flutter how to create an dart:ffi struct reference - flutter

Question

I created a struct with dart:ffi.
import 'dart:ffi';
import 'package:ffi/ffi.dart';
class TestStruct extends Struct{
external Pointer<Utf8> strText;
#Int32()
external int nNum;
#Bool()
external bool bIsTrue;
//contstruct
TestStruct(String str, int number, bool state){
strText = str as Pointer<Utf8>;
nNum = number as int;
bIsTrue = state as bool;
}
}
I want to create a reference of TestStruct and use it. So I wrote the code.
TestStruct test = TestStruct("Text", 10, true);
but this is an error
Subclasses of 'Struct' and 'Union' are backed by native memory, and can't be instantiated by a generative constructor.
Try allocating it via allocation, or load from a 'Pointer'.
I tried searching with the api documentation, but I didn't understand. Do you know how to create a struct as a reference?? thank you.

Answer 1

Example:
class InAddr extends Struct {
factory InAddr.allocate(int sAddr) =>
calloc<InAddr>().ref
..sAddr = sAddr;
#Uint32()
external int sAddr;
}
You can allocate this with calloc function
final Pointer<InAddr> inAddress = calloc<InAddr>();
And free the pointer with
calloc.free(inAddress);

Answer 2

While I am late to the party, writing a complete example so that it may be helpful for others new to the FFI scene.
If this is the dart representative struct (from C) -
import 'dart:ffi';
import 'package:ffi/ffi.dart';
class TestStruct extends Struct{
#Int32()
external int nNum;
#Bool()
external bool bIsTrue;
// IMPORTANT: DO NOT DEFINE A CONSTRUCTOR since struct/union are natively allocated
}
You can instantiate from dart using ffi.Pointer like below.
// my_native_helper.dart
final dynamicLibrary = ... ; // load .so/process
void some_function() {
final Pointer<TestStruct > inAddress = calloc<TestStruct >();
inAddress.ref.nNum= 1234;
inAddress.ref.bIsTrue = false;
// ffigen generated method replicating C extern function
// To auto generate this, define the C signature in a header file
dynamicLibrary.receiveFromDart(inAddress);
calloc.free(inAddress);
}
The C method will look like below.
extern "C" void recieveFromDart(TestStruct* structPtr) {
cout << structPtr->nNum << "/" << structPtr->bIsTrue << endl;
}

Answer 3

According to the doc you can't create instances of Struct classes:
https://api.flutter.dev/flutter/dart-ffi/Struct-class.html
However, you typically would need pointers. So you can come up with something like that:
static Pointer<InAddr> allocate(int sAddr) {
final pointer = calloc<InAddr>();
pointer.ref.sAddr = sAddr;
return pointer;
}