Send id<MTLTexture> from ios to unity - ios

Question

I'm writing a plugin for unity, and I need to send a texture from ios to unity.
There is a UnitySendMessage function which takes char* as a parameter, but I didn't find a way to convert id<MTLTexture> to char*.
How can I send id<MTLTexture> from ios and receive it in unity?
My current code :
//ios side ...
id<MTLTexture> _texture = CVMetalTextureGetTexture(texture);
UnitySendMessage(CALLBACK_OBJECT, CALLBACK_TEXTURE_READY,_texture);//error
//...
//unity side
private void OnTextureReady(string texture_str)
{
IntPtr texture = new IntPtr(Int32.Parse(texture_str));
int width = 256;
int height = 256;
rawImage.texture = Texture2D.CreateExternalTexture(width, height,
TextureFormat.ARGB32, false, false, texture);
}

Answer 1

iOS plugin documentation says that you can only pass strings using UnitySendMessage.
The workaround would be to create a mapping from string to texture objects in Objective-C side, pass the string key via UnitySendMessage and then retrieve the texture object using a custom DllImport function.
Declare you map:
// class field
{
NSMutableDictionary<NSString *, id<MTLTexture>> _textures;
}
// in constructor
_textures = [NSMutableDictionary new];
// in function code
NSString *textureName = #"cookies";
_textures[textureName] = texture; // save MTLTexture for later
UnitySendMessage(CALLBACK_OBJECT, CALLBACK_TEXTURE_READY, textureName);
On the C# side CreateExternalTexture requires a pointer to a texture object of type IntPtr. To obtain it you can declare a DllImport function that takes a texture name and returns IntPtr:
[DllImport("__Internal")]
static extern IntPtr GetMetalTexturePointerByName(string textureName);
and implement it on the iOS side like so:
return plugin->_textures[textureName];
Not sure if it works though in terms of what CreateExternalTexture expects.
See also this post, a guy is doing something similar (but reverse):
Convert uintptr_t to id<MTLTexture>