This document began its life as a post to gtk-perl-list about a redesign of the fundamentals of the bindings; today it is the reference documentation for the developers of the bindings.
To reduce confusion, refer to GLib, the C library, with a capital L, and Glib the perl module with a lower-case l. While the Gtk2 module is the primary client of Glib, it is not necessarily the only one; in fact, the perl bindings for the GStreamer library build directly atop Glib. Therefore, this document describes just the GLib/Glib basics. For details on how Gtk2 extends upon the concepts presented here, see Gtk2::devel.
In various places, we use the name GPerl to refer to the actual binding subsystem.
In order to avoid getting very quickly out of date, this document doesn't go into great detail on APIs. gperl.h is rather heavily commented, and should be considered the canonical source of correct API information.
Glib, as a perl module, must decide which portions of GLib's facilities to map to perl and which to abstract and encapsulate.
In the grand scheme, the bindings have been designed with a few basic tenets in mind:
The Glib module takes care of all the basic types handled by the GObject library --- GEnum, GFlags, GBoxed, GObject, GValue, GClosure --- as well has signal marshalling and such in GSignal. I'll discuss each of these separately.
In practice, you will rarely see direct calls to the functions that convert objects in and out of perl. Most code should use the C preprocessor to provide easier-to-remember names that follow the perl API style, e.g., newSVGObject(obj) rather than gperl_new_object(type,obj) and SvGObject(sv) instead of gperl_get_gobject(type, sv). The convention used in all of gtk2-perl is described in Gtk2::devel.
In order to use the C data structures from Perl, we need to wrap those objects up in Perl objects. In general, a Perl object is simply a blessed reference. A typical scheme for representing C objects in perl is bless a reference to a scalar holding the C pointer value; perl will destroy the reference-counted scalar when there are no more references to it, and one would normally destroy the underlying data structure at this point. However, GLib is a little more complex than your typical C library, so this easy, typical setup won't work for us.
GBoxed types are opaque wrappers for C structures, providing copy and free functions, to allow the types to be used generically. For the most part we can get away with using the typical scheme described above to provide an opaque object, but in some instances an opaque object is very alien in perl. The Glib::Boxed section explains how we get around this.
GObject, on the other hand, is a type-aware, reference-counted object with lifetime semantics that differ somewhat from perl SVs. Thus we need something a bit more sophisticated than a plain old opaque wrapper; in fact, we use a blessed hash reference with the pointer to the C object tucked away in attached magic, and a pointer to the SV stored in the GObject's user data. The combined perl/C object does some nifty reference-count borrowing to ensure that object lifetime is managed correctly.
If an object is created by a function that returns directly to perl, then the wrapper returned by that function should ``own'' the object. If no other code assumes ownership of that object (by ref'ing a GObject or copying a GBoxed), then the object should be destroyed when the perl scalar is destroyed (actually, as part of its destruction).
For various reasons, mostly to do with robustness and performance, there is a one-to-one mapping between GType classes and perl package names. These must be registered, usually as part of the module initialization process.
In addition, the type system tries as hard as it can to recover when things don't go well, using the GType system to its advantage. If you return a C object of a type that is not registered with Gperl, such as MyCustomTypeFoo, gperl_new_object (see below) will warn you that it has blessed the unknown MyCustomTypeFoo into the first known package in its ancestry, Gtk2::VBox.
GBoxed and GObject have distinct mapping registries to avoid cross-pollination and mistakes in the type system. See below.
To assist in handling inheritance that isn't specified directly by the GType system, the function gperl_set_isa allows you to add elements to the @ISA for a package. gperl_register_object does this for you, but you may need to add additional parents, e.g., for implementing GInterfaces. (see Gtk2/xs/GtkEntry.xs for an example)
You may be thinking that we could use substitution rules to map the GObject classes to perl packages. In practice, this is a bad idea, fraught with problems; the substitution rules are not easily extendable and are easily broken by extension packages which don't follow the naming conventions.
GPerl uses this mechanism to avoid the need to know integer values for enum and flag types at the perl level. An enum value is just a string; a bitfield of flag values is represented as a reference to an array of strings. These strings are the GLib-provided nicknames. For the convenience of a perl developer, the bindings treat '-' and '_' as equivalent when looking up the corresponding integer values during conversion.
A GEnum or GFlags type mapping should be registered with
void gperl_register_fundamental (GType gtype, const char * package);
so that their package names can be used where a GType is required (for example, as GObject property types or GtkTreeModel column types).
The basic functions for converting between C and perl values are
/* croak if val is not part of type, otherwise return * corresponding value. this is the general case. */ gint gperl_convert_enum (GType type, SV * val); /* return a scalar which is the nickname of the enum value * val, or croak if val is not a member of the enum. */ SV * gperl_convert_back_enum (GType type, gint val); /* collapse a list of strings to an integer with all the * correct bits set, croak if anything is invalid. */ gint gperl_convert_flags (GType type, SV * val); /* convert a bitfield to a list of strings, or croak. */ SV * gperl_convert_back_flags (GType type, gint val);
Other utility functions allow for finer-grained control, such as the ability to pass unknown values, which can be necessary in special cases. In general, each of these functions raises an exception when something goes wrong. To be helpful, they croak with a message listing the valid values when they encounter invalid input.
There are two functions for creating boxed wrappers:
SV * gperl_new_boxed (gpointer boxed, GType gtype, gboolean own); SV * gperl_new_boxed_copy (gpointer boxed, GType gtype);
If own is TRUE, the wrapper returned by gperl_new_boxed will take boxed with it when it dies. In the case of a copy, own is implied, so there's a separate function which doesn't need the own option.
To get a boxed pointer out of a scalar wrapper, you just call gperl_get_boxed_check --- this will croak if the sv is undef or not blessed into the specified package.
When you register a boxed type you get the option of supplying a table of function pointers describing how the boxed object should be wrapped, unwrapped, and destroyed. This allows you to decide in the wrapping function what subclass of the boxed type's class the wrapper should actually take (a trick used by Gtk2::Gdk::Event), or represent a boxed type as a native perl type (such as using array references for Gnome2::Canvas::Point objects). All of this happens automagically, behind the scenes, and most types assume the default wrapper class.
SV * gperl_new_object (GObject * object, gboolean own);
The wrapper SV will be blessed into the package corresponding to the gtype returned by G_OBJECT_TYPE (object), that is, the bottommost type in the inheritance chain. If that bottommost type is not known, the function walks back up the tree until it finds one that's known, blesses the reference into that package, and spits out a warning on stderr. To hush the warning, you need merely call
In general, this process will claim a reference on the GObject (with g_object_ref()), so that the C object stays alive so long as there is a perl wrapper for it. If <i>own</i> is set to TRUE, the perl wrapper will claim ownership of the C object by removing that reference; in theory, for a new GObject, fresh from a constructor, this leaves the object with a single reference owned by the perl object. The next question out of your mouth should be, ``But what about GObject derivatives that require sinking or other strange methods to claim ownership?'' For the answer, see the GtkObject section's description of sink functions.
void gperl_register_object (GType gtype, const char * package);
This magical function also sets up the @ISA for the package to point to the package corresponding to g_type_parent (gtype). [Since this requires the parent package to be registered, there is a simple deferral mechanism, which means your @ISA might not be set until the next call to gperl_register_object.]
There are two ways to get an object out of an SV (though I think only one is really needed):
GObject * gperl_get_object (SV * sv); GObject * gperl_get_object_check (SV * sv, GType gtype);
The second one is like the first, but croaks if the object is not derived from gtype.
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