How to serialize a reference to the managed peer of an Android Callable Wrapper in Xamarin - xamarin.android

Question

I have a native Android Activity that receives a callback interface as part of the Intent used to start it:
public interface ICallback : Serializable
{
void invoke(Result result);
}
I want to implement the callback in Xamarin as a lambda:
class CallbackWrapper : Java.Lang.Object, ICallback
{
private Action<Result> onInvoke;
public CallbackWrapper(Action<Result> onInvoke)
{
this.onInvoke = onInvoke;
}
public void Invoke(Result result)
{
this.onInvoke(result);
}
}
...
intent.PutExtra(CALLBACK_EXTRA, new CallbackWrapper(result => { ... }));
StartActivityForResult(intent);
The first problem is that when my callback gets deserialized from the intent bundle, I get the following exceptions:
System.NotSupportedException
Unable to activate instance of type CallbackWrapper from native handle 0xff...
System.MissingMethodException
No constructor found for CallbackWrapper::.ctor(System.IntPtr, Android.Runtime.JniHandleOwnership)
I add the constructor as explained in the exception:
class CallbackWrapper : Java.Lang.Object, ICallback
{
public CallbackWrapper(IntPtr handle, JniHandleOwnership transfer) : base(handle, transfer)
{
}
...
}
The exception is fixed, but now when the activity calls my handler, the onInvoke field is null. How do I get a reference to the onInvoke delegate that was used to create the Intent?

Answer 1

The solution - serialize a handle to the original object.
The first step is to enable object serialization. Serialization in Java is done using specially-named private methods, instead of through interface methods. Xamarin allows you to inject these methods into the generated Android callable wrappers using the Java.Interop.ExportAttribute attribute:
using Java.Interop;
class CallbackWrapper : Java.Lang.Object, ICallback
{
...
[Export("readObject", Throws = new[] { typeof(Java.IO.IOException), typeof(Java.Lang.ClassNotFoundException) })]
private void ReadObject(Java.IO.ObjectInputStream source)
{
}
[Export("writeObject", Throws = new[] { typeof(Java.IO.IOException), typeof(Java.Lang.ClassNotFoundException) })]
private void WriteObject(Java.IO.ObjectOutputStream destination)
{
}
}
Even if an ACW implements Serializable, the ACW itself has no useful fields - that why you need to serialize the managed state through the readObject/writeObject method pair.
Note that for this to work, your project needs to reference the Mono.Android.Export assembly, otherwise you'll get a build-time error.
The second part is getting a serializable reference to CallbackWrapper. This can be achieved using System.Runtime.InteropServices.GCHandle. The first step is to create a handle to the object and write it during serialization:
[Export("writeObject", Throws = new[] { typeof(Java.IO.IOException), typeof(Java.Lang.ClassNotFoundException) })]
private void WriteObject(Java.IO.ObjectOutputStream destination)
{
var handle = GCHandle.Alloc(this);
destination.WriteLong(GCHandle.ToIntPtr(handle).ToInt64());
}
The second step is deserialization:
[Export("readObject", Throws = new[] { typeof(Java.IO.IOException), typeof(Java.Lang.ClassNotFoundException) })]
private void ReadObject(Java.IO.ObjectInputStream source)
{
// deserialize GCHandle from stream
var handle = GCHandle.FromIntPtr(new IntPtr(source.ReadLong()));
// convert handle to object
var trueSelf = handle.Target as NativeValidationHandler;
// copy fields from original callback
this.onInvoke = trueSelf.onInvoke;
// free this handle
handle.Free();
}
The handle doesn't need to be a pinned handle, because we don't ever access the object's address, we just use the handle.
Note that in the above implementation you can only deserialize a callback once, because deserialization will free the handle. Alternatively you can allocate the handle once in the constructor and provide a Dispose method that frees that handle, if you wish to be able to deserialize the handle multiple times. Freeing the handle during deserialization also means that the object will never be collected if it's never deserialized, because the handle will prevent the object from being collected.

Answer 2

If you want to use Serializable than you are right. but i would recommend you to use Parcelable, because
Parcelable is a part of Android sdk and it's mainly made for parcelling purpose.
Parcelable is faster than Serializable because it doesn't use reflection while later does.
Although there is demerit that it has some boilerplate code.
Worth to read => https://android.jlelse.eu/parcelable-vs-serializable-6a2556d51538