TweetFollow Us on Twitter

Undo, TCommand
Volume Number:6
Issue Number:11
Column Tag:Jorg's Folder

TCommand and Undo

By Jörg Langowski, MacTutor Editorial Board

“MacApp - TCommand and Undo”

The example MacApp program in the August and September columns gives us a starting point for explaining how to extend a MacApp application. One of the most important operations that well-written Macintosh programs should support is Undo; so in this column I will show you in more detail how to implement undoing of commands in MacApp.

To undo an operation done on some document, we have to remember the state of the document before the operation and restore that state. Sounds simple; the problem is that in a usual Macintosh application many parts of the program will be affected. The structure beneath MacApp, however, makes it very simple to implement Undo. For each operation that should be undoable, a ‘command object’ is created which is intelligent enough to remember what it has done to the document, and which contains an Undo method that uses this information to reverse the operation.

Using command objects

Before we review our particular command object’s methods and variables, let’s see how some of these command objects are used. When the mouse is clicked inside a MacApp view, the DoMouseCommand method of that view is called. In your definition of that method, you must define the mouse behavior that you want. The previous example knew only one behavior; when the ‘drawing’ icon in the palette was selected, one could draw a rounded rectangle, using a cross cursor.

The standard way of implementing the mouse behavior would then be:

- determine which command you want to execute, depending on where the mouse was clicked;

- create a new command object corresponding to the operation to be done, and check for errors with FailNIL;

- initialize the command object by calling its initialization method;

- and, finally, exit the DoMouseCommand method, returning the command object to the calling routine.

That command object is then used by MacApp to track the mouse and implement undo/redo. When it is no longer used, it will be deleted (so you don’t have to care about it anymore).

In our new example, we want to implement dragging of the rectangles. When the mouse is clicked inside one of the rectangles, moving it should not create a new rectangle, but drag the old one around. Therefore, we first have to write a routine that checks whether the mouse was clicked inside an existing object. This is done by a global routine, FindBox, which is used as a parameter to the ForEachShapeDo method of the TEDocument class, passing the parameters to FindBox through the structure aFindBoxStruct. (The way in which a routine can be applied to a list of shapes has been described in the previous example - C++ makes it a little more complicated than Pascal).

When the mouse has been clicked inside an existing shape (aFindBoxStruct.myBox is non-nil), we use the Dragger instead of the Sketcher object to implement the mouse behavior (see listing).

The TCommand class

Command objects are implemented through MacApp’s TCommand class. If you want to define some undoable operation, you have to derive a subclass of TCommand which implements that specific operation. The class TDragger is implemented below. It includes the methods IDragger, TrackMouse, DoIt, UndoIt, and RedoIt.

In DoMouseCommand, we first create a new TDragger object, and then initialize it, telling it which object is going to be dragged (itsBox). In the initialization method, that object’s pointer is copied into an instance variable (fBox), and the instance variables oldLocation and newLocation are set to their initial values.

After we return the Dragger object from our DoMouseCommand, the MacApp system will call its TrackMouse method. The variable aTrackPhase indicates whether we just pushed the mouse button, whether we are in the middle of a drag, or just released the button. We need to take action only after the button has been released; it is then that the shape is moved to its new location on the screen. We set the object’s coordinates to their new values (calculated from the starting and ending points of the mouse drag), change newLocation, and return from the routine. this is returned to the calling routine, which means that on the next call the same mouse tracker will be used.

Note that all we need to do in the mouse tracker is to change the coordinates of the dragged object. The actual move will be done by the DoIt method, which is called after the mouse drag is completed. DoIt simply invalidates the object’s old and new bounds rectangles, so that these regions will be updated (this is also done automatically by MacApp).

In UndoIt, we simply reset the coordinates back to the old location and then call DoIt; RedoIt changes the coordinates back to the new location and DoesIt.

So in fact, you see that undoing a simple operation like moving an object is made very simple by MacApp. The command object exists as long as no other undoable command is chosen; when you perform another action that might be undoable, MacApp frees the last command so that its information is lost. But before that, all parameters necessary to undo/redo the operation should be stored inside the command object and used if necessary.

A final remark to the ‘cmnu’ 128 resource: This is a pseudo-menu (never displayed in the menu bar), which contains ‘buzzwords’ that should appear after Undo in the Edit menu so that the user can see which command can be undone/redone. The buzzword is associated with the proper command through the command number, we define cDragBox = 3001 for this purpose. This definition must of course be included in both the Rez file and the C++ program. It is used by the ICommand method, where the menu behavior is initialized.

When you execute the example and drag one of the rectangles around, you might notice that the mouse behavior is somewhat strange. The object is not dragged by moving its gray outline around, as you would expect, but instead a ‘rubber band’ rectangle is drawn connecting the starting and ending points of the drag. This is the standard behavior of mouse move feedback in MacApp, and if we want something more fancy, we have to define it ourselves. Next month, I’ll show you how to implement such a ‘track feedback’ routine. Until then, happy hacking.

Listing 1: Changes to the V6#8 example to support shape dragging

class TDragger : public TCommand {

public:
 TTEDocument   *fTEDocument;
 TTextView*fTextView;
 TBox   *fBox; // the object being dragged
 Rect   oldLocation; // initial location of object 
 Rect   newLocation; // new location of object 
 pascal void IDragger(TBox *itsBox, 
 TTEDocument *itsDocument, TTextView *itsView);
 pascal struct TCommand *TrackMouse
 (TrackPhase aTrackPhase,
  VPoint *anchorPoint, VPoint *previousPoint, 
  VPoint *nextPoint, Boolean mouseDidMove);
 pascal void DoIt();
 pascal void RedoIt();
 pascal void UndoIt();
#ifdef qDebug
 virtual pascal void Fields
 (pascal void (*DoToField) (StringPtr fieldName,
  Ptr fieldAddr, short fieldType, void *link), 
 void *link);
#endif
};

struct FindBoxStruct {
 Point theMouse;
 TBox *myBox;
};

const int cDragBox = 3001;
 // command number for Box dragger object}

pascal void FindBox
 (TBox *aBox, FindBoxStruct *aFindBoxStruct)
{
 if (PtInRect
 (aFindBoxStruct->theMouse,&aBox->fLocation))
 { aFindBoxStruct->myBox = aBox; };
}

pascal struct TCommand *TTextView::DoMouseCommand
 (Point *theMouse,  EventInfo *info, Point *hysteresis)
{ 
 TSketcher *aSketcher;
 TDragger  *aDragger;
 FindBoxStruct aFindBoxStruct;
 
 if (fPaletteView->fIconSelected == kBox-kBox) 
 {
 // dragging support, JL 9/90
 aFindBoxStruct.theMouse = *theMouse;
 aFindBoxStruct.myBox = nil;
 
 fTEDocument->ForEachShapeDo
 ((DoToObject)FindBox,&aFindBoxStruct);
 
 if (aFindBoxStruct.myBox != nil)
 { 
 SysBeep(5);// drag it
 aDragger = new TDragger;
 FailNIL(aDragger);
 aDragger->IDragger(aFindBoxStruct.myBox,
  fTEDocument, this);
 return aDragger;
 }
 else
 {
 aSketcher = new TSketcher;
 FailNIL(aSketcher);
 aSketcher->ISketcher(fTEDocument, this);
 return aSketcher;
 }
 }
 else
 return inherited::DoMouseCommand
 (theMouse,info,hysteresis);
}

pascal void TDragger::IDragger
 (TBox *itsBox, TTEDocument *itsDocument,
  TTextView *itsView)
{
 TScroller *aScroller;

 aScroller = itsView->GetScroller(true);
 ICommand(cDragBox, itsDocument, 
 itsView, aScroller);
 fTEDocument = itsDocument;
 fTextView = itsView;
 fBox = itsBox;
 oldLocation = fBox->fLocation;
 newLocation = fBox->fLocation;
}

pascal struct TCommand *TDragger::TrackMouse
 (TrackPhase aTrackPhase, VPoint *anchorPoint,
  VPoint *previousPoint,  VPoint *nextPoint, 
  Boolean mouseDidMove)
{
 
 if ((aTrackPhase == trackRelease) 
 && mouseDidMove)
 {
 newLocation.top = oldLocation.top 
 + nextPoint->v - anchorPoint->v;
 newLocation.left = oldLocation.left 
 + nextPoint->h - anchorPoint->h;
 newLocation.bottom = oldLocation.bottom 
 + nextPoint->v - anchorPoint->v;
 newLocation.right = oldLocation.right 
 + nextPoint->h - anchorPoint->h;
 fBox->fLocation = newLocation;
 }
 return this;
}

pascal void TDragger::DoIt()
{
 fTextView->InvalidRect(&newLocation);
 fTextView->InvalidRect(&oldLocation);
}

pascal void TDragger::RedoIt()  
{  
 fBox->fLocation = newLocation;
 DoIt(); 
}

pascal void TDragger::UndoIt()
{
 fBox->fLocation = oldLocation;
 DoIt(); 
}


#ifdef qDebug
pascal void TDragger::Fields(pascal void (*DoToField)
  (StringPtr fieldName, Ptr fieldAddr, 
 short fieldType, void *link), void *link)
{
 DoToField(“\pTDragger”, nil, bClass, link);
 DoToField(“\pfTEDocument”, 
 (Ptr) &fTEDocument, bObject, link);
 DoToField(“\pfTextView”, 
 (Ptr) &fTextView, bObject, link);
 DoToField(“\pfBox”, (Ptr) &fBox, bObject, link);
 DoToField(“\poldLocation”, 
 (Ptr) &oldLocation, bRect, link);
 DoToField(“\pnewLocation”, 
 (Ptr) &newLocation, bRect, link);
 inherited::Fields(DoToField, link);
}
#endif


#define cDragBox 3001

resource ‘cmnu’ (128) {
 128,
 textMenuProc,
 allEnabled,
 enabled,
 “Buzzwords”,  
 /* these words appear after Undo in the Edit menu */
  {
 “Page Setup Change”, noIcon, noKey, 
 noMark, plain, cChangePrinterStyle;
 “Typing”, noIcon, noKey, noMark, plain, cTyping;
 “Drawing”,  noIcon, noKey, 
 noMark, plain, cDrawBox;
 “Dragging”,  noIcon, noKey, 
 noMark, plain, cDragBox
 }
};

 
AAPL
$111.78
Apple Inc.
-0.87
MSFT
$47.66
Microsoft Corpora
+0.14
GOOG
$516.35
Google Inc.
+5.25

MacTech Search:
Community Search:

Software Updates via MacUpdate

NeoOffice 2014.6 - Mac-tailored, OpenOff...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
LibreOffice 4.3.5.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
CleanApp 5.0.0 Beta 5 - Application dein...
CleanApp is an application deinstaller and archiver.... Your hard drive gets fuller day by day, but do you know why? CleanApp 5 provides you with insights how to reclaim disk space. There are... Read more
Monolingual 1.6.2 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
NetShade 6.1 - Browse privately using an...
NetShade is an Internet security tool that conceals your IP address on the web. NetShade routes your Web connection through either a public anonymous proxy server, or one of NetShade's own dedicated... Read more
calibre 2.13 - Complete e-library manage...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Mellel 3.3.7 - Powerful word processor w...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
ScreenFlow 5.0.1 - Create screen recordi...
Save 10% with the exclusive MacUpdate coupon code: AFMacUpdate10 Buy now! ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your... Read more
Simon 4.0 - Monitor changes and crashes...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
BBEdit 11.0.2 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more

Latest Forum Discussions

See All

Galaxy Trucker Pocket (Games)
Galaxy Trucker Pocket 1.0.8 Device: iOS iPhone Category: Games Price: $2.99, Version: 1.0.8 (iTunes) Description: Galaxy Truckers Wanted!================================================================= (5/5) "Galaxy Trucker isn’t... | Read more »
Make your own Tribez Figures (and More)...
Make your own Tribez Figures (and More) with Toyze Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
So Many Holiday iOS Sales Oh My Goodness...
The holiday season is in full-swing, which means a whole lot of iOS apps and games are going on sale. A bunch already have, in fact. Naturally this means we’re putting together a hand-picked list of the best discounts and sales we can find in order... | Read more »
It’s Bird vs. Bird in the New PvP Mode f...
It’s Bird vs. Bird in the New PvP Mode for Angry Birds Epic Posted by Jessica Fisher on December 19th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Telltale Games and Mojang Announce Minec...
Telltale Games and Mojang Announce Minecraft: Story Mode – A Telltale Games Series Posted by Jessica Fisher on December 19th, 2014 [ permalink ] | Read more »
WarChest and Splash Damage Annouce Their...
WarChest and Splash Damage Annouce Their New Game: Tempo Posted by Jessica Fisher on December 19th, 2014 [ permalink ] WarChest Ltd and Splash Damage Ltd are teaming up again to work | Read more »
BulkyPix Celebrates its 6th Anniversary...
BulkyPix Celebrates its 6th Anniversary with a Bunch of Free Games Posted by Jessica Fisher on December 19th, 2014 [ permalink ] BulkyPix has | Read more »
Indulge in Japanese cuisine in Cooking F...
Indulge in Japanese cuisine in Cooking Fever’s new sushi-themed update Posted by Simon Reed on December 19th, 2014 [ permalink ] Lithuanian developer Nordcurrent has yet again updated its restaurant simulat | Read more »
Badland Daydream Level Pack Arrives to C...
Badland Daydream Level Pack Arrives to Celebrate 20 Million Downloads Posted by Ellis Spice on December 19th, 2014 [ permalink ] | Read more »
Far Cry 4, Assassin’s Creed Unity, Desti...
Far Cry 4, Assassin’s Creed Unity, Destiny, and Beyond – AppSpy Takes a Look at AAA Companion Apps Posted by Rob Rich on December 19th, 2014 [ permalink ] These day | Read more »

Price Scanner via MacPrices.net

Holiday sale: 13-inch 128GB MacBook Air for $...
 Best Buy has the 2014 13-inch 1.4GHz 128GB MacBook Air on sale for $849.99, or $150 off MSRP, on their online store. Choose free home shipping or free local store pickup (if available). Price valid... Read more
13-inch 2.6GHz Retina MacBook Pro on sale for...
Best Buy has lowered their price on the 2014 13″ 2.6GHz/128GB Retina MacBook Pro to $1149.99 on their online store for a limited time. That’s $150 off MSRP and the lowest price available for this... Read more
Kodak Returns to CES With New Consumer Produ...
Former photography colossus Kodak is returning to CES for the first time in three years where the Kodak booth (#21818 South Hall 1) will showcase a wide range of innovative, imaging-related products... Read more
Invaluable Launches New Eponymously -Named A...
Invaluable, the world’s largest online live auction marketplace, hhas announced the official launch of the Invaluable app for iPad, now available for download in the iTunes App Store. Invaluable... Read more
IDC Reveals Worldwide Mobile Enterprise Appli...
International Data Corporation (IDC) last week hosted the IDC FutureScape: Worldwide Mobile Enterprise Applications and Solutions 2015 Predictions Web conference. The session provided organizations... Read more
Hello Vino Wine App Launches “Safe Ride Home”...
Hello Vino has announced addition of a new “Get a Safe Ride Home” feature in its Food & Drink app with a direct connection to Uber, the technology platform that connects users with rides. The... Read more
DEVON-technologies Releases DEVONthink To Go...
Coeur d’Alene, Idaho based DEVON-technologies, LLC has updated DEVONthink To Go, its mobile companion to DEVONthink, to version 1.5. The update includes an iOS 8 extension, compatibility with the... Read more
The Apple Store offering free next-day shippi...
The Apple Store is now offering free next-day shipping on all in stock items if ordered before 12/23/14 at 10:00am PT. Local store pickup is also available within an hour of ordering for any in stock... Read more
It’s 1992 Again At Sony Pictures, Except For...
Techcrunch’s John Biggs interviewed a Sony Pictures Entertainment (SPE) employee, who quite understandably wished to remain anonymous, regarding post-hack conditions in SPE’s L.A office, explaining “... Read more
OtterBox Defender Series Case For iPad mini 3...
With their innovative Touch ID technology and ultrathin profile, the latest tranche of Apple iPads are more desirable than ever, and OtterBox has just announced the Defender Series custom-engineered... Read more

Jobs Board

*Apple* Store Leader Program (US) - Apple, I...
…Summary Learn and grow as you explore the art of leadership at the Apple Store. You'll master our retail business inside and out through training, hands-on experience, Read more
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Retail - Multiple Positions (US) - A...
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
*Apple* Retail - Multiple Positions (US) - A...
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
*Apple* Retail - Multiple Positions (US) - A...
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.