Move platform dependent code to its own class

lib/platform.dart

@@ -0,0 +1,50 @@
+import 'package:flutter/services.dart';
+
+/// Object representing a document in Storage Access Framework terms.
+class Document {
+  /// Unique document ID given by the SAF.
+  final String id;
+
+  /// Name of the document (i.e. file name).
+  final String name;
+
+  /// Document ID of the parent document.
+  final String parent;
+
+  /// Whether this document represents a directory.
+  final bool isDirectory;
+
+  // Use Map<dynamic, dynamic> here, as we get casting errors otherwise. This
+  // won't be typesafe anyway.
+  Document.fromJson(Map<dynamic, dynamic> json)
+      : id = json['id'],
+        name = json['name'],
+        parent = json['parent'],
+        isDirectory = json['isDirectory'];
+}
+
+/// Collection of methods that are implemented platform dependent.
+class Platform {
+  static const _platform = MethodChannel('de.johrpan.musicus/platform');
+
+  /// Get child documents.
+  ///
+  /// [treeId] is the base URI as requested from the SAF.
+  /// [parentId] is the document ID of the parent. If this is null, the children
+  /// of the tree base will be returned.
+  static Future<List<Document>> getChildren(
+      String treeUri, String parentId) async {
+    final List<Map<dynamic, dynamic>> childrenJson =
+        await _platform.invokeListMethod(
+      'getChildren',
+      {
+        'treeUri': treeUri,
+        'parentId': parentId,
+      },
+    );
+
+    return childrenJson
+        .map((childJson) => Document.fromJson(childJson))
+        .toList();
+  }
+}