[cairo] UserFontFace vs FtFontFace management (cairomm)

[Jonathon Jongsma]

Behdad Esfahbod wrote:
> Jonathon Jongsma wrote:
>> The problem is that the
>> implementation relies on the assumption that some of your callbacks will be NULL.  If I 
>> recall correctly, there are at least two callbacks that are mutually exclusive, so the 
>> internal logic is something like:
>> if (foo_callback != NULL)
>>    foo_callback();
>> else if (bar_callback != NULL)
>>    bar_callback();
>>
>> So as you can see, if you implement the callbacks as virtual functions and install 
>> callbacks in the underlying C object that will call your virtual function, you'll never be 
>> able to call bar_callback(), since foo_callback will always be non-null.  Did that 
>> explanation make any sense?  My memory is a little bit rusty regarding all of the details 
>> at the moment, but I know that the user-font implementation is quite C-specific and 
>> difficult to wrap in a natural way for object-oriented languages.  There may still be ways 
>> to improve the cairomm UserFont interface though, and I'd appreciate any ideas you may have.
> 
> 
> What you describe is very close to how it is perceived, but I think I was
> careful enough to not make that mistake.  The two methods that have
> interdependencies are text_to_glyphs and unicode_to_glyphs.  However, the docs
> for text_to_glyphs give you a way to essentially say "unimplemented":
> 
> [snip tens of lines of docs]
>  *
>  * The callback is optional.  If @num_glyphs is negative upon
>  * the callback returning, the unicode_to_glyph callback
>  * is tried.  See #cairo_user_scaled_font_unicode_to_glyph_func_t.
>  *
> 
> So all you need to do is to make sure your default implementation sets
> num_glyphs to -1.
> 
> Being able to implement user fonts in, say, Python is quite a nice feature and
> I expect one to simply subclass cairo.UserFontFace for that indeed.  At some
> point I'll update the binding chapter and clarify this, but so far my plans to
> tackle that part of the cairo tree have failed miserably.

Oh, that's good to know.  I must have missed that part of the documentation (or maybe the 
documentation changed after I had started looking at wrapping them?).  Anyway, I'm glad to 
hear it.  I'll definitely take a look at changing back to a subclass-UserFontFace 
implementation

-- 
jonner