How can I connect to metro bundler after adding flavors?

I'm experiencing an issue that I can't get my React Native app to connect to the metro bundler after I added flavors to the app. I searched a lot for possible solutions but couldn't make it work. This issue happens when I'm using a physical device connected through USB. USB debugging is set on and everything worked fine before I added flavors. I need to use a physical device to test some bluetooth features.

This is what I added to my gradle file:

debuggableVariants = ["localDebug"]

flavorDimensions "default"
    productFlavors {
        local {
            resValue "string", "app_name", "Note Ninja (Debug)"
            applicationIdSuffix ".debug"
            versionNameSuffix "-debug"
            buildConfigField "String", "FLAVOR_NAME", "\"local\""
        }
        internal {
            resValue "string", "app_name", "Note Ninja (Internal Testing)"
            versionNameSuffix "-internal"
            buildConfigField "String", "FLAVOR_NAME", "\"internal\""
        }
        prod {
            resValue "string", "app_name", "Note Ninja"
            buildConfigField "String", "FLAVOR_NAME", "\"prod\""
        }
    }

Now I adjusted my npm script to the following: "android:local": "react-native run-android --mode=localDebug --appId com.noteninja.debug",

When I run this script, metro is starting and the app is installed on the phone but I get the error message "Unable to load script. Make sure you're either running Metro or that bundle 'index.android.bundle' is packaged correctly for release. When I click reload I get the error "Could not connect to development server".

I tried to open localhost:8081 in the phone's browser and I can access metro so it does not seem like a firewall issue. I tried adb reverse tcp:8081 tcp:8081 but that also did not help (the device is connected as it's listed when I use adb devices). I uninstalled the app, I cleared gradle and metro cache but nothing seems to help.

Solution

This issue is a common headache when working with React Native flavors and connecting a physical device to the Metro bundler. Let me share a possible solution that can help you resolve it. Since you have already tried adb reverse and clearing the caches, let’s focus on a couple of other steps that might have been overlooked.

Possible Causes and Fixes

Bundle Mismatch for the Flavor
The index.android.bundle might not be correctly associated with your flavor. Ensure you have not accidentally created a production-style build for your custom flavors. Check your gradle.properties or build commands to ensure debuggableVariants includes the correct variant (localDebug).
- Add this in your build.gradle:
```
debuggable true
applicationVariants.all { variant ->
    if (variant.buildType.name == "debug" && variant.flavorName == "local") {
        variant.outputs.all {
            outputFileName = "index.android.bundle"
        }
    }
}
```
Ensure Flavors Are Passed Correctly to Metro
Metro bundler might not know which flavor is being used. Try explicitly specifying the flavor and mode when starting Metro.

Modify your package.json to add:
```
"start:local": "react-native start --reset-cache"
```
Then, run Metro before installing the app:
```
npm run start:local
```
React Native Flavors Configuration
Double-check your settings.gradle and build.gradle files. Ensure that your localDebug flavor is correctly mapped and resolves to the right build variant. Sometimes, mismatched flavor names can cause issues.
Device Connectivity
Run the following commands again to ensure proper connection:
```
adb reverse tcp:8081 tcp:8081
adb shell input keyevent 82 # Ensures the device stays awake
```
Also, check the ipconfig or ifconfig on your system and confirm that Metro is serving on the expected IP address (default is localhost:8081).
Enable Cleartext Traffic for Debug Flavors
Add this to your AndroidManifest.xml for the localDebug flavor:
```
<application android:usesCleartextTraffic="true" />
```
Check App Installation Path
Sometimes flavors generate different application IDs that do not match what the Metro bundler expects. Verify that your com.noteninja.debug is correctly being resolved.
Run Metro with Specific Configurations
Sometimes, Metro bundler needs explicit instructions. Run Metro like this:
```
npx react-native start --reset-cache --port 8081
```
Rebuild Gradle
If none of the above works, try a full rebuild:
```
cd android
./gradlew clean
cd ..
npm run android:local
```

Additional Debugging

Ensure the device is connected properly by checking adb devices.
Use adb logcat to check logs for more specific errors.
Open localhost:8081/debugger-ui on your browser and ensure Metro is running.

By following these steps, you should be able to resolve the issue. If the problem persists, try sharing the full logs for deeper analysis.