandler];
}

- (void)application:(UIApplication*)application
performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler {
    [_lifeCycleDelegate application:application performFetchWithCompletionHandler:completionHandler];
}

- (void)addApplicationLifeCycleDelegate:(NSObject<FlutterPlugin>*)delegate {
    [_lifeCycleDelegate addDelegate:delegate];
}
@end

Launch options

The examples demonstrate running Flutter using the default launch settings.

In order to customize your Flutter runtime, you can also specify the Dart entrypoint, library, and route.

Dart entrypoint

Calling run on a FlutterEngine, by default, runs the main() Dart function of your lib/main.dart file.

You can also run a different entrypoint function by using runWithEntrypoint with an NSString specifying a different Dart function.

Dart library

In addition to specifying a Dart function, you can specify an entrypoint function in a specific file.

For instance the following runs myOtherEntrypoint() in lib/other_file.dart instead of main() in lib/main.dart:

[flutterEngine runWithEntrypoint:@"myOtherEntrypoint" libraryURI:@"other_file.dart"];
flutterEngine.run(withEntrypoint: "myOtherEntrypoint", libraryURI: "other_file.dart")

Route

An initial route can be set for your Flutter WidgetsApp when constructing the engine.

FlutterEngine *flutterEngine =
    [[FlutterEngine alloc] initWithName:@"my flutter engine"];
[[flutterEngine navigationChannel] invokeMethod:@"setInitialRoute"
                                      arguments:@"/onboarding"];
[flutterEngine run];
let flutterEngine = FlutterEngine(name: "my flutter engine")
flutterEngine.navigationChannel.invokeMethod("setInitialRoute", arguments:"/onboarding")
flutterEngine.run()

This code sets your dart:ui’s window.defaultRouteName to "/onboarding" instead of "/".

See Navigation and routing for more about Flutter’s routes.

Other

The previous example only illustrates a few ways to customize how a Flutter instance is initiated. Using platform channels, you’re free to push data or prepare your Flutter environment in any way you’d like, before presenting the Flutter UI using a FlutterViewController.