TweetFollow Us on Twitter

Grokking OS X's Undo Support

Volume Number: 24 (2008)
Issue Number: 04
Column Tag: Programming

Grokking OS X's Undo Support

How to effectively use the Undo Manager built into Cocoa

by Marcus S. Zarra

Introduction

One of the great benefits of working with Cocoa is that the APIs give the developer numerous features "for free". One of those features is undo support. Any Cocoa application, without any work from the developer, will automatically get undo support at some basic level. In this article I walk through exactly how Undo support works in Cocoa and how you as the developer can take some control over undo to provide the functionality you are looking for.

Overview

Undo is one of those features that is rarely thought about. Most developers do not check it off as a feature in their application and most users do not immediately look for it when reviewing a new application. However, it is a feature that if not present when you need it, it is sorely missed. Fortunately, Apple has recognized this and included undo support for the developer so that, in most cases, we do not need to think about it.

Unfortunately, when your application strays from the beaten path then you need to roll up your sleeves and make sure that undo support works exactly as you expect it to. As it is said, the devil is in the details, and having inconsistent undo support is worse than having none at all.

What is Undo?

Apple defines undo as:

An undo operation is a method for reverting a change to an object, along with the arguments needed to revert the change (for example, its state before the change)

Undo is performed on a stack. What this means to those not familiar with the term is that each action, or event, that is "undoable" is added to the top of the stack and therefore can be removed from the stack. When you remove an item from the stack it is also removed from the top and thus reversing the order it was added. For example, if you were editing a paragraph of text in a Cocoa NSTextArea and you typed "Hello World", that text would be added to the stack as an undoable event. If you then selected "Hello" and hit ?B, the text would become bold and the action of bolding that selection of text would be added to the stack.


The reason that the "stack" aspect of this is important is when you want to undo something. When you choose undo then the first time is removed from the top of the stack (in this example, the bolding of "Hello") and undone. If the items were processed in a FIFO (First In, First Out) order then the text would be deleted instead, which is definitely not what the user would expect to happen.

The Undo Stack

What exactly is the stack? Probably the easiest way to describe the stack is to describe how to put things onto it. As an example, lets say that we have a value that we want to be undoable and therefore we want to add any changes to that value to the stack. To do that we would perform the following:

-(void)setName:(NSString*)newName;;
{
   [[self undoManager] registerUndoWithTarget:self 
      selector:@selector(setName:) 
      object:[self name]];
   [newName retain];
   [name release];
   name = newName;
}

For those familiar with Cocoa/Objective-C, this is a fairly standard setter call to set the attribute name. The change from the normal is the additional call to the NSUndoManager. Note that in this example I am passing it a reference to the object to call the method on (self), the method name to call (setName:) and the previous value of that attribute.

Internally, the NSUndoManager, is going to remember these values that you are passing in and retain them as an undo event. That event will then be stored on a stack (presumably in an NSArray) to be recalled at a later time. Therefore the stack is really an array of structures/objects that reference a target, a selector and another object. One thing that is interesting in this design is that the object (the previous value of name) is actually retained by the undo event. This guarantees that the value will still be around if the undo event is ever invoked.

When the user of the application invokes the undo command, the last item added to the NSUndoManager is retrieved and then the selector is called upon the target with the object that is being passed in, thereby reversing the event. Of note: When an event is removed from the undo stack, it is automatically added to the redo stack. This allows a user to undo and redo their actions as needed.

Grouping Undo

In addition to being able to undo and redo individual events such as the setter above, it is possible to group individual events together so that they are undone and redone as a single operation. An example of this would be an extension of the example above. For instance, lets imagine that the setName: method above actually breaks the string apart into a first name and last name. And instead of having an undo registration at the setName, we want to have the first and last name register events, then grouping would come into play when the full name is set.

-(void)setName:(NSString*)name;
{
   [[self undoManager] beginUndoGrouping];
   NSArray *words = [name componentsSeparatedByString@" "];
   [self setFirstName:[words objectAtIndex:0];
   [self setLastName:[words objectAtIndex:1];
   [[self undoManager] endUndoGrouping];
}
- (void)setFirstName:(NSString*)name
{
   [[self undoManager] registerUndoWithTarget:self 
      selector:@selector(setFirstName:) 
      object:[self firstName]];
   [name retain];
   [firstName release];
   firstName = name;
}
- (void)setLastName:(NSString*)name
{
   [[self undoManager] registerUndoWithTarget:self
      selector:@selector(setLastName:)
      object:[self lastName]];
   [name retain];
   [lastName release];
   lastName = name;
}

As can be seen in the listing, the individual parts of the name have stored their changes in the NSUndoManager's stack but the setName: method does not store its changes in the undo stack. Instead, it starts a group, sets the individual components and then ends the grouping. What this does is still the NSUndoManager that all undo events it receives between the begin and end are to be performed together. By using grouping in an example like this you do not risk polluting the stack with a setName:, setFirstName: and setLastName: call and thereby causing an issue when the user attempts to undo the action.

Naming the event

In addition to being able to control the undo/redo events, it is also possible to name these events. That name is then used by the undo and redo menu items to help explain to the user exactly what they will be undoing and redoing.

- (void)setFirstName:(NSString*)name
{
   [[self undoManager] registerUndoWithTarget:self 
      selector:@selector(setFirstName:) 
      object:[self firstName]];
   [[self undoManager] setActionName:
      NSLocalizedString(@"First Name", 
      @"First Name undo action")];
   [name retain];
   [firstName release];
   firstName = name;
}

The only change from the previous example is the addition of [undoManager setActionName:]. This call instructs the NSUndoManager to attach the passed in string as the name of the previous event. In this example, it attaches the name of "First Name" to the event of setFirstName:. If the user were to look in the edit menu after changing the first name they would see something like "Undo First Name". As you can see, we localized the action name so that we can easily go back and localize the name of the action at a later time.

It should be noted that groupings can also be named. If an event and a grouping are both named then the grouping name will win out. For example, if our setName: method also set the ActionName to "Name" then when the setName: method is called the edit menu would display "Undo Name" and not "Undo First Name" or "Undo Last Name".

Levels of Undo

As can be expected, since the NSUndoManager actually retains the objects being passed around it is quite possible to begin to have a memory issue. This is especially true if the object of the function call is in itself quite large. One option to help contain this is to limit the number or "levels" of undo available to the application. This is controlled by a simple call to:

[[self undoManager] setLevelsOfUndo:10];

which will limit the application to remembering only the last 10 undo events.

Blocking Undo Support

As you can probably imagine, since undo support is managed as such a low level (all the way down in the setters), it is possible to build up a large undo stack just by populating an object from another source. Fortunately, there is a way to avoid this. For example, imagine you are populating our example object from an XML feed. Naturally, you do not want to start building up the undo stack while setting the values from the xml document but you do want to add to the stack when a user changes something. How can you instruct your application to tell the difference?

The secret is to pause the undo registration while loading your document and then resuming it after your load is complete. See the following example:

- (MyObject*)buildMyObjectFromXML:(NSXMLDocument*)doc
{
   NSString *tempFirstName = ...;
   NSString *tempLastName = ...;
   NSUndoManager *undo = [self undoManager];
   [undo disableUndoRegistration];
   MyObject *object = [[MyObject alloc] init];
   [object setFirstName:tempFirstName];
   [object setLastName:tempLastName];
   [undo enableUndoRegistration];
   return [object autorelease];
}

In this example, we instruct the NSUndoManager to stop registering events and then we build the data object and set its attributes. The data object itself has no knowledge of whether or not the NSUndoManager is disabled (although we could check if needed for some reason) and continues to register events. The NSUndoManager receives these events and since it is disabled simply throws them away. When the object has been completely loaded we then turn the registration back on so that any events afterwards will be registered properly.

Initializing the NSUndoManager

In all of these examples, I have just been calling [self undoManager] to get a reference to the NSUndoManager. In an actual program you should have very few NSUndoManagers. Each NSUndoManager should be in a logical location. For instance, if you have a document style application then each document should have its own NSUndoManager. Otherwise you probably only want one NSUndoManager for the entire application.

To initialize an NSUndoManager, you simply call:

NSUndoManager *undoManager = [[NSUndoManger alloc] init];

Naturally, you are going to want to hold onto a reference to this object so that you can make it available to other parts of your application.

Getting the UI to use your NSUndoManager

If you are going to build and/or use your own NSUndoManager then you will want to let the user interface access that same NSUndoManager to avoid having multiple stacks trying to act on the same data. Fortunately, all views get their NSUndoManager from their window and the window gets its NSUndoManager from its delegate. Therefore if you have set up your controller (or another object) as the delegate for your window, add the following method call:

- (NSUndoManager *)windowWillReturnUndoManager:(NSWindow *)window
{
   return [self undoManager];
}

The window will then call this method to get its NSUndoManager. If you do not supply a delegate then the window will initialize its own NSUndoManager.

Conclusion

Once the curtain has been pulled back you can see that the NSUndoManager is in fact a simple array containing structures with two objects and a selector. That is all there is to Undo. However that simplicity belies the power of the design. The application of that simple data structure allows for the sense of wonder and awe that is an application with a properly implemented undo subsystem.


Marcus S. Zarra is the owner of Zarra Studios, based out of Colorado Springs, Colorado. He has been developing Cocoa software since 2003, Java software since 1996, and has been in the industry since 1985. Currently Marcus is producing software for OS X. In addition to writing software, he assists other developers by blogging about development and supplying code samples.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Bookends 12.7.8 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
VirtualBox 5.1.14 - x86 virtualization s...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
FileZilla 3.24.0 - Fast and reliable FTP...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface. Version 3.24.0: New The context menu for remote file search... Read more
BusyContacts 1.1.6 - Fast, efficient con...
BusyContacts is a contact manager for OS X that makes creating, finding, and managing contacts faster and more efficient. It brings to contact management the same power, flexibility, and sharing... Read more
BusyCal 3.1.4 - Powerful calendar app wi...
BusyCal is an award-winning desktop calendar that combines personal productivity features for individuals with powerful calendar sharing capabilities for families and workgroups. Its unique features... Read more
Duplicate Annihilator 5.8.3 - Find and d...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
MarsEdit 3.7.10 - Quick and convenient b...
MarsEdit is a blog editor for OS X that makes editing your blog like writing email, with spell-checking, drafts, multiple windows, and even AppleScript support. It works with with most blog services... Read more
WALTR 2 2.0.9 - $39.95
WALTR 2 helps you wirelessly drag-and-drop any music, ringtones, videos, PDF, and ePub files onto your iPhone, iPad, or iPod without iTunes. It is the second major version of Softorino's critically-... Read more
Paperless 2.3.9 - $49.95
Paperless is a digital documents manager. Remember when everyone talked about how we would soon be a paperless society? Now it seems like we use paper more than ever. Let's face it - we need and we... Read more
Adobe After Effects CC 2017 14.1 - Creat...
After Effects CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous After Effects customer). The new, more connected After Effects CC... Read more

Super Mario Run dashes onto Android in M...
Super Mario Run was one of the biggest mobile launches in 2016 before it was met with a lukewarm response by many. While the game itself plays a treat, it's pretty hard to swallow the steep price for the full game. With that said, Android users... | Read more »
WarFriends Beginner's Guide: How to...
Chillingo's new game, WarFriends, is finally available world wide, and so far it's a refreshing change from common mobile game trends. The game's a mix of tower defense, third person shooter, and collectible card game. There's a lot to unpack here... | Read more »
Super Gridland (Entertainment)
Super Gridland 1.0 Device: iOS Universal Category: Entertainment Price: $1.99, Version: 1.0 (iTunes) Description: Match. Build. Survive. "exquisitely tuned" - Rock Paper Shotgun No in-app purches, and no ads! | Read more »
Red's Kingdom (Games)
Red's Kingdom 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Mad King Mac has kidnapped your father and stolen your golden nut! Solve puzzles and battle goons as you explore and battle your... | Read more »
Turbo League Guide: How to tame the cont...
| Read more »
Fire Emblem: Heroes coming to Google Pla...
Nintendo gave us our first look at Fire Emblem: Heroes, the upcoming mobile Fire Emblem game the company hinted at last year. Revealed at the Fire Emblem Direct event held today, the game will condense the series' tactical RPG combat into bite-... | Read more »
ReSlice (Music)
ReSlice 1.0 Device: iOS Universal Category: Music Price: $9.99, Version: 1.0 (iTunes) Description: Audio Slice Machine Slice your audio samples with ReSlice and create flexible musical atoms which can be triggered by MIDI notes or... | Read more »
Stickman Surfer rides in with the tide t...
Stickson is back and this time he's taken up yet another extreme sport - surfing. Stickman Surfer is out this Thursday on both iOS and Android, so if you've been following the other Stickman adventures, you might be interested in picking this one... | Read more »
Z-Exemplar (Games)
Z-Exemplar 1.4 Device: iOS Universal Category: Games Price: $3.99, Version: 1.4 (iTunes) Description: | Read more »
5 dastardly difficult roguelikes like th...
Edmund McMillen's popular roguelike creation The Binding of Isaac: Rebirth has finally crawled onto mobile devices. It's a grotesque dual-stick shooter that tosses you into an endless, procedurally generated basement as you, the pitiable Isaac,... | Read more »

Price Scanner via MacPrices.net

Twelve South Releases RelaxedLeather Cases fo...
Inspired by the laid-back luxury of burnished leather boots and crafted in rich tones of taupe, herb and marsala, RelaxedLeather cases deliver smart, easy protection for the iPhone 7. Each genuine... Read more
Week’s Best Deal: New 2016 13-inch 2.0GHz Mac...
Amazon has the new 2016 13″ 2.0GHz non-Touch Bar MacBook Pros on sale for a limited time for $225 off MSRP including free shipping: - 13″ 2.0GHz MacBook Pro, Space Gray (MLL42LL/A): $1274.99 $225 off... Read more
Back in stock: Apple refurbished Mac minis fr...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $80 off MSRP - 2.6GHz Mac... Read more
Apple Ranked ‘Most Intimate Brand’
The top ranked ‘”intimate” brands continued to outperform the S&P and Fortune 500 indices in revenue and profit over the past 10 years, according to MBLM’s Brand Intimacy 2017 Report, the largest... Read more
B-Eng introduces SSD Health Check for Mac OS
Fehraltorf, Switzerland based independant Swiss company- B-Eng has announced the release and immediate availability of SSD Health Check 1.0, the company’s new hard drive utility for Mac OS X. As the... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: -... Read more
4-core 3.7GHz Mac Pro on sale for $2290, save...
Guitar Center has the 3.7GHz 4-core Mac Pro (MD253LL/A) on sale for $2289.97 including free shipping or free local store pickup (if available). Their price is a $710 savings over standard MSRP for... Read more
128GB Apple iPad Air 2, refurbished, availabl...
Apple has Certified Refurbished 128GB iPad Air 2s WiFis available for $419 including free shipping. That’s an $80 savings over standard MSRP for this model. A standard Apple one-year warranty is... Read more
13-inch 2.7GHz Retina MacBook Pro on sale for...
B&H Photo has the 2015 13″ 2.7GHz/128GB Retina Apple MacBook Pro on sale for $100 off MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro (MF839LL/A): $... Read more
Laptop Market – Flight To Quality? – The ‘Boo...
Preliminary quarterly PC shipments data released by Gartner Inc. last week reveal an interesting disparity between sales performance of major name PC vendors as opposed to that of less well-known... Read more

Jobs Board

*Apple* MAC Infrastructure Engineer - InnoCo...
Summary: Responsible for all aspects of Apple Desktop hardware. This includes research, design, test, and deploy technologies being researched by the desktop Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Manhattan, NY Introduction: We have immediate job openings for several Desktop Support Technicians with one of our most Read more
Senior Workstation Administrator - *Apple*...
…with extraordinary HR. QualificationsJOB SUMMARY/OVERVIEWThe Senior Workstation Administrator - Apple supports the mission of TriNet by providing advanced level Read more
Intermediate *Apple* macOS Systems Integrat...
**Position Summary:** SC3 is actively seeking an Intermediate Apple macOS systems integration administrator that will be responsible for providing Apple Mac Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Los Angeles, CA Introduction: We have immediate job openings for several Desktop Support Technicians with one of our Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.