Rework android support example (#3676)

Tries to address some of the issues in
#3675


* [ ] Various tasks like `sdkUrl` or `buildToolsVersion` should be
prefixed with `android`
* [x] Hardcoded `buildToolsVersion` defaults to `35.0.0`. I suggest to
just remove the default.
* [x] Hardcoded `platformsVersion` defaults to `android-35`. We could
derive it from `buildToolsVersion`.
* [ ] In general lots of various paths which are just sub-paths of
`buildToolsPath` have their own tasks. Instead we could have some
"config" class, which returns common paths from a `buildToolsPath`, e.g.
by applying a (version specific) layout/sub-paths.
* [x] `AndroidAppModule.androidSdkModule` should have the type
`ModuleRef[AndroidSdkModule]`.
* [x] `androidResources` hardcodes a path to
`src/main/AndroidManifest.xml`. It should either lookup the file from
`sources` task, or be it's own `androidManifest: Source` task.
* [x] `androidKeystore` is probably meant to be a persistent task

example/javalib/android/1-hello-world/build.mill

@@ -12,19 +12,23 @@ package build
 import mill._
 import mill.javalib.android.{AndroidAppModule, AndroidSdkModule}
 
-object `package` extends RootModule with AndroidAppModule {
-
-  // Create and configure an Android SDK module to manage Android SDK paths and tools.
-  object androidSdkModule extends AndroidSdkModule
+// Create and configure an Android SDK module to manage Android SDK paths and tools.
+object androidSdkModule0 extends AndroidSdkModule{
+  def buildToolsVersion = "35.0.0"
+}
 
+// Actual android application
+object app extends AndroidAppModule {
+  def androidSdkModule = mill.define.ModuleRef(androidSdkModule0)
 }
 
 ////SNIPPET:END
 
 
 /** Usage
 
-> ./mill androidApk
+> ./mill show app.androidApk
+".../out/app/androidApk.dest/app.apk"
 
 */
 
@@ -42,13 +46,9 @@ object `package` extends RootModule with AndroidAppModule {
 // ----
 // .
 // ├── build.mill
-// └── src
-//     └── main
-//         ├── AndroidManifest.xml
-//         └── java
-//             └── com
-//                 └── helloworld
-//                     └── app
-//                         └── MainActivity.java
+// ├── AndroidManifest.xml
+// └── src/main/java
+//     └── com/helloworld/app
+//         └── MainActivity.java
 // ----
 //

scalalib/src/mill/javalib/android/AndroidAppModule.scala

@@ -2,6 +2,7 @@ package mill.javalib.android
 
 import mill._
 import mill.api.PathRef
+import mill.define.ModuleRef
 import mill.scalalib.JavaModule
 
 /**
@@ -27,7 +28,12 @@ trait AndroidAppModule extends JavaModule {
    *
    * @return The Android SDK module that is used across the project.
    */
-  def androidSdkModule: AndroidSdkModule
+  def androidSdkModule: ModuleRef[AndroidSdkModule]
+
+  /**
+   * An XML file containing configuration and metadata about your android application
+   */
+  def androidManifest: Task[PathRef] = Task.Source(millSourcePath / "AndroidManifest.xml")
 
   /**
    * Generates the Android resources (such as layouts, strings, and other assets) needed
@@ -45,16 +51,16 @@ trait AndroidAppModule extends JavaModule {
     val genDir: os.Path = T.dest // Directory to store generated resources.
 
     os.call(Seq(
-      androidSdkModule.aaptPath().path.toString, // Call aapt tool
+      androidSdkModule().aaptPath().path.toString, // Call aapt tool
       "package",
       "-f",
       "-m",
       "-J",
       genDir.toString, // Generate R.java files
       "-M",
-      (millSourcePath / "src/main/AndroidManifest.xml").toString, // Use AndroidManifest.xml
+      androidManifest().path.toString, // Use AndroidManifest.xml
       "-I",
-      androidSdkModule.androidJarPath().path.toString // Include Android SDK JAR
+      androidSdkModule().androidJarPath().path.toString // Include Android SDK JAR
     ))
 
     PathRef(genDir)
@@ -64,7 +70,7 @@ trait AndroidAppModule extends JavaModule {
    * Adds the Android SDK JAR file to the classpath during the compilation process.
    */
   def unmanagedClasspath: T[Agg[PathRef]] = Task {
-    Agg(androidSdkModule.androidJarPath())
+    Agg(androidSdkModule().androidJarPath())
   }
 
   /**
@@ -93,7 +99,7 @@ trait AndroidAppModule extends JavaModule {
 
     os.call(
       Seq(
-        androidSdkModule.d8Path().path.toString, // Call d8 tool
+        androidSdkModule().d8Path().path.toString, // Call d8 tool
         "--output",
         jarFile.toString, // Output JAR file
         "--no-desugaring" // Disable desugaring
@@ -112,10 +118,10 @@ trait AndroidAppModule extends JavaModule {
     val dexOutputDir: os.Path = T.dest
 
     os.call(
-      Seq(androidSdkModule.d8Path().path.toString, "--output", dexOutputDir.toString) ++
+      Seq(androidSdkModule().d8Path().path.toString, "--output", dexOutputDir.toString) ++
         Seq(
           androidJar().path.toString, // Use the JAR file from the previous step
-          androidSdkModule.androidJarPath().path.toString // Include Android framework classes
+          androidSdkModule().androidJarPath().path.toString // Include Android framework classes
         )
     )
 
@@ -134,13 +140,13 @@ trait AndroidAppModule extends JavaModule {
 
     os.call(
       Seq(
-        androidSdkModule.aaptPath().path.toString,
+        androidSdkModule().aaptPath().path.toString,
         "package",
         "-f",
         "-M",
-        (millSourcePath / "src/main/AndroidManifest.xml").toString, // Path to AndroidManifest.xml
+        androidManifest().path.toString, // Path to AndroidManifest.xml
         "-I",
-        androidSdkModule.androidJarPath().path.toString, // Include Android JAR
+        androidSdkModule().androidJarPath().path.toString, // Include Android JAR
         "-F",
         unsignedApk.toString // Output APK
       ) ++ Seq(androidDex().path.toString) // Include DEX files
@@ -160,7 +166,7 @@ trait AndroidAppModule extends JavaModule {
 
     os.call(
       Seq(
-        androidSdkModule.zipalignPath().path.toString, // Call zipalign tool
+        androidSdkModule().zipalignPath().path.toString, // Call zipalign tool
         "-f",
         "-p",
         "4", // Force overwrite, align with 4-byte boundary
@@ -189,7 +195,7 @@ trait AndroidAppModule extends JavaModule {
 
     os.call(
       Seq(
-        androidSdkModule.apksignerPath().path.toString,
+        androidSdkModule().apksignerPath().path.toString,
         "sign", // Call apksigner tool
         "--ks",
         androidKeystore().path.toString, // Path to keystore
@@ -218,7 +224,7 @@ trait AndroidAppModule extends JavaModule {
    * For more details on the keytool utility, refer to:
    * [[https://docs.oracle.com/javase/8/docs/technotes/tools/windows/keytool.html keytool Documentation]]
    */
-  def androidKeystore: T[PathRef] = Task {
+  def androidKeystore: T[PathRef] = Task(persistent = true) {
     val keystoreFile: os.Path = T.dest / "keystore.jks"
 
     if (!os.exists(keystoreFile)) {

scalalib/src/mill/javalib/android/AndroidSdkModule.scala

@@ -27,12 +27,12 @@ trait AndroidSdkModule extends Module {
   /**
    * Specifies the version of the Android build tools to be used.
    */
-  def buildToolsVersion: T[String] = Task { "35.0.0" }
+  def buildToolsVersion: T[String]
 
   /**
    * Specifies the Android platform version (e.g., Android API level).
    */
-  def platformsVersion: T[String] = Task { "android-35" }
+  def platformsVersion: T[String] = Task { "android-" + buildToolsVersion().split('.').head }
 
   /**
    * Provides the path to the `android.jar` file, necessary for compiling Android apps.