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).

The datatypes.library implements datatypesclass from which all other datatype classes inherit from. The following BOOPSI class diagram illustrates how the various datatypes classes relate to each other.

Table 1: Datatypes Library Object Classes

Object Classes	Autodoc File Showing the Methods Supported	Type of Data Object
Picture class	picture_dtc.doc	IFF graphic image file
Sound class	sound_dtc.doc	IFF audio sample file
Text class	text_dtc.doc	ASCII characters
AmigaGuide class	amigaguide_dtc.doc	Hypertext databases
Animation class	animation_dtc.doc	Animations containing graphics and sound

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.

; /* Compiled with SAS/C 6.56.  Run from CLI only.
sc DATA=NEAR NMINC STRMERGE NOSTKCHK IGNORE=73 dt.c
slink from lib:c.o dt.o TO dt LIBRARY lib:sc.lib lib:amiga.lib
quit ; */
 
#include <exec/types.h>
#include <datatypes/datatypesclass.h>  /* This includes other files we need */
#include <stdio.h>
 
#include <clib/exec_protos.h>          /* Prototypes for system functions */
#include <clib/intuition_protos.h>
#include <clib/datatypes_protos.h>
 
#ifdef LATTICE                         /* Disable SAS/C CTRL-C handling */
int CXBRK(void)    { return(0); }
int chkabort(void) { return(0); }
#endif
 
 
 
 
struct Library *IntuitionBase=NULL;    /* System library bases */
struct Library *DataTypesBase=NULL;
 
VOID main(int argc, char **argv)
{
APTR dtobject=NULL;        /* Pointer to a datatypes object */
struct dtTrigger mydtt;    /* A trigger structure for the DTM_TRIGGER method */
ULONG dores;               /* Variable for return values    */
 
if (IntuitionBase=OpenLibrary("intuition.library",39L))
   {
   if(DataTypesBase=OpenLibrary("datatypes.library",39L) )
      {
      if(argc > 1 ) /* CLI only, at least one argument please. */
         {
         /* Attempt to make an 8svx sound object from the file name the user */
         /* specified in the command line.  For a list of possible error     */
         /* returns, see the Autodocs for NewDTObjectA().  The group ID tag  */
         /* will allow only sound datatype files to be accepted for the call.*/
         if (dtobject = NewDTObject(argv[1], DTA_GroupID, GID_SOUND,
                                             TAG_END) )
            {
            mydtt.MethodID     = DTM_TRIGGER; /* Fill in the dtTrigger struct */
            mydtt.dtt_GInfo    = NULL;
            mydtt.dtt_Function = STM_PLAY;
            mydtt.dtt_Data     = NULL;
 
            /* The return value of the DTM_TRIGGER method used with the 8svx */
            /* sound datatype is undefined in V39.  This is likely to change */
            /* in future versions of the Amiga operating system.             */
            dores = DoDTMethodA(dtobject, NULL, NULL, &mydtt);
 
            /* Let the 8svx sound finish playing.  Currently (V39) there is  */
            /* no programmatic way to find out when it is finished playing.  */
            Wait(SIGBREAKF_CTRL_C);
 
            DisposeDTObject(dtobject);
            }
         else printf("Couldn't create new object or not a sound data file\n");
 
         }
      else printf("Give a file name too.\n");
 
      CloseLibrary(DataTypesBase);
      }
   else printf("Can't open datatypes library\n");
 
   CloseLibrary(IntuitionBase);
   }
else printf("Can't open V39 Intuition\n");
}

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.

Determining Data Type

One of the main features of the DataTypes system is its ability to determine the data type of a block of data. This data block can reside in a file or the clipboard.

The following functions are used to determine the DataType of a data block:

ObtainDataTypeA()	Obtain the DataType descriptor for a data block.
ReleaseDataType()	Release the DataType descriptor for a data block.

The data type detection functions use the DataType structure.

struct DataType
{
    struct Node            dtn_Node1;
    struct Node            dtn_Node2;
    struct DataTypeHeader *dtn_Header;
    struct List            dtn_ToolList;
    STRPTR                 dtn_FunctionName;
    struct TagItem        *dtn_AttrList;
    ULONG                  dtn_Length;
};

The DataType structure is read-only. The only pertinent field is the dtn_Header field, which points to a DataTypeHeader structure.

struct DataTypeHeader
{
    STRPTR   dth_Name;
    STRPTR   dth_BaseName;
    STRPTR   dth_Pattern;
    WORD    *dth_Mask;
    ULONG    dth_GroupID;
    ULONG    dth_ID;
    WORD     dth_MaskLen;
    WORD     dth_Pad;
    UWORD    dth_Flags;
    WORD     dth_Priority;
};

The DataTypeHeader structure fields are as follows:

dth_Name

Descriptive name of the data type. For example, the description for an ILBM data type could possibly be "Amiga BitMap Picture".

dth_BaseName

This is the base name for the data type and is used to obtain the class that handles this data type.

dth_GroupID

This indicates the main type data that the object contains. Following are the possible values:

GID_SYSTEM	Fonts, Executables, Libraries, Devices, etc...
GID_TEXT	Formatted or unformatted text.
GID_DOCUMENT	Formatted text with embedded DataTypes (such as pictures).
GID_SOUND	Audio samples.
GID_INSTRUMENT	Audio samples used for playing music.
GID_MUSIC	Musical scores.
GID_PICTURE	Graphic picture or brush.
GID_ANIMATION	Moving picture or cartoon.
GID_MOVIE	Moving picture or cartoon with sound.

dth_ID

This is an individual indentifier for the DataType. For IFF files it is the same as the FORM type, for example ILBM for an Amiga BitMap picture. For non-IFF files, it is the first four characters of dth_Name.

dth_Flags

The flags field contains, among other information, the coarse type of data. The type can be obtained by ANDing DTF_TYPE_MASK with this field.

DTF_IFF	Interchange File Format
DTF_BINARY	Non-readable characters
DTF_ASCII	Readable characters
DTF_MISC	Disks and drawers

dth_Pattern

dth_Mask

dth_MaskLen

dth_Priority

These fields are used by the detection code in datatypes.library for determining the data type. See the "Defining a DataType Descriptor" section for more information.

Following is a code fragment that shows how to determine the data type of a file. This fragment uses functions from datatypes.library, dos.library, and iffparse.library.

    STRPTR name = "somefilename";
    BPTR lock;
 
    struct DataTypeHeader *dth;
    struct DataType *dtn;
    UBYTE idesc[5];
    STRPTR tdesc;
    STRPTR gdesc;
    UWORD ttype;
 
    /* Obtain a lock on the file that we want information on */
    if (lock = IDOS->Lock (name, ACCESS_READ))
    {
	/* Get a pointer to the appropriate DataType structure */
	if (dtn = IDataTypes->ObtainDataTypeA (DTST_FILE, (APTR)lock, NULL))
	{
	    /* Get a pointer to the DataTypeHeader structure */
	    dth = dtn->dtn_Header;
 
	    /* Get the coarse type */
	    ttype = dth->dth_Flags & DTF_TYPE_MASK;
 
	    /* Get a pointer to the text strings */
	    tdesc = IDataTypes->GetDTString (ttype + DTMSG_TYPE_OFFSET);
	    gdesc = IDataTypes->GetDTString (dth->dth_GroupID);
 
	    /* Convert the ID to a string. */
	    IIFFParse->IDtoStr (dth->dth_ID, idesc);
 
	    /* Display the information */
	    IDOS->Printf ("   Description: %s\n", dth->dth_Name);
	    IDOS->Printf ("     Base Name: %s\n", dth->dth_BaseName);
	    IDOS->Printf ("          Type: %d - %s\n", ttype, tdesc);
	    IDOS->Printf ("         Group: %s\n", gdesc);
	    IDOS->Printf ("            ID: %s\n", idesc);
 
	    /* Release the DataType structure now that we are done with it */
	    IDataTypes->ReleaseDataType (dtn);
	}
 
	/* Release the DOS lock on the file */
	IDOS->UnLock (lock);
    }

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.

; /* Compiled with SAS/C 6.56.  Run from CLI only.
sc DATA=NEAR NMINC STRMERGE NOSTKCHK IGNORE=73 dtpic.c
slink from lib:c.o dtpic.o TO dtpic LIBRARY lib:sc.lib lib:amiga.lib
quit ; */
 
#include <exec/types.h>
#include <datatypes/datatypes.h>      /* Datatypes definitions we need */
#include <datatypes/pictureclass.h>
#include <stdio.h>
 
#include <clib/exec_protos.h>         /* Prototypes for system functions */
#include <clib/intuition_protos.h>
#include <clib/alib_protos.h>
#include <clib/datatypes_protos.h>
#include <clib/graphics_protos.h>
 
#ifdef LATTICE
int CXBRK(void)    { return(0); }     /* Disable SAS/C CTRL-C handling */
int chkabort(void) { return(0); }
#endif
 
struct Library *IntuitionBase=NULL;   /* System library bases */
struct Library *GfxBase=NULL;
struct Library *DataTypesBase=NULL;
 
VOID main(int argc, char **argv)
{
APTR dtobject=NULL;                   /* Pointer to a datatypes object       */
ULONG res;                            /* Variable for function return values */
struct dtFrameBox mydtFrameBox;       /* Use this with DTM_FRAMEBOX method   */
struct FrameInfo myFrameInfo;         /* For info returned from DTM_FRAMEBOX */
struct gpLayout mygpLayout;           /* Use this with DTM_PROCLAYOUT method */
 
ULONG modeID = INVALID_ID;            /* Variables for storing the display */
struct Screen *myScreen=NULL;         /* environment information obtained  */
struct BitMap *bm = NULL;             /* the datatype object.              */
ULONG *cregs = NULL;
ULONG i,r,g,b,numcolors;
 
if (IntuitionBase=OpenLibrary("intuition.library",39L))
  {
  if (GfxBase=OpenLibrary("graphics.library",39L))
    {
    if(DataTypesBase=OpenLibrary("datatypes.library",0L))
        {
        if(argc > 1 ) /* CLI only, at least one argument please.  */
            {
            /* Attempt to create a picture object in memory from the file    */
            /* name given by the user in the command line.  If we wanted to  */
            /* show the picture in a screen set up ahead of time, we could   */
            /* set PDTA_Remap to TRUE and provide a pointer to the screen    */
            /* with the PDTA_Screen tag (datatypes.library handles the rest).*/
            /* However in this case we want to first find out the attributes */
            /* of the picture object and set up a matching screen and do the */
            /* remapping later.  Therefore PDTA_Remap is set to false.       */
            /* The group ID tag ensures that we get only a picture file type.*/
            if (dtobject = NewDTObject(argv[1], PDTA_Remap,  FALSE,
                                                DTA_GroupID, GID_PICTURE,
                                                TAG_END) )
                {
                /* Here we want to find the display environment required by  */
                /* this picture.  To do that, perform the DTM_FRAMEBOX method */
                /* on the object.  The datatypes library fills in the struct */
                /* FrameBox you give it with the info on the display needed.  */
                mydtFrameBox.MethodID         = DTM_FRAMEBOX;
                mydtFrameBox.dtf_GInfo        = NULL;
                mydtFrameBox.dtf_ContentsInfo = NULL;
                mydtFrameBox.dtf_FrameInfo    = &myFrameInfo;
                mydtFrameBox.dtf_SizeFrameInfo= sizeof (struct FrameInfo);
                mydtFrameBox.dtf_FrameFlags   = 0L;
 
                /* The return value from DTM_FRAMEBOX is currently undefined */
                res = DoMethodA(dtobject, &mydtFrameBox);
 
                /* OK, now do the layout (remap) of the object on our process */
                mygpLayout.MethodID   = DTM_PROCLAYOUT;
                mygpLayout.gpl_GInfo  = NULL;
                mygpLayout.gpl_Initial= 1L;
 
                /* The return value of DTM_PROCLAYOUT is non-zero for success */
                if( res = DoMethodA(dtobject, &mygpLayout) )
                   {
                   /* Get the attributes of this picture object.  You could  */
                   /* use a series of GetAttr() function calls here instead.  */
                   res = GetDTAttrs(dtobject, PDTA_ModeID, &modeID,
                                              PDTA_CRegs, &cregs,
                                              PDTA_BitMap, &bm,
                                              TAG_END);
 
                   /* Did we get all threee attributes? */
                   if( (modeID!=INVALID_ID) && (cregs) && (bm) )
                       {
                       /* Open a screen that matches the picture object */
                       if( myScreen = OpenScreenTags( NULL,
                           SA_Width,     myFrameInfo.fri_Dimensions.Width,
                           SA_Height,    myFrameInfo.fri_Dimensions.Height,
                           SA_Depth,     myFrameInfo.fri_Dimensions.Depth,
                           SA_DisplayID, modeID,
                           SA_BitMap,    bm,
                           TAG_END) )
                           {
                           /* Now fill in the color registers for this screen */
                           numcolors = 2<<(myFrameInfo.fri_Dimensions.Depth-1);
                           for( i=0; i < numcolors; i++ )
                              {
                              r = cregs[i * 3 + 0];
                              g = cregs[i * 3 + 1];
                              b = cregs[i * 3 + 2];
                              SetRGB32(&myScreen->ViewPort, i, r, g, b);
                              }
 
                           printf("Ctrl-C in this window to quit\n");
                           /* Wait for the user to have a look...  */
                           Wait(SIGBREAKF_CTRL_C);
 
                           CloseScreen(myScreen);
                           }
                       else printf("Couldn't open required screen\n");
                       }
                   else printf("Couldn't get picture attributes\n");
 
                  DisposeDTObject(dtobject);
                  }
               else printf("Couldn't perform PROC_LAYOUT\n");
               }
            else printf("Couldn't create new object or not a picture file\n");
            }
        else printf("Give a file name too.\n");
 
        CloseLibrary(DataTypesBase);
        }
    else printf("Can't open datatypes library\n");
 
    CloseLibrary(GfxBase);
    }
  else printf("Can't open V39 graphics\n");
 
  CloseLibrary(IntuitionBase);
  }
else printf("Can't open V39 Intuition\n");
}

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 section.