feat: add general ui handler

flutter/lib/plugin/handlers.dart

@@ -0,0 +1,43 @@
+import 'dart:ffi';
+
+abstract class NativeHandler {
+  bool onEvent(Map<String, dynamic> evt);
+}
+
+typedef OnSelectPeersCallback = Bool Function(Int returnCode,
+    Pointer<Void> data, Uint64 dataLength, Pointer<Void> userData);
+typedef OnSelectPeersCallbackDart = bool Function(
+    int returnCode, Pointer<Void> data, int dataLength, Pointer<Void> userData);
+
+class NativeUiHandler extends NativeHandler {
+  NativeUiHandler._();
+
+  static NativeUiHandler instance = NativeUiHandler._();
+
+  @override
+  bool onEvent(Map<String, dynamic> evt) {
+    final name = evt['name'];
+    final action = evt['action'];
+    if (name != "native_ui") {
+      return false;
+    }
+    switch (action) {
+      case "select_peers":
+        int cb = evt['cb'];
+        int userData = evt['user_data'] ?? 0;
+        final cbFuncNative = Pointer.fromAddress(cb)
+            .cast<NativeFunction<OnSelectPeersCallback>>();
+        final cbFuncDart = cbFuncNative.asFunction<OnSelectPeersCallbackDart>();
+        onSelectPeers(cbFuncDart, userData);
+        break;
+      default:
+        return false;
+    }
+    return true;
+  }
+
+  void onSelectPeers(OnSelectPeersCallbackDart cb, int userData) async {
+    // TODO: design a UI interface to pick peers.
+    cb(0, Pointer.fromAddress(0), 0, Pointer.fromAddress(userData));
+  }
+}