fix Android compatibility with AGP > 9 hosts in desktop_drop

[loucass]

Follow-up to #495 — @ekuleshov reported that 0.8.2 (Gradle 9.1.0 + Flutter 3.44.8) still fails with the same Could not find method kotlin() error that #495 was meant to fix.

Problem

0.8.2 made the apply plugin: 'kotlin-android' line conditional on AGP major < 9, but left the Kotlin configuration block unconditional:

• android { kotlinOptions { jvmTarget } } — the pre-AGP-9 form, removed in AGP 9

Result: AGP 8 hosts now passed, but AGP 9.1 hosts failed on the kotlin {} DSL itself (kotlin() not found when KGP wasn't applied).

Root cause

Host	Who compiles Kotlin?	kotlin {} unconditional	Conditional (this PR)
AGP < 9	nobody → plugin must apply KGP itself	❌ kotlin() not found*	✅ android.kotlinOptions branch
AGP ≥ 9	AGP built-in Kotlin	✅ works	✅ kotlin.compilerOptions branch

*kotlin extension only exists after KGP is applied.

Fix

Make the configuration block itself conditional:

def agpMajor = com.android.Version.ANDROID_GRADLE_PLUGIN_VERSION.tokenize('.')[0] as int
if (agpMajor < 9) {
    apply plugin: 'kotlin-android'
    android {
        kotlinOptions {
            jvmTarget = JavaVersion.VERSION_1_8.toString()
        }
    }
} else {
    kotlin {
        compilerOptions {
            jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_1_8
        }
    }
}

Verification

• AGP < 9 host: LocalSend debug APK on Flutter 3.41.9 · AGP 8.12.1 · Gradle 8.13 — run (✓ Built app-debug.apk)
• AGP 9 host: desktop_drop example on AGP 9.1 · Gradle 9.3.1 · Flutter 3.47.1 — run

[boyan01]

Thanks for the fix and the detailed verification. Looks good to me.

[ekuleshov]

hmm. I'm still getting these errors with 0.8.3 release

1: Task failed with an exception.
-----------
* Where:
Build file '**/.pub-cache/hosted/pub.dev/desktop_drop-0.8.3/android/build.gradle' line: 36

* What went wrong:
A problem occurred evaluating project ':desktop_drop'.
> Could not find method kotlin() for arguments [..] on project ':desktop_drop' of type org.gradle.api.Project.                                                                                                                                              

2: Task failed with an exception.
-----------
* What went wrong:
A problem occurred configuring project ':desktop_drop'.
> Failed to notify project evaluation listener.
   > java.lang.NullPointerException (no error message)
   > java.lang.NullPointerException (no error message)
   > Android Gradle Plugin: project ':desktop_drop' does not specify `compileSdk` in build.gradle (**/.pub-cache/hosted/pub.dev/desktop_drop-0.8.3/android/build.gradle).                                                                                                                                

[loucass]

@ekuleshov ok thanks. Could you try my branch fix/agp-kotlin-tasks directly to test new fixes ?

dependency_overrides:
  desktop_drop:
    git:
      url: https://github.com/loucass/flutter-plugins.git
      path: packages/desktop_drop
      ref: fix/agp-kotlin-tasks

[ekuleshov]

@loucass with that override the plugin is being reported as non-compliant:

WARNING: Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): desktop_drop
Future versions of Flutter will fail to build if your app uses plugins that apply KGP.

and then build fail with this:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:compileReleaseJavaWithJavac'.
> Compilation failed; see the compiler output below.
  **/android/app/src/main/java/io/flutter/plugins/GeneratedPluginRegistrant.java:39: error: cannot find symbol                                                                                                                                                 
        flutterEngine.getPlugins().add(new one.mixin.desktop.drop.DesktopDropPlugin());
                                                                 ^
    symbol:   class DesktopDropPlugin
    location: package one.mixin.desktop.drop
  1 error

[loucass]

@ekuleshov ok since all the if/else approach didn't work either
I've reverted to the unconditional apply plugin: 'kotlin-android' approach from 0.7.1. Could you test branch fix/agp-kotlin-tasks again please to verify this approach ?

[ekuleshov]

@loucass it builds with the same warning

WARNING: Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): desktop_drop
Future versions of Flutter will fail to build if your app uses plugins that apply KGP.

take a look at this one:

https://github.com/781flyingdutchman/background_downloader/blob/7bbf0f15330ef22427bbc019049a5ed80ae867b2/android/build.gradle#L6-L10

[boyan01]

WARNING: Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): desktop_drop
Future versions of Flutter will fail to build if your app uses plugins that apply KGP.

Ignore this warning. Flutter uses regular expression scanning, and it will issue a warning whenever apply plugin: 'kotlin-android' is present. We've already handled this, so let Flutter fix it themselves.

relate to flutter/flutter#189770

[loucass]

so happy to hear that it finally have built !!
Thanks again @ekuleshov for testing all the iterations with me and confirming the fix builds on your AGP 9.1 setup.

The unconditional KGP apply approach is the final solution same pattern as background_downloader and proven on both AGP 8 and 9. PR with the full fix is at #500 @boyan01