Refactor flutter to support Android (#1072)

flutter/sherpa_onnx/lib/sherpa_onnx.dart

@@ -0,0 +1,56 @@
+// Copyright (c)  2024  Xiaomi Corporation
+import 'dart:io';
+import 'dart:ffi';
+
+export 'src/feature_config.dart';
+export 'src/offline_recognizer.dart';
+export 'src/offline_stream.dart';
+export 'src/online_recognizer.dart';
+export 'src/online_stream.dart';
+export 'src/speaker_identification.dart';
+export 'src/tts.dart';
+export 'src/vad.dart';
+export 'src/wave_reader.dart';
+export 'src/wave_writer.dart';
+
+import 'src/sherpa_onnx_bindings.dart';
+
+String? _path;
+
+// see also
+// https://github.com/flutter/codelabs/blob/main/ffigen_codelab/step_05/lib/ffigen_app.dart
+final DynamicLibrary _dylib = () {
+  if (Platform.isIOS) {
+    throw UnsupportedError('Unknown platform: ${Platform.operatingSystem}');
+  }
+  if (Platform.isMacOS) {
+    if (_path == null) {
+      return DynamicLibrary.open('libsherpa-onnx-c-api.dylib');
+    } else {
+      return DynamicLibrary.open('$_path/libsherpa-onnx-c-api.dylib');
+    }
+  }
+
+  if (Platform.isAndroid || Platform.isLinux) {
+    if (_path == null) {
+      return DynamicLibrary.open('libsherpa-onnx-c-api.so');
+    } else {
+      return DynamicLibrary.open('$_path/libsherpa-onnx-c-api.so');
+    }
+  }
+
+  if (Platform.isWindows) {
+    if (_path == null) {
+      return DynamicLibrary.open('sherpa-onnx-c-api.dll');
+    } else {
+      return DynamicLibrary.open('$_path\\sherpa-onnx-c-api.dll');
+    }
+  }
+
+  throw UnsupportedError('Unknown platform: ${Platform.operatingSystem}');
+}();
+
+void initBindings([String? p]) {
+  _path ??= p;
+  SherpaOnnxBindings.init(_dylib);
+}