How to access LibraryExtension in custom gradle plugin

Question

I would like to write a custom gradle plugin that manipulates either com.android.build.gradle.AppExtension or com.android.build.gradle.LibraryExtension. The basic form of the plugin is:

class AndroidLibrary : Plugin<Project> {
   override fun apply(project: Project) {

      var lib: LibraryExtension = project.extensions.getByName("android")  as LibraryExtension
      lib.minSdkVersion = "26"
   }
}

The problem is that the class LibraryExtension cannot be resolved. That class is contained in "com.android.tools.build:gradle:7.1.2". Note, there is no problem accessing either of these classes in a gradle.build.kts script file. I just can't access either of those classes from within a custom plugin like seen above. I've tried adding a dependency to com.android.tools.build.gradle 7.1.2 in the build script, but that doesn't work. I also tried adding it as a buildscript dependency, and that didn't help either. I also got the com.android.tools.build:gradle jar file and added it as a dependency, but that too didn't work.

It seems to me to be reasonable to expect a custom plugin of being able to manipulate the android build settings but I just can't find a way to resolve either LibraryExtension or AppExtension from within the custom plugin project.

If someone knows what I need to do to resolve those two classes, that would be greatly appreciated. I'm currently under the impression, that this is an architectural limitation of with android's gradle plugin and that I really can't access the "android" build section using LibraryExtension or AppExtension from a custom plugin and dsl. If that is the case, that too would be useful to know.

Answer 1

Initially I was building the plugin using IDEA. I could build the project by using a "gradlew build" command. I noticed that I would get the following message whenever I tried to sync the project:

 This version of the Android Support plugin for IntelliJ IDEA (or Android Studio) cannot open this project, please retry with version 2020.1.1 or newer

I had been ignoring that issue since I could successfully build the project using gradle commands. Here's a link the explains what the message means: https://intellij-support.jetbrains.com/hc/en-us/community/posts/4405168877202-This-version-of-the-Android-Support-plugin-for-IntelliJ-IDEA-or-Android-Studio-cannot-open-this-project-please-retry-with-version-2020-3-1-or-newer-

One of the suggestions to this problem was to build the project in Android Studio instead of waiting for IDEA to incorporate later android build. I did that and that indeed fixed that issue. The additional side effect was that I could now build the plugin. I was able to add dependencies for both gradleApi and to com.android.build.gradle and was able to resolve AppExtension of LibraryExtension. I had added the same dependencies I when I was using IDEA IDE. I had been customarily building my custom gradle plugin using IDEA. This was my first custom gradle plugin that I was going to manipulate an "android" build configuration.