language-bindings.xml

按照官方的说法：Cairo is a vector graphics library with cross-device output support. 翻译过来

<appendix id="language-bindings">
  <title>Creating a language binding for cairo</title>
  <para>
    While cairo is implemented and C, and has a C API, it is expected
    that many users of cairo will be using it from languages other
    than C. The glue that connects the core cairo library to another
    language is known as a <firstterm>language
    binding</firstterm>. This appendix attempts to collect together
    issues that come up when creating a language bindings for cairo
    and present standardized solutions to promote consistency among
    the different language bindings.
  </para>
  <sect1 id="bindings-general">
    <title>General considerations</title>
    <para>
      The naming of the central <link
	linkend="cairo-t"><type>cairo_t</type></link> type is a
      special exception. The object is “a cairo context” not “a
      cairo”, and names such as <type>cairo_t</type> rather than
      <type>cairo_context_t</type> and
      <function>cairo_set_source()</function> rather than
      <function>cairo_context_set_source()</function> are simply
      abbreviations to make the C API more palatable. In languages
      which have object-oriented syntax, this abbreviation is much
      less useful. In fact, if ‘Cairo’ is used as a namespace, then
      in many languages, you'd end up with a ridiculous type name
      like ‘Cairo.Cairo’. For this reason, and for inter-language
      consistency all object-oriented languages should name this
      type as if it were <type>cairo_context_t</type>.
    </para>
    <para>
      The punctuation and casing of the type names and
      method names of cairo should be changed to match the general
      convention of the language. In Java, where type names are written
      in StudlyCaps and method names in javaCaps, cairo_font_extents_t
      will become FontExtents and
      <literal>cairo_set_source(cr,source)</literal>,
      <literal>cr.setSource(source)</literal>.
      As compared to changing the punctuation, and casing, much
      more reluctance should be used in changing the method names
      themselves. Even if get is usually omitted from getters in
      your language, you shouldn't bind cairo_get_source() as
      cr.source().
    </para>
  </sect1>
  <sect1 id="bindings-memory">
    <title>Memory Management</title>
    <para>
      The objects in cairo can roughly be divided into two types:
      reference-counted, opaque types like
      <link
      linkend="cairo-surface-t"><type>cairo_surface_t</type></link>
      and plain structures like
      <link
	linkend="cairo-glyph-t"><type>cairo_glyph_t</type></link>.
      <link
	linkend="cairo-path-t"><type>cairo_path_t</type></link>
      and 
      <link
	linkend="cairo-path-data-t"><type>cairo_path_data_t</type></link>
      are special cases and are treated separately in this appendix.
    </para>
    <para>
      Refcounted opaque types all have a
      <function>..._reference()</function>
      function to increase the refcount by one and a
      <function>..._destroy()</function> to decrease the refcount
      by one. These should not be exposed to the user of the language
      binding, but rather used to implement memory management within
      the language binding. The simplest way to do memory management
      for a language binding is to treat the language binding object
      as a simple handle to the cairo object. The language binding
      object references the cairo object, and unreferences it when
      finalized. This is the recommended method, though there are
      a couple of caveats to be noted:
    </para>
    <itemizedlist>
      <listitem>
	<para>
	  Equality won't work as expected. You can have two language
	  objects for the same cairo and they won't necessarily
	  compare equal. If the language allows customizing the
	  equality operation, then this is fixable by comparing
	  the underlying pointers. It also can be fixed by creating
	  at most one language object per cairo object, and
	  uniquifying via a <firstterm>pin table</firstterm> (a hash
	  table that goes from cairo object to language object).
	  For <type>cairo_surface_t</type> you can use also 
	  <link
	  linkend="cairo-surface-set-user-data"><function>cairo_surface_set_user_data()</function></link>
	  instead of a separate pin table.
	</para>
      </listitem>
      <listitem>
	<para>
	  Derivation from the language object doesn't work because
	  you can lose the language object while keeping the Cairo
	  object. Code like:
	</para>
<programlisting>
public class MySurface (ImageSurface) {
   public MySurface (width, height) {
      super (Format.ARGB32, width, height);
   }
   public int get42 () {
      return 42;	  
   }
}

   cr = Cairo(MySurface(width, height));
   surface = cr.getTarget();
</programlisting>
	<para>
	  Can result in <varname>surface</varname> containing an
	  <classname>ImageSurface</classname> not a <classname>MySurface</classname>.
	  This is not easily fixable without creating memory leaks,
	  and it's probably best to simply forbid deriving from the
	  language objects.
	</para>
      </listitem>
    </itemizedlist>
    <para>
      When a plain structure is used as a return value from cairo,
      this is done by passing it as a “out parameter”.
    </para>
<programlisting>
cairo_font_extents_t extents;      

cairo_font_extents (cr, &amp;extents);</programlisting>
    <para>
      In a language binding, this should typically be treated
      as a return value:
    </para>
<programlisting>
FontExtents extents = cr.fontExtents ();</programlisting>
    <para>
      A language binding has a choice in how it implements the
      language objects for plain structures. It can use a pure
      language object with fields corresponding to those of the C
      structure, and convert from and to the C structure when calling
      cairo functions or converting cairo return values. Or it
      can keep a pointer to the C structure internally and wrap
      it inside a language object much like occurs for refcounted
      objects. The choice should be invisible to the user: they should
      be able to imagine that it is implemented as a pure language
      object.
    </para>
  </sect1>
  <sect1 id="bindings-return-values">
    <title>Multiple return values</title>
    <para>
      There are a number of functions in the cairo API that have
      multiple <firstterm>out parameters</firstterm> or
      <firstterm>in-out parameters</firstterm>. In some languages
      these can be translated into multiple return values. In Python,
      what is:
    </para>
    <programlisting>
cairo_user_to_device (cr, &amp;x, &amp;y);</programlisting>
    <para>
      can by mapped to:
    </para>
    <programlisting>
(x, y) = cr.user_to_device (cr, x, y);</programlisting>
    <para>
      but many languages don't have provisions for multiple return
      values, so it is necessary to introduce auxiliary types.
      Most of the functions that require the auxiliary types
      require a type that would, in C, look like
    </para>
    <programlisting>
typedef struct _cairo_point cairo_point_t;
struct _cairo_point {
    double x;
    double y;
}</programlisting>
    <para>
      The same type should be used both for functions that use a pair
      of coordinates as an absolute position, and functions that use
      a pair of coordinates as a displacement. While an argument could
      be made that having a separate “distance” type is more correct,
      it is more likely just to confuse users.
    </para>
    <programlisting>
void
cairo_user_to_device (cairo_t *cr, double *x, double *y);

void
cairo_user_to_device_distance (cairo_t *cr, double *dx, double *dy);

void
cairo_device_to_user (cairo_t *cr, double *x, double *y);

void
cairo_device_to_user_distance (cairo_t *cr, double *dx, double *dy);

void
cairo_matrix_transform_distance (cairo_matrix_t *matrix, double *dx, double *dy);

void
cairo_matrix_transform_point (cairo_matrix_t *matrix, double *x, double *y);

void
cairo_get_current_point (cairo_t *cr, double *x, double *y);
    </programlisting>
    <para>
      There are also a couple of functions that return four values
      representing a rectangle. These should be mapped to a
      “rectangle” type that looks like:
    </para>
    <programlisting>
typedef struct _cairo_rectangle cairo_rectangle_t;
struct _cairo_rectangle {
    double x;
    double y;
    double width;
    double height;
}</programlisting>
    <para>
      The C function returns the rectangle as a set of two points to
      facilitate rounding to integral extents, but this isn't worth
      adding a “box” type to go along with the more obvious
      “rectangle” representation.
    </para>
    <remark>
      Q: Would it make sense here to define a standard
      <function>cairo_rectangle_round()</function> method
      that language bindings should map?
    </remark>
    <programlisting>
void
cairo_stroke_extents (cairo_t *cr,
		      double *x1, double *y1,
		      double *x2, double *y2);

void
cairo_fill_extents (cairo_t *cr,
		    double *x1, double *y1,
		    double *x2, double *y2);
    </programlisting>
  </sect1>
  <sect1 id="bindings-overloading">
    <title>Overloading and optional arguments</title>
    <para>
      Function overloading (having a several variants of a function
      with the same name and different arguments) is a language
      feature available in many languages but not in C.
    </para>
    <para>
      In general, language binding authors should use restraint in
      combining functions in the cairo API via function
      overloading. What may seem like an obvious overload now may
      turn out to be strange with future additions to cairo.
      It might seem logical to make
      <link
      linkend="cairo-set-source-rgb"><function>cairo_set_source_rgb()</function></link>
	an overload of <function>cairo_set_source()</function>, but future plans to add
	<function>cairo_set_source_rgb_premultiplied()</function>,
      which will also take three doubles make this a bad idea. For
      this reason, only the following pairs of functions should
      be combined via overloading
    </para>
    <programlisting>
void
cairo_set_source (cairo_t *cr, cairo_pattern_t *source);

void
cairo_set_source_surface (cairo_t          *cr,
                          cairo_surface_t  *source,
                          double            surface_x,
                          double            surface_y);
      
void
cairo_mask (cairo_t         *cr,
	    cairo_pattern_t *pattern);

void
cairo_mask_surface (cairo_t         *cr,
		    cairo_surface_t *surface,
		    double           surface_x,
		    double           surface_y);
      
cairo_surface_t *
cairo_image_surface_create (cairo_format_t	format,
			    int			width,
			    int			height);
cairo_surface_t *
cairo_image_surface_create_for_data (unsigned char	       *data,
				     cairo_format_t		format,
				     int			width,
				     int			height,
				     int			stride);

cairo_status_t
cairo_surface_write_to_png (cairo_surface_t	*surface,
			    const char		*filename);

cairo_status_t
cairo_surface_write_to_png_stream (cairo_surface_t	*surface,
				   cairo_write_func_t	write_func,
				   void			*closure);

cairo_surface_t *
cairo_image_surface_create_from_png (const char	*filename);

cairo_surface_t *
cairo_image_surface_create_from_png_stream (cairo_read_func_t	read_func,
					    void		*closure);
    </programlisting>
    <para>
      Note that there are cases where all constructors for a type
      aren't overloaded together. For example
      <link
	linkend="cairo-image-surface-create-from-png"><function>cairo_image_surface_create_from_png()</function></link>
      should <emphasis>not</emphasis> be overloaded together with
      <link
	linkend="cairo-image-surface-create"><function>cairo_image_surface_create()</function></link>.
      In such cases, the remaining constructors will typically need to
      be bound as static methods. In Java, for example, we might have:
    </para>
<programlisting>
Surface surface1 = ImageSurface(Format.RGB24, 100, 100);
Surface surface2 = ImageSurface.createFromPNG("camera.png");</programlisting>
    <para>
      Some other overloads that add combinations not found in C may be
      convenient for users for language bindings that provide
      <type>cairo_point_t</type> and <type>cairo_rectangle_t</type>
      types, for example:
    </para>
    <programlisting>
void
cairo_move_to (cairo_t       *cr,
               cairo_point_t *point);
void
cairo_rectangle (cairo_t           *cr,
                 cairo_rectangle_t *rectangle);
    </programlisting>
  </sect1>
  <sect1 id="bindings-streams">
    <title>Streams and File I/O</title>
    <para>
      Various places in the cairo API deal with reading and writing
      data, whether from and to files, or to other sources and
      destinations. In these cases, what is typically provided in the
      C API is a simple version that just takes a filename, and a
      complex version that takes a callback function.
      An example is the PNG handling functions:
    </para>
<programlisting>
cairo_surface_t *
cairo_image_surface_create_from_png (const char	*filename);

cairo_surface_t *
cairo_image_surface_create_from_png_stream (cairo_read_func_t read_func,
					    void             *closure);

cairo_status_t
cairo_surface_write_to_png (cairo_surface_t	*surface,
			    const char		*filename);

cairo_status_t
cairo_surface_write_to_png_stream (cairo_surface_t	*surface,
				   cairo_write_func_t	write_func,
				   void			*closure);</programlisting>