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.

{* File: SendMeAMessage.p *}
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; 


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

PROCEDURE EntryPoint( paramPtr: XCmdPtr);

 Str31  = String[31];
PROCEDURE SendMeAMessage(paramPtr:XCmdPtr);

{-------------- EntryPoint --------------}
PROCEDURE EntryPoint(paramPtr: XCmdPtr);

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

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:

 global myData
 GetHomeInfo “myData”

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

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?

*file: FCreateXFCN.c *

C -q2 -g FCreateXFCN.c
link  -sn Main=FCreateXFCN 
 -sn STDIO=FCreateXFCN 
  -sn INTENV=FCreateXFCN 
  -rt XFCN=300 
  FCreateXFCN.c.o 
  “{CLibraries}”CInterface.o 
  -o testXCMDs
#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;
 NumToStr( paramPtr, (long)error, &str );
 paramPtr->returnValue = 
 PasToZero( paramPtr, &str );

#include <>

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

Microsoft Office 2016 16.11 - Popular pr...
Microsoft Office 2016 - Unmistakably Office, designed for Mac. The new versions of Word, Excel, PowerPoint, Outlook, and OneNote provide the best of both worlds for Mac users - the familiar Office... Read more
Adobe Photoshop CC 2018 19.1.2 - Profess...
Photoshop CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Photoshop customer). Adobe Photoshop CC 2018, the industry standard... Read more
Adobe Dreamweaver CC 2018 -...
Dreamweaver CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Dreamweaver customer). Adobe Dreamweaver CC 2018 allows you to... Read more
Adobe Flash Player - Plug-in...
Adobe Flash Player is a cross-platform, browser-based application runtime that provides uncompromised viewing of expressive applications, content, and videos across browsers and operating systems.... Read more
Drive Genius 5.2.0 - $79.00
Drive Genius features a comprehensive Malware Scan. Automate your malware protection. Protect your investment from any threat. The Malware Scan is part of the automated DrivePulse utility. DrivePulse... Read more
MegaSeg 6.0.6 - Professional DJ and radi...
MegaSeg is a complete solution for pro audio/video DJ mixing, radio automation, and music scheduling with rock-solid performance and an easy-to-use design. Mix with visual waveforms and Magic... Read more
ffWorks 1.0.7 - Convert multimedia files...
ffWorks (was iFFmpeg), focused on simplicity, brings a fresh approach to the use of FFmpeg, allowing you to create ultra-high-quality movies without the need to write a single line of code on the... Read more
Dash 4.1.5 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
Evernote 7.0.3 - Create searchable notes...
Evernote allows you to easily capture information in any environment using whatever device or platform you find most convenient, and makes this information accessible and searchable at anytime, from... Read more
jAlbum Pro 15.3 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more

Latest Forum Discussions

See All

All the best games on sale for iPhone an...
It might not have been the greatest week for new releases on the App Store, but don't let that get you down, because there are some truly incredible games on sale for iPhone and iPad right now. Seriously, you could buy anything on this list and I... | Read more »
Everything You Need to Know About The Fo...
In just over a week, Epic Games has made a flurry of announcements. First, they revealed that Fortnite—their ultra-popular PUBG competitor—is coming to mobile. This was followed by brief sign-up period for interested beta testers before sending out... | Read more »
The best games that came out for iPhone...
It's not been the best week for games on the App Store. There are a few decent ones here and there, but nothing that's really going to make you throw down what you're doing and run to the nearest WiFi hotspot in order to download it. That's not to... | Read more »
Death Coming (Games)
Death Coming Device: iOS Universal Category: Games Price: $1.99, Version: (iTunes) Description: --- Background Story ---You Died. Pure and simple, but death was not the end. You have become an agent of Death: a... | Read more »
Hints, tips, and tricks for Empires and...
Empires and Puzzles is a slick match-stuff RPG that mixes in a bunch of city-building aspects to keep things fresh. And it's currently the Game of the Day over on the App Store. So, if you're picking it up for the first time today, we thought it'd... | Read more »
What You Need to Know About Sam Barlow’s...
Sam Barlow’s follow up to Her Story is #WarGames, an interactive video series that reimagines the 1983 film WarGames in a more present day context. It’s not exactly a game, but it’s definitely still interesting. Here are the top things you should... | Read more »
Pixel Plex Guide - How to Build Better T...
Pixel Plex is the latest city builder that has come to the App Store, and it takes a pretty different tact than the ones that came before it. Instead of being in charge of your own city by yourself, you have to work together with other players to... | Read more »
Fortnite Will Be Better Than PUBG on Mob...
Before last week, if you asked me which game I prefer between Fortnite Battle Royale and PlayerUnknown’s Battlegrounds (PUBG), I’d choose the latter just about 100% of the time. Now that we know that both games are primed to hit our mobile screens... | Read more »
Siege of Dragonspear (Games)
Siege of Dragonspear 2.5.12 Device: iOS Universal Category: Games Price: $9.99, Version: 2.5.12 (iTunes) Description: Experience the Siege of Dragonspear, an epic Baldur’s Gate tale, filled with with intrigue, magic, and monsters.... | Read more »
7 Wonders Guide - Should You Buy The Lea...
The fantastic mobile version of 7 Wonders just got updated with an expansion that adds Leaders to the game. This new content adds a whole layer of depth to the game, but before you spend $1.99 to buy it blindly, check out this breakdown of exactly... | Read more »

Price Scanner via

B&H drops prices on 15″ MacBook Pros up t...
B&H Photo has dropped prices on new 2017 15″ MacBook Pros, now up to $300 off MSRP and matching Adorama’s price drop yesterday. Shipping is free, and B&H charges sales tax for NY & NJ... Read more
Apple restocks Certified Refurbished 2017 13″...
Apple has restocked Certified Refurbished 2017 13″ 2.3GHz MacBook Pros for $200-$230 off MSRP. A standard Apple one-year warranty is included with each MacBook, models receive new outer cases, and... Read more
13″ Space Gray Touch Bar MacBook Pros on sale...
Adorama has new 2017 13″ Space Gray Touch Bar MacBook Pros on sale for $150 off MSRP. Shipping is free, and Adorama charges sales tax in NY & NJ only: – 13″ 3.1GHz/256GB Space Gray MacBook Pro (... Read more
Best deal of the year on 15″ Apple MacBook Pr...
Adorama has New 2017 15″ MacBook Pros on sale for up to $300 off MSRP. Shipping is free, and Adorama charges sales tax in NJ and NY only: – 15″ 2.8GHz Touch Bar MacBook Pro Space Gray (MPTR2LL/A): $... Read more
Save $100-$150+ on 13″ Touch Bar MacBook Pros...
B&H Photo has 13″ Touch Bar MacBook Pros on sale for $100-$150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13″ 3.1GHz/256GB Space Gray MacBook Pro... Read more
Current deals on 27″ Apple iMacs, models up t...
B&H Photo has 27″ iMacs on sale for up to $150 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 27″ 3.8GHz iMac (MNED2LL/A): $2149 $150 off MSRP – 27″ 3... Read more
Thursday Deal: 13″ 2.3GHz MacBook Pro for $11...
B&H Photo has the 13″ 2.3GHz/128GB Space Gray MacBook Pro on sale for $100 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 13-inch 2.3GHz/128GB Space... Read more
How to save $100-$190 on 10″ & 12″ iPad P...
Apple is now offering Certified Refurbished 2017 10″ and 12″ iPad Pros for $100-$190 off MSRP, depending on the model. An Apple one-year warranty is included with each model, and shipping is free: –... Read more
Silver 12″ 1.3GHz MacBook on sale at B&H...
B&H Photo has the 2017 12″ 1.3GHz Silver MacBook on sale for $1399.99 including free shipping plus sales tax for NY & NJ residents only. Their price is $200 off MSRP, and it’s the lowest... Read more
Amazon offers 21″ Apple iMacs for up to $150...
Amazon 21″ iMacs on sale today for $50-$150 off MSRP, depending on the model. Shipping is free: – 21″ 3.4GHz 4K iMac (MNE02LL/A): $1349.99 $150 off MSRP – 21″ 3.0GHz iMac (MNDY2LL/A): $1199 $100 off... Read more

Jobs Board

Data Scientist, *Apple* Ecosystem (AMP Anal...
# Data Scientist, Apple Ecosystem (AMP Analytics) Job Number: 113428728 Santa Clara Valley, California, United States Posted: 24-Jan-2018 Weekly Hours: 40.00 **Job Read more
*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 113523441 Orange, CA, California, United States Posted: 21-Feb-2018 Weekly Hours: 40.00 **Job Summary** Are you passionate Read more
*Apple* Professional Learning Specialist - A...
# Apple Professional Learning Specialist Job Number: 113456892 Englewood, NJ, New Jersey, United States Posted: 02-Feb-2018 Weekly Hours: 40.00 **Job Summary** The Read more
*Apple* Retail Online, Senior Financial Anal...
# Apple Retail Online, Senior Financial Analyst Job Number: 57228835 Santa Clara Valley, California, United States Posted: 12-Feb-2018 Weekly Hours: 40.00 **Job Read more
Site Reliability Engineer, *Apple* Pay - Ap...
# Site Reliability Engineer, Apple Pay Job Number: 113356036 Santa Clara Valley, California, United States Posted: 12-Jan-2018 Weekly Hours: 40.00 **Job Summary** Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.