Use the storage access framework

Everything related to file system access has been rewritten to make use
of the storage access framework. This means that the
WRITE_EXTERNAL_STORAGE is no longer needed. Because of that, the
dependency on permission_handler could be dropped and all code related
to permission handling has been removed. To be able to open a whole
document tree, the minSdkVersion was bumped to 21. Finally the file
selector was rewritten using custom platform dependent code.

android/app/build.gradle

@@ -38,7 +38,7 @@ android {
 
     defaultConfig {
         applicationId "de.johrpan.musicus"
-        minSdkVersion 16
+        minSdkVersion 21
         targetSdkVersion 29
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName

android/app/src/main/AndroidManifest.xml

@@ -1,8 +1,6 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="de.johrpan.musicus">
 
-    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
-
     <application
         android:name="io.flutter.app.FlutterApplication"
         android:label="Musicus"

android/app/src/main/kotlin/de/johrpan/musicus/MainActivity.kt

@@ -1,39 +1,117 @@
 package de.johrpan.musicus
 
-import androidx.annotation.NonNull;
-import androidx.core.content.ContextCompat
+import android.app.Activity
+import android.content.Intent
+import android.net.Uri
+import android.provider.DocumentsContract
+import androidx.annotation.NonNull
 import io.flutter.embedding.android.FlutterActivity
 import io.flutter.embedding.engine.FlutterEngine
 import io.flutter.plugin.common.MethodChannel
 import io.flutter.plugins.GeneratedPluginRegistrant
 
+class Document(private val id: String, private val name: String, private val parentId: String?, private val isDirectory: Boolean) {
+    fun toMap(): Map<String, Any?> {
+        return mapOf(
+                "id" to id,
+                "name" to name,
+                "parentId" to parentId,
+                "isDirectory" to isDirectory
+        )
+    }
+}
+
 class MainActivity : FlutterActivity() {
     private val CHANNEL = "de.johrpan.musicus/platform"
+    private val AODT_REQUEST = 0
+
+    private var aodtResult: MethodChannel.Result? = null
 
     override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
         GeneratedPluginRegistrant.registerWith(flutterEngine)
 
         MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler { call, result ->
-            if (call.method == "getStorageRoots") {
-                result.success(getStorageRoots())
+            if (call.method == "openTree") {
+                aodtResult = result
+                // We will get the result within onActivityResult
+                openTree()
+            } else if (call.method == "getChildren") {
+                val treeUri = Uri.parse(call.argument<String>("treeUri"))
+                val parentId = call.argument<String>("parentId")
+                val children = getChildren(treeUri, parentId)
+                result.success(children.map { it.toMap() })
             } else {
                 result.notImplemented()
             }
         }
     }
 
-    private fun getStorageRoots(): ArrayList<String> {
-        val result = ArrayList<String>()
+    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
+        super.onActivityResult(requestCode, resultCode, data)
 
-        ContextCompat.getExternalFilesDirs(this, null).forEach {
-            val path = it.absolutePath;
-            val index = path.lastIndexOf("/Android/data/")
+        if (requestCode == AODT_REQUEST) {
+            if (resultCode == Activity.RESULT_OK && data?.data != null) {
+                // Drop all old URIs
+                contentResolver.persistedUriPermissions.forEach {
+                    contentResolver.releasePersistableUriPermission(it.uri, Intent.FLAG_GRANT_WRITE_URI_PERMISSION)
+                }
 
-            if (index > 0) {
-                result.add(path.substring(0, index))
+                // We already checked for null
+                val uri = data.data!!
+                contentResolver.takePersistableUriPermission(uri, Intent.FLAG_GRANT_WRITE_URI_PERMISSION)
+
+                aodtResult?.success(uri.toString())
+            } else {
+                aodtResult?.success(null)
             }
         }
+    }
 
-        return result
+    /**
+     * Open a document tree using the storage access framework
+     *
+     * The result is handled within [onActivityResult]
+     */
+    private fun openTree() {
+        val intent = Intent(Intent.ACTION_OPEN_DOCUMENT_TREE)
+        intent.addFlags(Intent.FLAG_GRANT_WRITE_URI_PERMISSION)
+        intent.addFlags(Intent.FLAG_GRANT_PERSISTABLE_URI_PERMISSION)
+
+        startActivityForResult(intent, AODT_REQUEST)
+    }
+
+    /**
+     * List children of a directory
+     *
+     * @param treeUri The treeUri from the ACTION_OPEN_DOCUMENT_TREE request
+     * @param parentId Document ID of the parent directory or null for the top level directory
+     * @return List of directories and files within the directory
+     */
+    private fun getChildren(treeUri: Uri, parentId: String?): List<Document> {
+        val realParentId = parentId ?: DocumentsContract.getTreeDocumentId(treeUri)
+        val children = ArrayList<Document>()
+        val childrenUri = DocumentsContract.buildChildDocumentsUriUsingTree(treeUri, realParentId)
+
+        val cursor = contentResolver.query(
+                childrenUri,
+                arrayOf(DocumentsContract.Document.COLUMN_DOCUMENT_ID,
+                        DocumentsContract.Document.COLUMN_DISPLAY_NAME,
+                        DocumentsContract.Document.COLUMN_MIME_TYPE),
+                null, null, null)
+
+        if (cursor != null) {
+            while (cursor.moveToNext()) {
+                val id = cursor.getString(0)
+                val name = cursor.getString(1)
+                val isDirectory = cursor.getString(2) == DocumentsContract.Document.MIME_TYPE_DIR
+
+                // Use parentId here to let the consumer know that we are at the top level.
+                children.add(Document(id, name, parentId, isDirectory))
+            }
+
+            cursor.close()
+        }
+
+        return children
     }
 }