Datatypes Library

Introduction to the Datatypes Library

The purpose of the datatypes library is to provide software tools for handling data in an object-oriented way. The object-oriented approach means that your application can work with numerous data file standards without having to worry about the complex details of each one. Instead you need only understand the simple conventions of the datatypes library.

The datatypes library is built on Intuition's BOOPSI facility (BOOPSI is an acronym for Basic Object-Oriented Programming System for Intuition). Although not required, it is very helpful to know a little about how BOOPSI works before trying to use the datatypes library. For information on BOOPSI, refer to BOOPSI. Some familiarity with object-oriented theory and practice is also helpful, though not required.

Since the datatypes library uses the TagItem structure for passing parameters to functions, you will have to understand how TagItems work before you can call the functions in the datatypes library. For more information on TagItems refer to the Utility Library.

Why Use the Datatypes Library?

One practical benefit of the datatypes library is that it allows you to quickly add support for IFF data files (this article will show you how). However, the goals of the datatypes library are much more ambitious than that. Here's a summary:

• Consistent, simple handling of multiple data standards - Most of the details of dealing with the various data standards are hidden. Once you have learned how to handle one type of data with the datatypes library, you will find that the other types are handled in much the same way.

• Extensible - You can add your own types of data objects to those already supported by the datatypes library. Datatypes has functions that allow other applications to find out about and work with your data object, without having to understand the internal details of the data.

• Automatic support of IFF and clipboard - The initial version of the datatypes library (V39) provides support for 8SVX sound data and ILBM graphic data. These are the two most widely used data file standards on the Amiga. Developers who want to support these IFF standards no longer have to become IFF experts. Similarly, the datatypes library provides a consistent and easy-to-use interface to the Amiga's clipboard device to encourage data sharing between applications.

• Intuition gadget support - Because the datatypes library is implemented with boopsi, the data objects it handles can also be treated as gadgets. Gadget operations can be performed on data objects within Intuition's task context, the same as other boopsi gadgets.

• Automatic conversion from one format to another - Future versions of the datatypes library will support other types of data objects. Conversion from one format to another will be automatically handled by the library.

• Validation - Datatypes lets you easily check if a given file is a valid instance of one of the data objects it supports. For example, you can check to see if a file is a valid ILBM or not.

Classes, Objects and Methods

The jargon used to describe the datatypes library may be a little confusing if you have never worked with object-oriented systems before. For instance, the kinds of data supported by the library are divided into ``classes and ``sub-classes. The term ``class is used here in a familiar way; the members of a class simply have a common set of properties. The members of a sub-class have all the properties of the parent class and additional properties specific to the sub-class. (Each sub-class could be further broken down into sub-sub-classes and so on.)

Class	Ungulate	Has hooves, can run.
Sub-class	Cow	Has udder, can be milked (also has hooves and can run).
Object	Daisy	An instance of class Cow; can run and can be milked.

An actual instance of a class or sub-class is referred to as an ``object. The term ``object is appropriate because in general we want to ignore the details of each individual case and concentrate instead on what we can do with an object based on its class. In the example above the Daisy object can run and can be milked. The operations that can be performed with an object are referred to as ``methods and the object is said to ``inherit the methods and other attributes of its parent class (which in turn inherits the methods and attributes of its parent class, if it has one).

Currently, there are only four object classes (see Table 1) in the datatypes library. More will be implemented in future versions of the Amiga OS.

Table 1: Datatypes Library Object Classes in Release 3.0 (V39)

Object Classes and Sub-classes	Autodoc File Showing the Methods Supported	Type of Data Object
Picture class ILBM sub-class	picture_dtc.doc	IFF graphic image file
Sound class 8SVX sub-class	sound_dtc.doc	IFF audio sample file
Text class ASCII sub-class	text_dtc.doc ASCII_dtc.doc	ASCII characters
AmigaGuide class	amigaguide_dtc.doc	Hypertext databases

The examples programs listed below demonstrate how to perform some basic ``methods on ILBM and 8SVX class objects.

Datatypes Class Attributes

Datatype library classes have other attributes in addition to the methods (operations) that they support. For each attribute, there is a corresponding TagItem defined in the datatypes library that you can use to examine or set that attribute in a particular object

For example, picture objects have a display mode attribute. The tag that controls this attribute is named PDTA_ModeID and is described in the Autodoc file picture_dtc.doc. See the Autodoc files for each class (as shown in Table 1) for a complete list of all class attributes.

The class attribute descriptions in the include files also have a set of codes that indicate the ``applicability of the attribute. The codes are as follows:

I - Initialize	You can initialize the attribute when the object is created
S - Set	You can set the attribute to a new value after the object is created
G - Get	You can get the value of the attribute after the object is created
N - Notify	Changing the attribute triggers the object to send notification
U - Update	Attribute can be set using the object's OM_UPDATE method

These codes may seem a little mysterious until you have actually tried using the datatypes library. The N and U codes in particular are for special applications that want to implement their own object classes, an advanced topic beyond the scope of this article.

Basic Functions of the Datatypes Library

If all these new concepts seem a little daunting, rest assured; the datatypes library uses conventional C language function calls to get the job done. The calls you will be using most often are listed below. Notice that for each of these basic functions of datatypes library there is an equivalent BOOPSI call in the Intuition Library.

Function Name	Library	Purpose
NewDTObject() NewObject()	datatypes.library intuition.library	Create a datatype object in memory from a file or clip
DisposeDTObject() DisposeObject()	datatypes.library intuition.library	Free an object created earlier with NewDTObject() (or NewObject() )
GetDTAttrs() GetAttr()	datatypes.library intuition.library	Get attributes of a datatype object
SetDTAttrs() SetAttrs()	datatypes.library intuition.library	Set attributes for a datatype object
DoDTMethod() IDoMethod()	datatypes.library intuition.library	Perform the given method (operation) with a datatype object

In a typical application the sequence of calls might be performed like this:

1. Use NewDTObject() to create an object in memory from given data.
2. Get (or perhaps set) attributes of the object using GetDTAttr() (or SetDTAttrs() ).
3. Perform ``methods (operations) with the object using DoDTMethod().
4. Free the object and any memory or other resources it was using with the DisposeDTObject() call.

Basic Structures of the Datatypes Library

There are a lot of structures used with datatypes library function calls; too many to summarize in this article. However, here's a listing of the relevant include files that contain the structure definitions of interest to class users.

<datatypes/datatypes.h>	Group IDs, error numbers plus library overhead
<datatypes/datatypesclass.h>	Defines datatype methods and associated structures
<datatypes/picture.h>	Structures specific to the picture class
<datatypes/sound.h>	Structures specific to the sound class
<datatypes/text.h>	Structures specific to the text class
<libraries/amigaguide.h>	Structures and methods for AmigaGuide databases
<intuition/classusr.h>	Defines general boopsi object methods
<intuition/gadgetclass.h>	Defines gadget methods and associated structures

The two most important definitions in these include files appear in <intuition/classusr.h>. The objects used with datatypes library functions (and the boopsi functions in Intuition) are defined as follows:

typedef ULONG   Object;     /* abstract handle */

Since we want to treat objects as black boxes and don't really care how they are implemented, this definition is very appropriate. When a method is performed with an object, the parameter used to identify the method is a Msg structure defined as follows:

typedef struct {
    ULONG MethodID;
    /* method-specific data goes here */
} *Msg;

Some methods require more information than just the method identifier. Such methods have a custom structure defined in the include files. All method structures, however, begin with a field that contains the method ID.

A Simple Datatypes Example

The example program listed here should clarify some of the concepts discussed so far. Suppose you have a communications program and want to add the capability of playing back a user-specified 8SVX sample file for the bell sound (Ctrl-G). The program below shows how to play a sound with the datatypes library.

In this program, objects are of class 8SVX (a sub-class of the sound datatype). The method performed with the object is named DTM_TRIGGER (described in the Autodoc file sound_dtc.doc). The DTM_TRIGGER method (with type set to STM_PLAY) causes a sampled sound to be played on the Amiga's audio hardware. Since the DTM_TRIGGER method requires other information in addition to the method ID, a dtTrigger structure is used. This structure is defined in <datatypes/datatypesclass.h>.

Note that if the sound datatype is enhanced to support other types of sound files in a future version of the Amiga OS, the code given here will automatically support the new type. This example expects the file name and path to a sound file.

dt.c 

In addition to playing back a sampled sound, the datatypes library allows sound objects to become gadgets (the library includes default imagery for a sound gadget). Since all datatypes object classes are implemented as a sub-class of the boopsi ``gadget class, they all support the methods of gadget objects as described in the boopsi chapter of the Libraries manual.

A Picture Class Example

Here is a second, more complex example showing how to use all the datatypes library functions described so far. In this example, the objects used are of class ILBM, a sub-class of picture.

Two methods will be performed with the object, DTM_PROCLAYOUT and DTM_FRAMEBOX. Both these methods have associated structures (gpLayout and dtFrameBox respectively). DTM_PROCLAYOUT makes the object available within the context of your application task (as opposed to Intuition's). DTM_FRAMEBOX queries the display environment required by the picture.

Other attributes of the picture are obtained with a call to GetDTAttrs() and then a matching Intuition screen is created and the ILBM object is displayed. This example expects the file and path name of a picture file.

dtpic.c 

As with 8SVX objects, the datatypes library allows ILBM objects to be treated as gadgets. Remember that all datatypes object classes a sub-class of the boopsi ``gadget class and therefore support the gadget methods described in the boopsi chapter of the Amiga ROM Kernel Reference Manual: Libraries.