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();
+  }
+}

lib/selectors/files.dart

@@ -1,20 +1,7 @@
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
 
 import '../backend.dart';
-
+import '../platform.dart';
-class Document {
-  final String id;
-  final String name;
-  final String parent;
-  final bool isDirectory;
-
-  Document.fromMap(Map<dynamic, dynamic> map)
-      : id = map['id'],
-        name = map['name'],
-        parent = map['parent'],
-        isDirectory = map['isDirectory'];
-}
 
 class FilesSelector extends StatefulWidget {
   @override
@@ -22,8 +9,6 @@ class FilesSelector extends StatefulWidget {
 }
 
 class _FilesSelectorState extends State<FilesSelector> {
-  static const platform = MethodChannel('de.johrpan.musicus/platform');
-
   BackendState backend;
   List<Document> history = [];
   List<Document> children = [];
@@ -120,15 +105,9 @@ class _FilesSelectorState extends State<FilesSelector> {
       children = [];
     });
 
-    final childrenMaps = await platform.invokeListMethod<Map<dynamic, dynamic>>(
+    final newChildren = await Platform.getChildren(
-      'getChildren',
+        backend.musicLibraryUri, history.isNotEmpty ? history.last.id : null);
-      {
-        'treeUri': backend.musicLibraryUri,
-        'parentId': history.isNotEmpty ? history.last.id : null,
-      },
-    );
 
-    final newChildren = childrenMaps.map((m) => Document.fromMap(m)).toList();
     newChildren.sort((d1, d2) {
       if (d1.isDirectory != d2.isDirectory) {
         return d1.isDirectory ? -1 : 1;