Name
GdkPixbufLoader -- Application-driven progressive image loading.
Description
GdkPixbufLoader provides a way for applications to drive the
process of loading an image, by letting them send the image data
directly to the loader instead of having the loader read the data
from a file. Applications can use this functionality instead of
gdk_pixbuf_new_from_file() when they need to parse image data in
small chunks. For example, it should be used when reading an
image from a (potentially) slow network connection, or when
loading an extremely large file.
To use GdkPixbufLoader to load an image, just create a new one,
and call gdk_pixbuf_loader_write() to send the data to it. When
done, gdk_pixbuf_loader_close() should be called to end the stream
and finalize everything. The loader will emit two important
signals throughout the process. The first, "area_prepared",
will be called as soon as the image has enough information to
determine the size of the image to be used. It will pass a
GdkPixbuf in. If you want to use it, you can simply ref it. In
addition, no actual information will be passed in yet, so the
pixbuf can be safely filled with any temporary graphics (or an
initial color) as needed. You can also call the
gdk_pixbuf_loader_get_pixbuf() once this signal has been emitted
and get the same pixbuf.
The other signal, "area_updated" gets
called every time a region is updated. This way you can update a
partially completed image. Note that you do not know anything
about the completeness of an image from the area updated. For
example, in an interlaced image, you need to make several passes
before the image is done loading.
Loading an animation
Loading an animation is a little more complex then loading an
image. In addition to the above signals, there is also a "frame_done" signal,
as well as an "animation_done"
signal. The first lets the application know that it is dealing
with an animation, instead of a static image. It also passes a
GdkPixbufFrame in the signal. As before, if you want to keep
the frame, you need to ref it. Once the first "frame_done" signal
has been emitted, you can call gdk_pixbuf_loader_get_animation()
to get the GdkPixbufAnimation struct. Each subsequent frame
goes through a similar lifecycle. For example "area_prepared" is
re-emitted. Then "area_updated" is
emitted as many times as necessary. Finally, "animation_done"
is emitted as soon as all frames are done.
Details
gdk_pixbuf_loader_new ()
Creates a new pixbuf loader object.
gdk_pixbuf_loader_new_with_type ()
Creates a new pixbuf loader object that always attempts to parse
image data as if it were an image of type image_type, instead of
identifying the type automatically. Useful if you want an error if
the image isn't the expected type, for loading image formats
that can't be reliably identified by looking at the data, or if
the user manually forces a specific type.
gdk_pixbuf_loader_write ()
This will cause a pixbuf loader to parse the next count bytes of
an image. It will return TRUE if the data was loaded successfully,
and FALSE if an error occurred. In the latter case, the loader
will be closed, and will not accept further writes. If FALSE is
returned, error will be set to an error from the GDK_PIXBUF_ERROR
or G_FILE_ERROR domains.
gdk_pixbuf_loader_get_pixbuf ()
Queries the GdkPixbuf that a pixbuf loader is currently creating.
In general it only makes sense to call this function after the
"area_prepared" signal has been emitted by the loader; this means
that enough data has been read to know the size of the image that
will be allocated. If the loader has not received enough data via
gdk_pixbuf_loader_write(), then this function returns NULL. The
returned pixbuf will be the same in all future calls to the loader,
so simply calling g_object_ref() should be sufficient to continue
using it. Additionally, if the loader is an animation, it will
return the "static image" of the animation
(see gdk_pixbuf_animation_get_static_image()).
gdk_pixbuf_loader_get_animation ()
GdkPixbufAnimation* gdk_pixbuf_loader_get_animation
(GdkPixbufLoader *loader); |
Queries the GdkPixbufAnimation that a pixbuf loader is currently creating.
In general it only makes sense to call this function after the "area_prepared"
signal has been emitted by the loader. If the loader doesn't have enough
bytes yet (hasn't emitted the "area_prepared" signal) this function will
return NULL.
gdk_pixbuf_loader_close ()
Informs a pixbuf loader that no further writes with
gdk_pixbuf_loader_write() will occur, so that it can free its
internal loading structures. Also, tries to parse any data that
hasn't yet been parsed; if the remaining data is partial or
corrupt, an error will be returned. If FALSE is returned, error
will be set to an error from the GDK_PIXBUF_ERROR or G_FILE_ERROR
domains. If you're just cancelling a load rather than expecting it
to be finished, passing NULL for error to ignore it is
reasonable.
Signals
The "area-prepared" signal
This signal is emitted when the pixbuf loader has been fed the
initial amount of data that is required to figure out the size and
format of the image that it will create. After this signal is
emitted, applications can call gdk_pixbuf_loader_get_pixbuf() to
fetch the partially-loaded pixbuf.
The "area-updated" signal
This signal is emitted when a significant area of the image being
loaded has been updated. Normally it means that a complete
scanline has been read in, but it could be a different area as
well. Applications can use this signal to know when to repaint
areas of an image that is being loaded.
The "closed" signal
This signal is emitted when gdk_pixbuf_loader_close() is called.
It can be used by different parts of an application to receive
notification when an image loader is closed by the code that
drives it.