The Be Book - System Overview - The Translation Kit
The Translation Kit provides a framework for converting data streams between different media formats. A word processor, for example, could use the Translation Kit to import and export documents in a variety of formats, including HTML, PostScript, and plain ASCII, while working in its own native format.
The translation itself is performed by translator add-ons (or, less
globally, BTranslator
objects). You can create your own translator add-ons and
BTranslator
subclasses, or you can use the translators that
are supplied by Be and third parties.
The Translation Kit operates on
BPositionIO objects and their
descendents. Since
BFile
derives from
BPositionIO,
the Translation Kit is
naturally suited for importing and exporting files. However, it's far
more general; the Translation Kit could be used, for example, to
translate input directly to or from a network connection, provided a
suitable BPositionIO
subclass was available. More importantly,
BMemoryIO
and
BMallocIO
can be used to load data directly into or out of memory.
Applications primarily interact with the Translation Kit through the
BTranslatorRoster
class. This class encapsulates the functionality
required to load and unload translators, discover their capabilities,
configure the translators, and execute the translations.
Applications may also use the
BTranslationUtils
class, a collection of
static utility functions designed to simplify access to the Translation
Kit. Currently the class only defines members for loading bitmap images
from files, resources, and general
BPositionIO
data streams.
The Translation Kit identifies a particular media format by a media group
and format type. Media groups identify the class of media represented by
a format. Definitions for the common media groups can be found in
translation/TranslatorFormats.h:
| Media Group | Constant |
|---|---|
| Bitmap | B_TRANSLATOR_BITMAP |
BPicture data | B_TRANSLATOR_PICTURE |
| Text | B_TRANSLATOR_TEXT |
| Sound | B_TRANSLATOR_SOUND |
| Standard MIDI | B_TRANSLATOR_MIDI |
| Streaming media | B_TRANSLATOR_MEDIA |
| nothing | B_TRANSLATOR_NONE |
A format type is a type constant identifying the specific data format,
i.e. tiff, aiff, mpeg. The standard type constants can be found in
support/TypeConstants.h.
It is often convenient to define a baseline format to which all translators of a given media group adhere. An application that understands the baseline format could then use all the translators for that group. This also simplifies the job of the translator, which minimally needs only to translate between the baseline and the new formats. Well-behaved translators will always be able to translate to and from the default media format (if one exists for its media group).
The Translation Kit defines a default media format for each existing
media group. They are explained at length in the section
"Default Media
Formats". The format type constant for each default format is the same as
the media group constant; for example, the format type for the default
bitmap format is B_TRANSLATOR_BITMAP.
