Step 2: Build the app

What you’ll do: create an empty Flutter app, add the CARP packages, and do the small Android and iPhone setup CARP needs.

You need:

• Flutter installed: Install Flutter. When flutter doctor shows green checkmarks for Android or iOS, you’re ready.
• A phone set up for development: Android: turn on USB debugging, or iPhone: set up Xcode (iPhone apps can only be built on a Mac).

Tip

Use a real phone if you can. Simulators have no step counter or real battery, so they produce very little data.

Steps

1. Create an empty Flutter app. In a terminal:

   flutter create my_first_carp_app
   cd my_first_carp_app

2. Add the CARP packages.

   flutter pub add carp_core carp_mobile_sensing

3. Do the platform setup.

   Android

   In android/app/src/main/AndroidManifest.xml, add this just above <application, so the app may count steps:

   android/app/src/main/AndroidManifest.xml

   <manifest xmlns:android="http://schemas.android.com/apk/res/android">
   
       <uses-permission android:name="android.permission.ACTIVITY_RECOGNITION"/>
   
       <application ...>
   </manifest>

   In android/app/build.gradle.kts, add the two marked parts:

   android/app/build.gradle.kts

   android {
       compileOptions {
           isCoreLibraryDesugaringEnabled = true
           // ...leave the existing lines as they are
       }
       // ...
   }
   
   dependencies {
       coreLibraryDesugaring("com.android.tools:desugar_jdk_libs:2.1.4")
   }

   iPhone

   Open ios/Runner.xcworkspace in Xcode, select Runner → General, and set Minimum Deployments to 15.0. Then add this to ios/Runner/Info.plist inside <dict>:

   ios/Runner/Info.plist

   <plist version="1.0">
   <dict>
       ...
       <key>NSMotionUsageDescription</key>
       <string>This app counts your steps for a research study.</string>
   </dict>
   </plist>

4. Add your protocol. Create lib/protocol.dart with the code from Step 1:

   lib/protocol.dart

   import 'package:carp_core/carp_core.dart' hide Smartphone;
   import 'package:carp_mobile_sensing/carp_mobile_sensing.dart';
   
   SmartphoneStudyProtocol createProtocol() {
     final protocol = SmartphoneStudyProtocol(
       name: 'My first study',
       dataEndPoint: SQLiteDataEndPoint(),
     );
     final phone = Smartphone();
     protocol.addPrimaryDevice(phone);
     protocol.addParticipantRole(ParticipantRole('Participant'));
     protocol.addTaskControl(
       ImmediateTrigger(),
       BackgroundTask(name: 'Steps and battery', measures: [
         Measure(type: SensorSamplingPackage.STEP_EVENT),
         Measure(type: DeviceSamplingPackage.BATTERY_STATE),
       ]),
       phone,
     );
     return protocol;
   }

   Smartphone exists in both packages; hide tells Dart to use the one from carp_mobile_sensing.

Check it

flutter run

The default Flutter counter app starts on your phone. CARP isn’t running yet; that’s the next step.

Next: Step 3: Deploy