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.

 
AAPL
$102.08
Apple Inc.
+2.32
MSFT
$44.83
Microsoft Corpora
+0.75
GOOG
$523.95
Google Inc.
+3.11

MacTech Search:
Community Search:

Software Updates via MacUpdate

RapidWeaver 6.0 - Create template-based...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
NTFS 12.0.39 - Provides full read and wr...
Paragon NTFS breaks down the barriers between Windows and OS X. Paragon NTFS effectively solves the communication problems between the Mac system and NTFS, providing full read and write access to... Read more
RestoreMeNot 2.0.3 - Disable window rest...
RestoreMeNot provides a simple way to disable the window restoration for individual applications so that you can fine-tune this behavior to suit your needs. Please note that RestoreMeNot is designed... Read more
Macgo Blu-ray Player 2.10.9.1750 - Blu-r...
Macgo Mac Blu-ray Player can bring you the most unforgettable Blu-ray experience on your Mac. Overview Macgo Mac Blu-ray Player can satisfy just about every need you could possibly have in a Blu-ray... Read more
Apple iOS 8.1 - The latest version of Ap...
The latest version of iOS can be downloaded through iTunes. Apple iOS 8 comes with big updates to apps you use every day, like Messages and Photos. A whole new way to share content with your family.... Read more
TechTool Pro 7.0.5 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PDFKey Pro 4.0.2 - Edit and print passwo...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Yasu 2.9.1 - System maintenance app; per...
Yasu was originally created with System Administrators who service large groups of workstations in mind, Yasu (Yet Another System Utility) was made to do a specific group of maintenance tasks... Read more
Hazel 3.3 - Create rules for organizing...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a... Read more
Autopano Giga 3.7 - Stitch multiple imag...
Autopano Giga allows you to stitch 2, 20, or 2,000 images. Version 3.0 integrates impressive new features that will definitely make you adopt Autopano Pro or Autopano Giga: Choose between 9... Read more

Latest Forum Discussions

See All

Tilt to Live: Gauntlet’s Revenge is Almo...
Tilt to Live: Gauntlet’s Revenge is Almost Here Posted by Jessica Fisher on October 21st, 2014 [ permalink ] One Man Left has announced the official release date of Tilt to Live: Gauntlet’s Re | Read more »
Sago Mini Monsters Celebrates Halloween...
Sago Mini Monsters Celebrates Halloween with Fun Costumes and Special Treats. Posted by Jessica Fisher on October 21st, 2014 [ permal | Read more »
Inferno 2 Review
Inferno 2 Review By Andrew Fisher on October 21st, 2014 Our Rating: :: TWIN STICK GOODNESSUniversal App - Designed for iPhone and iPad With tight controls and awesome, stark visuals, Inferno 2 is loads of fun.   | Read more »
Clips Review
Clips Review By Jennifer Allen on October 21st, 2014 Our Rating: :: CONVENIENT PASTINGUniversal App - Designed for iPhone and iPad Making copying and pasting more powerful than usual, Clips is a great way to move stuff around.   | Read more »
MonSense Review
MonSense Review By Jennifer Allen on October 21st, 2014 Our Rating: :: ORGANIZED FINANCESiPhone App - Designed for the iPhone, compatible with the iPad Organize your finances with the quick and easy to use, MonSense.   | Read more »
This Week at 148Apps: October 13-17, 201...
Expert App Reviewers   So little time and so very many apps. What’s a poor iPhone/iPad lover to do? Fortunately, 148Apps is here to give you the rundown on the latest and greatest releases. And we even have a tremendous back catalog of reviews; just... | Read more »
Angry Birds Transformers Review
Angry Birds Transformers Review By Jennifer Allen on October 20th, 2014 Our Rating: :: TRANSFORMED BIRDSUniversal App - Designed for iPhone and iPad Transformed in a way you wouldn’t expect, Angry Birds Transformers is a quite... | Read more »
GAMEVIL Announces the Upcoming Launch of...
GAMEVIL Announces the Upcoming Launch of Mark of the Dragon Posted by Jessica Fisher on October 20th, 2014 [ permalink ] Mark of the Dragon, by GAMEVIL, put | Read more »
Interview With the Angry Birds Transform...
Angry Birds Transformers recently transformed and rolled out worldwide. This run-and-gun title is a hit with young Transformers fans, but the ample references to classic Transformers fandom has also earned it a place in the hearts of long-time... | Read more »
Hail to the King: Deathbat Review
Hail to the King: Deathbat Review By Rob Thomas on October 20th, 2014 Our Rating: :: SO FAR AWAYUniversal App - Designed for iPhone and iPad Hail to the King: Deathbat may feel like “Coming Home” for Avenged Sevenfold’s faithful,... | Read more »

Price Scanner via MacPrices.net

Select MacBook Airs $100 off MSRP, free shipp...
B&H Photo has 2014 a couple of MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. They also include free copies of Parallels Desktop and LoJack for... Read more
13-inch 2.5GHz MacBook Pro on sale for $100 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $999.99 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Read more
Strong iPhone, Mac And App Store Sales Drive...
Apple on Monday announced financial results for its fiscal 2014 fourth quarter ended September 27, 2014. The Company posted quarterly revenue of $42.1 billion and quarterly net profit of $8.5 billion... Read more
Apple Posts How-To For OS X Recovery
OS X 10.7 Lion and later include OS X Recovery. This feature includes all of the tools you need to reinstall OS X, repair your disk, and even restore from a Time Machine backup. OS X Recovery... Read more
Mac OS X Versions (Builds) Supported By Vario...
Apple Support has posted a handy resource explaining which Mac OS X versions (builds) originally shipped with or are available for your computer via retail discs, downloads, or Software Update. Apple... Read more
Deals on 2011 13-inch MacBook Airs, from $649
Daily Steals has the Mid-2011 13″ 1.7GHz i5 MacBook Air (4GB/128GB) available for $699 with a 90 day warranty. The Mid-2011 13″ 1.7GHz i5 MacBook Air (4GB/128GB SSD) is available for $649 at Other... Read more
2013 15-inch 2.0GHz Retina MacBook Pro availa...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
Updated iPad Prices
We’ve updated our iPad Air Price Tracker and our iPad mini Price Tracker with the latest information on prices and availability from Apple and other resellers, including the new iPad Air 2 and the... Read more
Apple Pay Available to Millions of Visa Cardh...
Visa Inc. brings secure, convenient payments to iPad Air 2 and iPad mini 3as well as iPhone 6 and 6 Plus. Starting October 20th, eligible Visa cardholders in the U.S. will be able to use Apple Pay,... Read more
Textkraft Pocket – the missing TextEdit for i...
infovole GmbH has announced the release and immediate availability of Textkraft Pocket 1.0, a professional text editor and note taking app for Apple’s iPhone. In March 2014 rumors were all about... Read more

Jobs Board

Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work 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
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.