Hmm, I think I have it figured out, after reading a bit of Custom Gradle Plugin ID not found .

The thing is – I could build SpannedGridLayoutManager with ./gradlew assemble; but the error message I get, appears specifically when I build it as a dependent project.

The thing is: when I build SpannedGridLayoutManager standalone, first SpannedGridLayoutManager/build.gradle gets called, and defines classpaths in dependencies in buildscript – then when SpannedGridLayoutManager/spannedgridlayoutmanager/build.gradle, the plugin ids get resolved according to the classpaths defined earlier.

When I build my project, of course first MyProject/build.gradle runs – but it did not have the same classpaths set; so then when MyProject/../SpannedGridLayoutManager/spannedgridlayoutmanager/build.gradle gets processed – as a dependent project – the classpaths are not defined, and the plugin lookup fails.

So in my project, I basically had to add these two lines in build.gradle – here shown as a diff:

$ git diff build.gradle
diff --git a/build.gradle b/build.gradle
index b730fda..6a696c9 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1,5 +1,6 @@
 buildscript {
     ext.kotlin_version = "1.4.0"
+    ext.dokka_version = '0.9.18'
     repositories {
         google()
         jcenter()
@@ -10,6 +11,7 @@ buildscript {
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
         classpath "com.android.tools.build:gradle:4.0.1"
         classpath "com.jfrog.bintray.gradle:gradle-bintray-plugin:1.8.4"
+        classpath "org.jetbrains.dokka:dokka-android-gradle-plugin:$dokka_version"
     }
 }

Of course, these two lines are copied verbatim from the original build file, SpannedGridLayoutManager/build.gradle.

Well, nice to get this fixed – but it would have been even better, with a better error message (or even, if ./gradlew --debug would say something like searching classpaths for ... and dump out everything where it looked, and what it found).

I am trying to build a project that uses: https://github.com/Arasthel/SpannedGridLayoutManager

When I run ./gradlew --assemble, I get:

1: Task failed with an exception.
-----------
* Where:
Build file '/path/to/SpannedGridLayoutManager/spannedgridlayoutmanager/build.gradle' line: 3

* What went wrong:
A problem occurred evaluating project ':spannedlm'.
> Plugin with id 'org.jetbrains.dokka-android' not found.

* Try:
Run with --stacktrace option to get the stack trace. Run with --debug option to get more log output. Run with --scan to get full insights.

This is SpannedGridLayoutManager/spannedgridlayoutmanager/build.gradle:

apply plugin: 'com.android.library'
apply plugin: 'kotlin-android'
apply plugin: 'org.jetbrains.dokka-android'

task dokkaJavadoc(type: org.jetbrains.dokka.gradle.DokkaAndroidTask) {
    delete "$rootDir/docs"

    outputFormat = 'html'
    outputDirectory = "$rootDir/docs"

    externalDocumentationLink {
        url = new URL("https://developer.android.com/reference/")
    }

    includeNonPublic = false

    // Always recreate documentation
    outputs.upToDateWhen { return false }
}

afterEvaluate {
    if (project.hasProperty("javadocJar")) {
        tasks.javadocJar.dependsOn dokkaJavadoc
    }
}

ext {
    bintrayRepo = 'maven'
    bintrayName = 'spannedgridlayoutmanager'

    publishedGroupId = 'com.arasthel'
    libraryName = 'SpannedGridLayoutManager'
    artifact = 'spannedgridlayoutmanager'

    libraryDescription = 'A layout manager that will resize and reorder views based on a provided SpanSize.'

    siteUrl = 'https://github.com/Arasthel/SpannedGridLayoutManager'
    gitUrl = 'https://github.com/Arasthel/SpannedGridLayoutManager.git'

    libraryVersion = '3.0.2'

    developerId = 'Arasthel'
    developerName = 'Jorge Martín Epsinosa'
    developerEmail = '[email protected]'

    licenseName = 'MIT'
    licenseUrl = 'https://opensource.org/licenses/MIT'
    allLicenses = ["MIT"]
}

android {
    //compileSdkVersion 28
    compileSdkVersion 29
    buildToolsVersion buildtools_version

    defaultConfig {
        minSdkVersion 14
        //targetSdkVersion 28
        targetSdkVersion 28
        versionCode 1
        versionName libraryVersion
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

tasks.withType(Javadoc) {
    excludes = ['**/*.kt']
}

dependencies {
    implementation "com.android.support:recyclerview-v7:$support_library_version"
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
}

// Bintray Upload
apply from: '../gradle/tools/bintrayv1.gradle'
apply from: '../gradle/tools/installv1.gradle'

This is SpannedGridLayoutManager/build.gradle:

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    //ext.kotlin_version = '1.3.10'
    ext.kotlin_version = '1.3.61'
    ext.support_library_version = '28.0.0'
    //ext.dokka_version = '0.9.17'
    ext.dokka_version = '0.9.18'
    ext.buildtools_version = '28.0.3'
    repositories {
        jcenter()
        google()
    }
    dependencies {
        //classpath 'com.android.tools.build:gradle:3.2.1'
        classpath 'com.android.tools.build:gradle:4.0.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "org.jetbrains.dokka:dokka-android-gradle-plugin:$dokka_version"

        classpath 'com.jfrog.bintray.gradle:gradle-bintray-plugin:1.8.4'
        classpath 'com.github.dcendents:android-maven-gradle-plugin:2.1'
    }
}

allprojects {
    repositories {
        jcenter()
        google()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

What do I need to do to get this to work?

Aside – this looks like some plugin, that gradlew cannot retrieve online from its repository.

Is there a command to see all available plugins visible to gradlew for a given configuration? Something like apt-show-versions -R -a would list all packages available to apt in Debian?