Add Dart API for VAD (#904)

sherpa-onnx/flutter/example/lib/main.dart

@@ -1,7 +1,9 @@
 // Copyright (c)  2024  Xiaomi Corporation
 import 'package:sherpa_onnx/sherpa_onnx.dart' as sherpa_onnx;
 import 'package:flutter/material.dart';
+
 import "./speaker_identification_test.dart";
+import "./vad_test.dart";
 
 void main() {
   runApp(const MyApp());
@@ -51,6 +53,7 @@ class _MyHomePageState extends State<MyHomePage> {
     if (_counter <= 10) {
       sherpa_onnx.initBindings();
       await testSpeakerID();
+      // await testVad();
     }
 
     setState(() {

sherpa-onnx/flutter/example/lib/vad_test.dart

@@ -0,0 +1,62 @@
+// Copyright (c)  2024  Xiaomi Corporation
+import 'dart:typed_data';
+import 'package:sherpa_onnx/sherpa_onnx.dart' as sherpa_onnx;
+import './utils.dart';
+
+Future<void> testVad() async {
+  final src = 'assets/silero_vad.onnx';
+  final modelPath = await copyAssetFile(src: src, dst: 'silero_vad.onnx');
+
+  final sileroVadConfig = sherpa_onnx.SileroVadModelConfig(model: modelPath);
+  final config = sherpa_onnx.VadModelConfig(
+    sileroVad: sileroVadConfig,
+    numThreads: 1,
+    debug: true,
+  );
+
+  final vad = sherpa_onnx.VoiceActivityDetector(
+      config: config, bufferSizeInSeconds: 10);
+  print('before vad.free(): ${vad.ptr}');
+  vad.free();
+  print('after vad.free(): ${vad.ptr}');
+
+  final buffer = sherpa_onnx.CircularBuffer(capacity: 16000 * 2);
+
+  final d = Float32List.fromList([0, 10, 20, 30]);
+  buffer.push(d);
+  assert(d.length == buffer.size, '${d.length} vs ${buffer.size}');
+
+  final f = Float32List.fromList([-5, 100.25, 599]);
+  buffer.push(f);
+
+  assert(buffer.size == d.length + f.length);
+  final g = buffer.get(startIndex: 0, n: 5);
+
+  assert(g.length == 5);
+  assert(g[0] == 0);
+  assert(g[1] == 10);
+  assert(g[2] == 20);
+  assert(g[3] == 30);
+  assert(g[4] == -5);
+
+  assert(buffer.size == d.length + f.length);
+
+  buffer.pop(3);
+  assert(buffer.size == d.length + f.length - 3);
+
+  final h = buffer.get(startIndex: buffer.head, n: 4);
+  assert(h.length == 4);
+  assert(h[0] == 30);
+  assert(h[1] == -5);
+  assert(h[2] == 100.25);
+  assert(h[3] == 599);
+
+  buffer.reset();
+
+  assert(buffer.size == 0);
+  assert(buffer.head == 0);
+
+  print('before free: ${buffer.ptr}');
+  buffer.free();
+  print('after free: ${buffer.ptr}');
+}