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

CSR Racing 2: Your guide to what's...
CSR Racing 2, or CSR2, as it likes to call itself, has finally arrived. The follow-up to the immensely popular drag racing game CSR Racing is the first release from NaturalMotion since the studio's acquisition by Zynga in early 2014. [Read more] | Read more »
Nanuleu (Games)
Nanuleu 1.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.1 (iTunes) Description: Nanuleu is a strategy game where you take control of ancient magical trees that protect the land from an invading dark force. A... | Read more »
The Slaughter: Act One (Games)
The Slaughter: Act One 1.0.323 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.323 (iTunes) Description: “The game mixes realism and surrealism to create a story that can cause just as much laughter as fear. A-” -... | Read more »
NEO TURF MASTERS (Games)
NEO TURF MASTERS 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: NEOGEO’s legendary golf game is back, in a brand-new mobile version with touch controls! NEO TURF MASTERS (also known as “BIG... | Read more »
How to send money to a friend with the F...
Facebook is already a lot of things to a lot of people. It's a social network, sure, but also a tool for catologing memories, chatting with friends, and much more. [Read more] | Read more »
Branch out into the Chinese market with...
Created a game that you’re super enthused about, only to find yourself struggling to get it the exposure that you believe it duly deserves? With the time, energy and money that goes into development, it’s only right that games should be given the... | Read more »
Pocket Arcade Story (Games)
Pocket Arcade Story 1.00 Device: iOS Universal Category: Games Price: $4.99, Version: 1.00 (iTunes) Description: Here comes a new challenger: it's the arcade simulation game you've been waiting for! Build your very own gaming... | Read more »
How to get coins faster in Rodeo Stamped...
There comes a time in a cowboy or cowgirl's life when all the riding and lassoing skills in the world aren't enough. You're going to need some cold, hard cash to keep your sky zoo expanding in Rodeo Stampede. [Read more] | Read more »
How to out-do Cam Newton in Can You Dab?
The thing about dance crazes is that you're never really sure when they've run their course. Take the Dab, for instance. Propelled by its adoption as the touchdown celebration of choice for Carolina Panthers quarterback Cam Newton, the Dab seemed... | Read more »
Artik Games releases Splashy Cats for An...
Splashy Cats had us hooked from the title alone, and when we found out the game was literally just zig-zagging one of our favourite pop-culture references, guised as a playable cat character, down a river – our appetites were whetted to say the... | Read more »

Price Scanner via MacPrices.net

Apple refurbished clearance 15-inch Retina Ma...
Apple has Certified Refurbished 2014 15″ 2.2GHz Retina MacBook Pros available for $1609, $390 off original MSRP. Apple’s one-year warranty is included, and shipping is free. They have refurbished 15... Read more
9-inch 128GB Silver iPad Pro on sale for $50...
B&H Photo has the 9.7″ 128GB Silver Apple iPad Pro on sale for $699 including free shipping plus NY tax only. Their price is $50 off MSRP. Read more
Why Use Indie Opera And Vivaldi Instead Of Sa...
For many years my web browser workhorses were various permutations and spinoffs of the Netscape/Mozilla/Firefox Open Source platform, and the Norwegian indie browser Opera, which I took a shine to... Read more
Western Digital Launches Worlds Fastest 256GB...
At the Mobile World Congress in Shanghai Western Digital Corporation this week introduced a new suite of 256 gigabyte (GB) microSD cards, which includes the new 256GB SanDisk Extreme microSDXC UHS-I... Read more
KeyCue 8.1 Integrates With Typinator To Displ...
Ergonis Software has released KeyCue 8.1, a new version of the company’s keyboard shortcut cheat sheet. KeyCue 8 introduced a new way to define a wide variety of triggers, which can be used to... Read more
Save up to $600 with Apple refurbished Mac Pr...
Apple has Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The following... Read more
21-inch 2.8GHz iMac on sale for $1199, save $...
Amazon has the 21″ 2.8GHz iMac (model #MK442LL/A) on sale for $1199.99 including free shipping. Their price is $100 off MSRP, and it’s the lowest price available for this model. Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros... Read more
MacBook Airs on sale for up to $50-$100 off M...
B&H Photo has 13″ and 11″ MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $849 $50 off list price - 11″ 1.... Read more
Brexit Vote Result Forecast To Slash UK 2016...
Uncertainty and economic volatility can be expected to increase over the next nine months, as the Brexit and concerns over the future of the EU hit IT investment, say Canalys market analysts, with... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* iPhone 6s and New Products Tester Ne...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
*Apple* iPhone 6s and New Products Tester Ne...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
*Apple* iPhone 6s and New Products Tester Ne...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
*Apple* Retail - Multiple Positions, Willow...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.