txt: ## dist_cpan-CTILMES-NativeHelpers-Callback.md
  # NAME
  
  NativeHelpers::Callback - Helper for looking up perl objects from C callbacks
  
  # SYNOPSIS
  
  ```
  use NativeHelpers::Callback;
  
  class thing is repr('CPointer') {}
  
  sub make_a_thing(--> thing) is native {}
  sub setcallback(&callback (int64 --> int32), int64) is native {}
  sub callit(--> int32) is native('./callback') {}
  
  class PerlObject
  {
      has thing $.thing;
      has int32 $.number;
  }
  
  sub my-callback(int64 $user-data --> int32)
  {
      NativeHelpers::Callback.lookup($user-data).number
  }
  
  my $object = PerlObject.new(thing => make_a_thing, number => 12);
  
  NativeHelpers::Callback.store($object, $object.thing);
  
  setcallback(&my-callback, NativeHelpers::Callback.id($object.thing));
  
  my $ret = callit();
  
  # You can also use option ":cb" to get a shorthand "cb":
  
  cb.store($object, $object.thing);
  cb.lookup($id);
  setcallback(&my-callback, cb.id($object.thing));
  cb.remove($object.thing);
  
  ```
  
  # DESCRIPTION
  
  C libraries often have callback routines allowing you to pass in an extra `void *` parameter of user data from which you are supposed to link into whatever functionality you need from within the callback.
  
  When using Perl routines as callbacks, naturally you want to pass in your Perl object. This is complicated by the fact that the Garbage Collector can potentially move Perl objects around so they may not be found where you initially put them.
  
  This simple helper object associates a perl object with some object that can be cast to a `int64` type so you can easily register the object (`store`/`remove`), and `lookup` from within a callback routine, associated with an `id` of that thing.
  
  Note this is 64-bit architecture specific, and assumes `void *` pointers are interchangeable with `int64`.
  
  Wherever you see a `void *` in a library.h file, just use `int64` for the Perl NativeCall subroutine that calls it.
  
  For example:
  
  ```
  typedef int (*callback)(void *);
  void setcallback(callback cb, void *user_data);
  
  ```
  
  goes to:
  
  ```
  sub setcallback(&callback (int64 --> int32), int64)
       is native('./callback') {}
  
  ```
  
  # COPYRIGHT and LICENSE
  
  Copyright 2019 Curt Tilmes
  
  This module is free software; you can redistribute it and/or modify it under the Artistic License 2.0.
  