Add Flutter text to speech demo (#1087)

flutter-examples/tts/lib/utils.dart

@@ -0,0 +1,56 @@
+// Copyright (c)  2024  Xiaomi Corporation
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:flutter/services.dart';
+import 'package:path/path.dart' as p;
+import 'package:path_provider/path_provider.dart';
+
+Future<String> generateWaveFilename([String suffix = '']) async {
+  final Directory directory = await getApplicationDocumentsDirectory();
+  DateTime now = DateTime.now();
+  final filename =
+      '${now.year.toString()}-${now.month.toString().padLeft(2, '0')}-${now.day.toString().padLeft(2, '0')}-${now.hour.toString().padLeft(2, '0')}-${now.minute.toString().padLeft(2, '0')}-${now.second.toString().padLeft(2, '0')}$suffix.wav';
+
+  return p.join(directory.path, filename);
+}
+
+// https://stackoverflow.com/questions/68862225/flutter-how-to-get-all-files-from-assets-folder-in-one-list
+Future<List<String>> getAllAssetFiles() async {
+  final AssetManifest assetManifest =
+      await AssetManifest.loadFromAssetBundle(rootBundle);
+  final List<String> assets = assetManifest.listAssets();
+  return assets;
+}
+
+String stripLeadingDirectory(String src, {int n = 1}) {
+  return p.joinAll(p.split(src).sublist(n));
+}
+
+Future<void> copyAllAssetFiles() async {
+  final allFiles = await getAllAssetFiles();
+  for (final src in allFiles) {
+    final dst = stripLeadingDirectory(src);
+    await copyAssetFile(src, dst);
+  }
+}
+
+// Copy the asset file from src to dst.
+// If dst already exists, then just skip the copy
+Future<String> copyAssetFile(String src, [String? dst]) async {
+  final Directory directory = await getApplicationDocumentsDirectory();
+  if (dst == null) {
+    dst = p.basename(src);
+  }
+  final target = p.join(directory.path, dst);
+  bool exists = await new File(target).exists();
+
+  if (!exists) {
+    final data = await rootBundle.load(src);
+    final List<int> bytes =
+        data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
+    await (await File(target).create(recursive: true)).writeAsBytes(bytes);
+  }
+
+  return target;
+}