Refactor flutter to support Android (#1072)

flutter/sherpa_onnx/lib/src/offline_stream.dart

@@ -0,0 +1,37 @@
+// Copyright (c)  2024  Xiaomi Corporation
+import 'dart:ffi';
+import 'dart:typed_data';
+import 'package:ffi/ffi.dart';
+
+import './sherpa_onnx_bindings.dart';
+
+class OfflineStream {
+  /// The user has to call OfflineStream.free() to avoid memory leak.
+  OfflineStream({required this.ptr});
+
+  void free() {
+    SherpaOnnxBindings.destroyOfflineStream?.call(ptr);
+    ptr = nullptr;
+  }
+
+  /// If you have List<double> data, then you can use
+  /// Float32List.fromList(data) to convert data to Float32List
+  ///
+  /// See
+  ///  https://api.flutter.dev/flutter/dart-core/List-class.html
+  /// and
+  ///  https://api.flutter.dev/flutter/dart-typed_data/Float32List-class.html
+  void acceptWaveform({required Float32List samples, required int sampleRate}) {
+    final n = samples.length;
+    final Pointer<Float> p = calloc<Float>(n);
+
+    final pList = p.asTypedList(n);
+    pList.setAll(0, samples);
+
+    SherpaOnnxBindings.acceptWaveformOffline?.call(ptr, sampleRate, p, n);
+
+    calloc.free(p);
+  }
+
+  Pointer<SherpaOnnxOfflineStream> ptr;
+}