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How to use the binding infrastructure

Pass the result of winapi calls to checknz, checkh and friends according to what constitutes an error in the result: you get automatic error handling and clear code.

Use own(object, finalizer) on all newly created objects but call disown(object) right after any successful api call that assigns that object an owner responsible for freeing it, and use own() again every time a call leaves an object without an owner. Doing this consistently will complicate the implementation sometimes but it prevents leaks and you get automatic object lifetime management (for what is worth, given the non-deterministic nature of the gc).

Avoid surfacing ABI boilerplate like buffers, buffer sizes and internal data structures. Sometimes you may want to reuse a buffer to avoid heap trashing especially on a function you know it could be called repeatedly many times. In this case add the buffer as an optional trailing argument - if given it will be used, if not, an internal buffer will be created. If there's a need to pass state around beyond this, make a class (that is, do it in the object layer).

Use wcs(arg) on all string args: if arg is a Lua string, it's assumed to be an utf8 encoded string and it's converted to wcs, otherwise it's assumed a wcs and passed through as is. This makes the API accept both Lua strings and wcs strings transparently.

Use flags(arg) on all flag args so that you can pass a string of the form 'FLAG1 FLAG2 ...' as an alternative to bit.bor(FLAG1, FLAG2, ...). It also changes nil into 0 to allow for optional flag args to be passed where winapi expects an int.

Count from 1! Use countfrom0 on all positional args: this will decrement the arg but only if it's strictly > 0 so that negative numbers are passed through as they are since we want to preserve values with special meaning like -1.

Simple type constructors

Use arg = types.FOO(arg) instead of arg ='FOO', arg). This allows passing a pre-allocated FOO as argument. Publish common types in the winapi namespace: FOO = types.FOO and then use FOO instead of types.FOO.

Array constructors

Use arg = arrays.FOO(arg) instead of arg ='FOO[?]', #arg, arg). This allows passing in a pre-allocated array as argument, and when passing in a table, the array size will be #arg.

Struct constructors

Don't allocate structs with'FOO', arg). Instead, make a struct constructor FOO = struct{...}, and pass all FOO args through it: arg = FOO(arg).

This can enable some magic, depending on how much you add to your definition:

  • the user can pass in a pre-allocated FOO which will be passed through.
  • if passing in a table, as it's usually the case,
    • the size (usually cbSize) field (if any) can be set automatically.
    • the struct's mask field (if any) can be set automatically to reflect that only certain fields (those present in the table) need to be set.
    • default values (eg. a version field) can be set automatically.
  • virtual fields with a getter and setter can be added which will be available alongside the cdata fields for all cdata of that type.

Virtual fields

Making a struct definition sets a metatable on the underlying ctype (using ffi.metatype()), making any virtual fields available to all cdata of that ctype. Accessing a struct through the virtual fields instead of the C fields has some advantages:

  • the struct's mask field, if any, will be set based on which fields are set, provided a bitmask is specified in the struct's definition of that field. Setting a masked field to nil will clear its bitmask in the mask field. Getting the value of a field with its mask cleared returns nil, regardless of its data type.

  • bits of masked bitfields can be read and set individually, provided you define the data field, the mask field, and the prefix for the mask constants in the struct definition.

  • output (an in/out) buffers can be allocated and anchored to the struct automatically: you can add those in an init function.


FOO = struct{
   ctype = 'FOO',    --the C struct that is to be created.
   size = 'cbSize',  --the field that must be set to sizeof(FOO), if any.
   mask = 'fMask',   --the field that masks other fields, if any.
   defaults = {
      nVersion = 1,  --set on creation.
   fields = mfields{ --mfields is the field def constructor for masked fields.
      'bar_field',    'barField',    MASK_BAR, setter, getter,
      'baz_field',    'bazField',    MASK_BAZ, pass,   pass,     -- setting baz_field sets MASK_BAZ in fMask.
      'zup_field',    'zupField',    MASK_ZUP, wcs,    mbs,      -- zup_field works with Lua strings.
      '__state',      'stateField',     MASK_STATE, pass, pass,  -- bitfield, see below
      '__stateMask',  'stateMaskField', MASK_STATE, pass, pass,
   bitfields = {
      -- setting state_FOO sets or clears the mask PREFIX_FOO in __state,
      -- and sets PREFIX_FOO to __stateMask (and sets MASK_STATE in fMask).
      -- getting state_FOO checks the mask PREFIX_FOO in __stateMask,
      -- and if set, checks the mask PREFIX_FOO in __state.
      state = {'__state', '__stateMask', 'PREFIX'},

Naming virtual fields

Use the "lowercase with underscores" naming convention for virtual field names. Use names like caption, x, y, w, h, pos, parent, etc. consistently throughout.

How to use the OO system

The easiest way to bind a new control is to use the code of an existing control as a template. Basically, you subclass from Control (or a specific control, if your control is a refinement of a standard control) after you define the style bitmasks, default values, and event name mappings, if any. You override the constructor and/or any methods and define any new properties by way of getters and setters.