TweetFollow Us on Twitter

XCMD Corner
Volume Number:4
Issue Number:7
Column Tag:HyperChat®

XCMD Corner

By Donald Koscheka, Apple Computers, Inc.

--Don Koscheka’s XCMD Corner

Last month I introduced XCMD programming. I explained the parameter block and discussed the interface between HyperTalk and the code that you write in either Pascal, “C” or the language of your choice. If you’re an experienced Macintosh programmer, that’s enough information to get started. The designers of HyperCard didn’t stop with just defining the interface for you. They went beyond what might be reasonably expected and provided some HyperTalk programming capabilities to the XCMD programmer. The XCMD programmer gains access to these capabilities through callbacks. Callbacks are procedures and functions that you call from your XCMD. Callbacks literally jump into Hypercard to perform some function.

Once you get the hang of XCMD programming, you’ll come to rely on some of the callbacks quite frequently. For example, Pascal programmers are accustomed to strings that are preceded by a length byte and that have a maximum of 255 characters (the Str255 type in Pascal) while HyperCard uses strings that have no length byte and are terminated with zero ( referred to as Zero-strings, zero-terminated strings or “C” strings). HyperTalk provides callbacks to convert from zero-terminated strings to Pascal and back again. The XCMD programmer can also use callbacks to retrieve and set the contents of fields and global containers as well as to send messages back to Hypercard.

One of the reasons that this access to HyperCard containers is so important is that XCMDs do not have access to the application globals. When Hypercard starts up, it takes control of its application globals and heap just as any application would. With Hypercard in control of the heap, your XCMD becomes a “guest” of Hypercard. You don’t have access to the globals from your XCMD so you’ll need a safe place to store information that you want to keep around.

A good candidate is a global container. Although Hypercard itself prefers to see text strings in containers, it’s not particular about what you put into a container; you can use the SetGlobal callback to put data into a global container, and GetGlobal to retrieve that data. Make sure that you declare any global containers in Hypercard before accessing them in an XCMD. Your XCMD may need to be backward compatible earlier versions of Hypercard that bombed if you called SetGlobal with an undeclared container name.

An interesting example of the use of callbacks is to send a card message back to HyperCard from a callback. The XCMD in listing 1, “SendMeAMessage”, takes one parameter which is the message to send back to HyperCard. Since messages are sent back as Pascal-format strings, we must convert the input string into a pascal string and then call SendCardMessage to send the message. As a matter of form, we set the return value to NIL indicating that this XCMD doesn’t have a result code (Hypercard will interpret NIL as empty).

The first callback that SendMeAMessage invokes, ZeroToPas, converts input parameter 1 from a zero-terminated string to a pascal-string. Input parameters are passed as handles so the parameter needs to be dereferenced one-time to convert the value to a pointer. ZeroToPas also expects you to pass a reference to the pascal-string into which you’ll store the converted Pascal-string. The second callback, SendCardMessage, sends the message back to Hypercard. To invoke this XCMD from a script use the form:

 SendMeAMessage “go next card”

Because Hypercard already handles message passing, this XCMD may not seem terribly useful. Nonetheless, it is a good illustration of callbacks and the technique is useful if you need to alert the user that some asynchronous event has completed.

{1}
{******************************}
{* File: SendMeAMessage.p *}
{******************************}
(******************************
 BUILD SEQUENCE
      (IGNORE LINK WARNINGS)
pascal SendMeAMessage.p
link -m ENTRYPOINT -rt XCMD=65534 
 -sn Main=SendMeAMessage 
 SendMeAMessage.p.o 
 “{Libraries}”Interface.o 
 “{PLibraries}”Paslib.o 
 -o “{xcmds}”testxcmds
******************************)

{$S SendMeAMessage }

UNIT Donald_Koscheka; 

{--------------INTERFACE----------------}
INTERFACE

USES  MemTypes, QuickDraw, OSIntf, ToolIntf, 
 PackIntf, HyperXCmd;

PROCEDURE EntryPoint( paramPtr: XCmdPtr);

{----------IMPLEMENTATION--------------}
IMPLEMENTATION
{$R-}
 
TYPE
 Str31  = String[31];
 
PROCEDURE SendMeAMessage(paramPtr:XCmdPtr);
 FORWARD;

{-------------- EntryPoint --------------}
PROCEDURE EntryPoint(paramPtr: XCmdPtr);
 BEGIN
 SendMeAMessage(paramPtr);
 END;

{------------ SendMeAMessage ------------}
PROCEDURE SendMeAMessage(paramPtr: XCmdPtr);
VAR
 theMessage : Str255;
 
{$I XCmdGlue.inc }
 
BEGIN {*** Body of XCMD ***}
 ZeroToPas( paramPtr^.params[1]^, theMessage );
 SendCardMessage( theMessage );
 paramPtr^.returnValue := NIL;
END; {*** Body of XCMD ***}
END.

Listing 1. SendACardMessage

XCMDs can also use callbacks to get the contents of a field or a global container. Listing 2, GetHomeInfo, uses two new callbacks (1) GetFieldByNum, to get the contents of background field 1 on the home card and (2) SetGlobal to set the contents of some global container.

GetHomeInfo Takes one parameter, the name of the global container to set. First, convert the parameter to a pascal-string. Next, use a series of callbacks to push the current card and go to the home stack. Once we get to the home stack, we call GetFieldByNum to get field data. The first parameter that GetFieldByNum takes is set to TRUE if you want to retrieve the contents of a card field and set to FALSE to retrieve the contents of a background field. The second parameter is the number of the field to retrieve. Alternatively, you could use GetFieldByID if you knew the id of the field or GetFieldByName if you knew the name.

GetFieldByNum returns a handle to the zero-terminated data that was stored in background field 1 of the home card. We then invoke SetGlobal to set the contents of the global container to whatever is stored in fieldData. Finally, we pop the current card to get back to where we started. A typical invocation of this XCMD is:


{2}
 global myData
 GetHomeInfo “myData”



PROCEDURE GetHomeInfo(paramPtr: XCmdPtr);
VAR
 globalName : Str255;
 fieldData: Handle;
 
{$I XCmdGlue.inc }
 
BEGIN
 WITH paramPtr^ DO
 BEGIN
 ZeroToPas( params[1]^, globalName );
 SendCardMessage( ‘Push Card’ );
 SendCardMessage( ‘Go Home’);
 
 fieldData := GetFieldByNum( FALSE, 1 );
 SetGlobal( globalName, fieldData );
 
 SendCardMessage( ‘Pop Card’);
 returnValue := NIL;
 END;
END;

Listing 2. Get Home Info.

So far I’ve showed you XCMDs that don’t do anything that you can’t already do in HyperTalk. A key feature of XCMD programming is that you can write your own commands to augment or add capabilities to Hypercard. Listing 3, “FCreateXFCN”, lets you create a file of any type from HyperCard. This XFCN demonstrates how XCMDs provide greater access to the toolbox than is available to the HyperTalk script writer.

FCreateXFCN introduces two new callbacks. NumToStr returns a result code to the script. NumToStr converts a signed long integer to a pascal-format string. If you don’t want a signed entity, use LongToStr instead which will convert the long integer without regard to sign.

The second callback introduced in this XFCN is PasToZero which takes a pascal format string and returns handle to a zero-terminated string. PasToZero is a handy way of copying from a pascal-string back to a zero-terminated string to return text to Hypercard.

I wrote the XFCN in “C” to show the difference in formats between Pascal and “C” XCMDS and to provide a template for “C” programmers. An important difference is that the parameter list, params, starts at index 0 for “C” and index 1 for Pascal.

FCreateXFCN first converts parameter 1 (params[0]) into a pascal-string and then moves the first four bytes of parameters 2 and 3 into the variables creator and type respectively. These two variables are of type OSType which is a special Macintosh type containing four consecutive ASCII characters. All four characters in this type are significant.

The result code will be empty if no error occurred and the file was created properly, otherwise it will return the OSErr number that occurred. First, convert the result code back to a pascal-string and then call PasToZero to convert that string into a handle to a zero-terminated string.

The XFCN would be more useful if it returned a brief description of the error in English, but I think I’ll leave that as an exercise for the student. Call FCreateXFCN with the following script:

Put FCreateXFCN( “New File Name”, “WILD”,  “STAK” )

The first parameter is the name of the file you wish to create, the second parameter is the creator and the last parameter is the file type. The foregoing invocation will create an empty stack. What can you do with that?

{3}
/*****************************\
*file: FCreateXFCN.c *
\*****************************/

/*****************************
 BUILD SEQUENCE
 
C -q2 -g FCreateXFCN.c
link  -sn Main=FCreateXFCN 
 -sn STDIO=FCreateXFCN 
  -sn INTENV=FCreateXFCN 
  -rt XFCN=300 
  -m FCREATEXFCN 
  FCreateXFCN.c.o 
  “{CLibraries}”CInterface.o 
  -o testXCMDs
 
*****************************/
#include<Types.h>
#include<OSUtils.h>
#include<Memory.h>
#include<Files.h>
#include<Resources.h>
#include  “HyperXCmd.h”

pascal void FCreateXFCN( paramPtr )
 XCmdBlockPtr  paramPtr;
/****************************
* In:   Paramblock:
*param[0] == filename
*param[1] == TYPE
*param[2] == CREATOR
* Out:  result code in returnValue
****************************/
{
char    *fname;
OSType  type, creator;
Str31 str;
char    vName[33];
short   error, vRefnum;

/** coerce the volume reference  ***
***number from the system **/
GetVol( vName, &vRefnum );
 
/**extract the filename from  ***
***the parameter list     **/
fname   = *(paramPtr->params[0]);
BlockMove(*(paramPtr->params[1]),&creator,4 );
BlockMove(*(paramPtr->params[2]),&type,4 );
 
error = Create( fname, vRefnum, 
 creator, type );
 
FlushVol( 0L, vRefnum );
 
if( !error )
 paramPtr->returnValue = 0L;
else{
 NumToStr( paramPtr, (long)error, &str );
 paramPtr->returnValue = 
 PasToZero( paramPtr, &str );
 }
}

#include <XCmdGlue.inc.c>

Listing 3. FCreateXFCN

Although this article covered some of the more frequently used callbacks, my objective was to present you with the spirit of the callback mechanism. You should have no trouble using any of the callbacks that are currently defined in the HyperCard Developer’s ToolKit (available from APDA). The most important lesson here is that before you go off and write an XCMD, check the callback list to see how you might incorporate them into your XCMDs. Try to get the callbacks to do as much work as possible for you so that you can concentrate on the code that you are trying to write.

Next Month: SortList, an XCMD that sorts a field by line.

end HyperChat
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

The beginner's guide to Warbits
Warbits is a turn-based strategy that's clearly inspired by Nintendo's Advance Wars series. Since turn-based strategy games can be kind of tricky to dive into, see below for a few tips to help you in the beginning. Positioning is crucial [Read... | Read more »
How to upgrade your character in Spellsp...
So you’ve mastered the basics of Spellspire. By which I mean you’ve realised it’s all about spelling things in a spire. What next? Well you’re going to need to figure out how to toughen up your character. It’s all well and good being able to spell... | Read more »
5 slither.io mash-ups we'd love to...
If there's one thing that slither.io has proved, it's that the addictive gameplay of Agar.io can be transplanted onto basically anything and it will still be good fun. It wouldn't be surprising if we saw other developers jumping on the bandwagon,... | Read more »
How to navigate the terrain in Sky Charm...
Sky Charms is a whimsical match-'em up adventure that uses creative level design to really ramp up the difficulty. [Read more] | Read more »
Victorious Knight (Games)
Victorious Knight 1.3 Device: iOS Universal Category: Games Price: $1.99, Version: 1.3 (iTunes) Description: New challenges awaits you! Experience fresh RPG experience with a unique combat mechanic, packed with high quality 3D... | Read more »
Agent Gumball - Roguelike Spy Game (Gam...
Agent Gumball - Roguelike Spy Game 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Someone’s been spying on Gumball. What the what?! Two can play at that game! GO UNDERCOVERSneak past enemy... | Read more »
Runaway Toad (Games)
Runaway Toad 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: It ain’t easy bein’ green! Tap, hold, and swipe to help Toad hop to safety in this gorgeous new action game from the creators of... | Read more »
PsyCard (Games)
PsyCard 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: From the makers och Card City Nights, Progress To 100 and Ittle Dew PSYCARD is a minesweeper-like game set in a cozy cyberpunk... | Read more »
Sago Mini Robot Party (Education)
Sago Mini Robot Party 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: -- Children's Technology Review Editor's Choice -- | Read more »
Egz – The Origin of the Universe (Games...
Egz – The Origin of the Universe 1.0.2 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.2 (iTunes) Description: ►►► Special offer until 2nd may : get the game at 2.99€ instead of 3.99€ ! ◄◄◄ Egz is a mesmerizing mix... | Read more »

Price Scanner via MacPrices.net

Price drops on clearance 12-inch Retina MacBo...
B&H Photo has dropped prices on leftover 2015 12″ Retina MacBooks with models now available starting at $999. Shipping is free, and B&H charges NY tax only: - 12″ 1.1GHz Gray Retina MacBook... Read more
15-inch Retina MacBook Pros available for $20...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Target offers Apple Watch Sport for $50 off M...
Target has Apple Watch Sports on sale for $50 off MSRP for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary... Read more
Apple restocks Certified Refurbished Mac mini...
Apple has restocked Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
Amazon.com has the 15″ 2.2GHz Retina MacBook Pro on sale for $1699.99 including free shipping. Their price is $300 off MSRP, and it’s the lowest price available for this model from any reseller (and... Read more
Apple Beats Microsoft at Own Game; Amazon Pri...
First quarter seasonality combined with an overall disinterested customer base led to an annual decline of 14.7% in worldwide tablet shipments during the first quarter of 2016 (1Q16). Worldwide... Read more
Tablets Had Worst Quarter Since 2012, says St...
The global tablet market began 2016 just as 2015 left off, down. Tablet shipments fell 10% to 46.5 million units during the Q1 2016, according to the new “Preliminary Global Tablet Shipments and... Read more
Clearance 13-inch MacBook Airs, Apple refurbi...
Apple recently dropped prices on certified refurbished 2015 13″ MacBook Airs with 4GB of RAM with models now available starting at $759. An Apple one-year warranty is included with each MacBook, and... Read more
Clearance 12-inch Retina MacBooks, Apple refu...
Apple has dropped prices on Certified Refurbished 2015 12″ Retina MacBooks with models now available starting at $929. Apple will include a standard one-year warranty with each MacBook, and shipping... Read more
Aleratec Releases Mac Software Upgrade for 1...
California based Aleratec Inc., designer, developer and manufacturer of Portable Device Management (PDM) charge/sync products for mobile devices and professional-grade duplicators for hard disk... Read more

Jobs Board

Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
*Apple* Retail - Multiple Positions - Apple,...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.