opt: reduce copy and malloc times for both of flutter and rust

flutter/lib/models/model.dart

@@ -1,10 +1,12 @@
 import 'dart:async';
 import 'dart:convert';
+import 'dart:ffi' hide Size;
 import 'dart:io';
 import 'dart:math';
 import 'dart:typed_data';
 import 'dart:ui' as ui;
 
+import 'package:ffi/ffi.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_hbb/consts.dart';
@@ -1367,6 +1369,9 @@ class FFI {
     final stream = bind.sessionStart(id: id);
     final cb = ffiModel.startEventListener(id);
     () async {
+      // Preserved for the rgba data.
+      Pointer<Uint8>? buffer;
+      int? bufferSize;
       await for (final message in stream) {
         if (message is EventToUI_Event) {
           try {
@@ -1377,13 +1382,30 @@ class FFI {
           }
         } else if (message is EventToUI_Rgba) {
           // Fetch the image buffer from rust codes.
-          bind.sessionGetRgba(id: id).then((rgba)  {
-            if (rgba != null) {
-              imageModel.onRgba(rgba);
+          final sz = platformFFI.getRgbaSize(id);
+          if (sz == null) {
+            return;
+          }
+          // The buffer does not exists or the bufferSize is not
+          // equal to the required size.
+          if (buffer == null || bufferSize != sz) {
+            // reallocate buffer
+            if (buffer != null) {
+              malloc.free(buffer);
             }
-          });
+            buffer = malloc.allocate(sz);
+            bufferSize = sz;
+          }
+          final rgba = platformFFI.getRgba(id, buffer, bufferSize!);
+          if (rgba != null) {
+            imageModel.onRgba(rgba);
+          }
         }
       }
+      // Free the buffer allocated on the heap.
+      if (buffer != null) {
+        malloc.free(buffer);
+      }
     }();
     // every instance will bind a stream
     this.id = id;

flutter/lib/models/native_model.dart

@@ -23,7 +23,10 @@ class RgbaFrame extends Struct {
 }
 
 typedef F2 = Pointer<Utf8> Function(Pointer<Utf8>, Pointer<Utf8>);
-typedef F3 = void Function(Pointer<Utf8>, Pointer<Utf8>);
+typedef F3 = Void Function(Pointer<Utf8>, Pointer<Uint8>);
+typedef F3Dart = void Function(Pointer<Utf8>, Pointer<Uint8>);
+typedef F4 = Uint64 Function(Pointer<Utf8>);
+typedef F4Dart = int Function(Pointer<Utf8>);
 typedef HandleEvent = Future<void> Function(Map<String, dynamic> evt);
 
 /// FFI wrapper around the native Rust core.
@@ -44,6 +47,8 @@ class PlatformFFI {
   final _toAndroidChannel = const MethodChannel('mChannel');
 
   RustdeskImpl get ffiBind => _ffiBind;
+  F3Dart? _session_get_rgba;
+  F4Dart? _session_get_rgba_size;
 
   static get localeName => Platform.localeName;
 
@@ -92,6 +97,23 @@ class PlatformFFI {
     return res;
   }
 
+  Uint8List? getRgba(String id, Pointer<Uint8> buffer, int bufSize) {
+    if (_session_get_rgba == null) return null;
+    var a = id.toNativeUtf8();
+    _session_get_rgba!(a, buffer);
+    final data = buffer.asTypedList(bufSize);
+    malloc.free(a);
+    return data;
+  }
+
+  int? getRgbaSize(String id) {
+    if (_session_get_rgba_size == null) return null;
+    var a = id.toNativeUtf8();
+    final bufferSize = _session_get_rgba_size!(a);
+    malloc.free(a);
+    return bufferSize;
+  }
+
   /// Init the FFI class, loads the native Rust core library.
   Future<void> init(String appType) async {
     _appType = appType;
@@ -107,6 +129,8 @@ class PlatformFFI {
     debugPrint('initializing FFI $_appType');
     try {
       _translate = dylib.lookupFunction<F2, F2>('translate');
+      _session_get_rgba = dylib.lookupFunction<F3, F3Dart>("session_get_rgba");
+      _session_get_rgba_size = dylib.lookupFunction<F4, F4Dart>("session_get_rgba_size");
       try {
         // SYSTEM user failed
         _dir = (await getApplicationDocumentsDirectory()).path;