Party Clicker Tutorial

For a quick intro to how Doppl works, try this simple tutorial app. We’ll walk you through the steps to get set up and running, and explain a few concepts along the way.


Who reads? Watch the setup video.


  1. A Mac OSX PC, with a few gigs of free drive space
  2. Android Studio
  3. Xcode

Install J2objc

Doppl is based on J2objc, which converts Java to Objective-C. Doppl is almost stock J2objc, but has some local tweaks, which means we have to distribute our own slightly altered version. The changes are mostly about file organization, rather than anything with the runtime. Read more here.

Download the Doppl J2objc Runtime, and extract the zip file to your local drive. This is your j2objc dist directory.

Get the Sample

Clone the sample app.

git clone

Open the sample app in Android Studio.

Note If you’re not an Android developer, that means go to File > Open, then open the PartyClickerSample directory. There is no project or “workspace” file to open.

Configure Android Studio

Edit the file. If it doesn’t exist, create it in your project root. Add a property point to your j2objc dist

j2objc.home=[j2objc dist]

If you’re having trouble finding, watch the video:

Assuming you have a device or emulator to test on, you should be able to run the Android app at this point.

Configure Xcode

Xcode also needs to know where J2objc is installed. Open XCode > Preferences, Locations > Custom Paths. Add ‘J2OBJC_LOCAL_PATH’ and set Path to j2objc dist (no trailing slash).

Various settings in the project use this property to point to J2objc, and if set up properly, your project should be portable among developers with different local drive location configurations. See Xcode Setup for more details.

Run Sample

Assuming everything is set up correctly, you should be able to navigate to the ios folder, open the project, and run it on an iPhone simulator. Make sure you have the ios application target set with a valid simulator.

If the sample doesn’t run, something in the setup isn’t working. Retrace steps and/or watch the full demo video.

The Parts

Back in Android Studio, find the root directory build.gradle file. This contains references to our artifact repo, which will eventually move to jcenter, and the classpath to the Doppl Gradle plugin.


The app’s build.gradle is set up like most Android applications, with a few additions.

2-3: Apply retrolambda and Doppl plugins

28-53: Project dependencies. Notice parallel doppl and testDoppl dependencies (Doppl is for Doppelgänger)

57-58: Where to copy Objective-C to. Can be multiple for each type.

60: Generate bridging headers for Swift, if desired.

64-67: Shorten prefixes. More detail here.

69: testAnnotationProcessor doesn’t advertise it’s output dir, so you need to specify.

71-76: Tell Doppl what to translate

Running Tests

Currently the iOS app itself and the iOS JUnit tests are in different Xcode projects. They don’t need to be, but for ease of organization, they were split out.

J2objc ships with JUnit. Doppl has some helper classes and Gradle task support to help setting this up. How all that works is a longer topic, but you can open and run the test project to see how it works.

Find the iostest folder in the root folder of the sample project. Open the Xcode project in that folder. Verify that you have the ‘iostest’ target and a valid simulator selected in Xcode.

Xcode target select

In the top menu, click Product > Test. This will probably take a while but will run a single XCTest that runs the set of JUnit tests.

Also notice at the bottom of the app’s build.gradle file, there’s an Exec task called runIOSTests. It can run the iOS unit tests from gradle, if you’re looking to automate things.

Modifying Java Code

The shared code in the project is flagged with the translatePattern file pattern in build.gradle. Any code that matches that pattern will be pushed through J2objc and output to the folders specified copyMainOutput or copyMainOutput. To trigger the J2objc code transform run

./gradlew dopplDeploy

Assuming no errors, this will push code changes to the ios project(s).

Dealing with Xcode

If you add or remove Java code files, Xcode will need to know about the new code. This is just one of those things you need to get used to, or try some alternate project structure. See here for more info.

Quick Tour of Code

Java Side

The Android application starts up with a custom Application class that initializes code for the shared library. PartyClickerApplication passes in itself, the Context, and a Dagger component. This will be referenced from the shared code. The iOS side with do a similar things in it’s AppDelegate.

The data package contains database definition and management code. The presenter package contains presenter logic, as well as Dagger definitions and other interfaces. These packages are both fully shared to iOS. You should also note that code generated with annotation processing is found in [root]/app/build/generated/source/apt/debug. This code is also converted during dopplDeploy.

Shared code communicates with platform specific code by way of interfaces (in this case. You could do a more reactive model). An interface representing the UI is defined in both PartyListPresenter and PartyPresenter. On the Android side, these are implemented by PartyListActivity and PartyActivity respectively.

Xcode Side

The UI code for the sample app is written in Swift. The entry point for the application is AppDelegate.swift.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {


    let context = AndroidContentIOSContext()
    let appModule = PPAppModule(androidAppApplication: context, with: PPLogCrashReporter())

    PAppManager.init__(with: context, with: PPDaggerDaggerComponent.builder()
        .appModule(with: appModule).build())

    // Override point for customization after application launch.
    return true

It creates the iOS version of the Android Context, as well as initializes the Dagger component, just like PartyClickerApplication.

Note one small but very important detail. DopplRuntime.start(). This app uses Doppl’s extended Android runtime support, which includes attaching to the main thread, and some static data initialization. This is handled in DopplRuntime.

As with Android, we implement the presenter UI interfaces in PartyListViewController and PartyViewController.

What’s Shared?

Although there are some duplicate artifacts (PartyListActivity and PartyListViewController, for example), you’ll notice that there’s not a lot of duplicate logic. The code in these classes is wiring the shared logic to the actual UI. If you’re familiar with mobile development, you’ll notice that pretty much everything that’s considered “logic” is shared. This includes main/background thread changes and SQLite database management. Were this a more complex example, that could include networking, Shared preferences, local files, and much more.

When something needs to be platform specific, creating a shared interface and implementing that on both platforms is, although perhaps verbose, simple. For example, the CrashReporter interface can be used extensively in the shared code, and in a live app, is intended to be implemented by each platforms specific library. There may be some delegate boilerplate involved, but again no real “logic”.

The goal of Doppl is to be able to push the level of what can be shared as far as is reasonable, but to allow the developer to decide what will be shared and to allow that line to be drawn in a natural and uncomplicated way.

What’s Next?

List of available libraries