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
$102.50
Apple Inc.
+0.25
MSFT
$45.43
Microsoft Corpora
+0.55
GOOG
$571.60
Google Inc.
+2.40

MacTech Search:
Community Search:

Software Updates via MacUpdate

Path Finder 6.5.5 - Powerful, award-winn...
Path Finder is a file browser that combines the familiar Finder interface with the powerful utilities and innovative features. Just a small selection of the Path Finder 6 feature set: Dual pane... Read more
QuarkXPress 10.2.1 - Desktop publishing...
With QuarkXPress, you can communicate in all the ways you need to -- and always look professional -- in print and digital media, all in a single tool. Features include: Easy to Use -- QuarkXPress is... Read more
Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - 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
SlingPlayer Plugin 3.3.20.505 - Browser...
SlingPlayer is the screen interface software that works hand-in-hand with the hardware inside the Slingbox to make your TV viewing experience just like that at home. It features an array of... Read more
Get Lyrical 3.8 - Auto-magically adds ly...
Get Lyrical auto-magically add lyrics to songs in iTunes. You can choose either a selection of tracks, or the current track. Or turn on "Active Tagging" to get lyrics for songs as you play them.... Read more
Viber 4.2.2 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more

Latest Forum Discussions

See All

Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »
Alien Creeps TD is Now Available Worldwi...
Alien Creeps TD is Now Available Worldwide Posted by Ellis Spice on August 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Dodo Master Review
Dodo Master Review By Jordan Minor on August 29th, 2014 Our Rating: :: NEST EGGiPad Only App - Designed for the iPad Dodo Master is tough but fair, and that’s what makes it a joy to play.   | Read more »
Motorsport Manager Review
Motorsport Manager Review By Lee Hamlet on August 29th, 2014 Our Rating: :: MARVELOUS MANAGEMENTUniversal App - Designed for iPhone and iPad Despite its depth and sense of tactical freedom, Motorsport Manager is one of the most... | Read more »
Motorsport Manager – Beginner Tips, Tric...
The world of Motorsport management can be an unforgiving and merciless one, so to help with some of the stress that comes with running a successful race team, here are a few hints and tips to leave your opponents in the dust. | Read more »
CalPal Update Brings the App to 2.0, Add...
CalPal Update Brings the App to 2.0, Adds Lots of New Stuff Posted by Ellis Spice on August 29th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Apple now offering refurbished 21-inch 1.4GHz...
The Apple Store is now offering Apple Certified Refurbished 21″ 1.4GHz iMacs for $929 including free shipping plus Apple’s standard one-year warranty. Their price is $170 off the cost of new models,... Read more
Save $50 on the 2.5GHz Mac mini, on sale for...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Bad Driver Database App Allows Good Drivers t...
Bad Driver Database 1.4 by Facile Group is a new iOS and Android app that lets users instantly input and see how many times a careless, reckless or just plain stupid driver has been added to the... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more
A&D Medical Launches Its WellnessConnecte...
For consumers and the healthcare providers and loved ones who care for them, A&D Medical, a leader in connected health and biometric measurement devices and services, has launched its... Read more
Anand Lal Shimpi Retires From AnandTech
Anand Lal Shimpi, whose AnandTech Website is famous for its meticulously detailed and thoroughgoing reviews and analysis, is packing it in. Lal Shimpi, who founded the tech site at age 14 in 1997,... Read more
2.5GHz Mac mini, Apple refurbished, in stock...
The Apple Store has Apple Certified Refurbished 2.5GHz Mac minis available for $509, $90 off MSRP. Apple’s one-year warranty is included, and shipping is free. Read more
13-inch 2.5GHz MacBook Pro on sale for $999,...
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

Jobs Board

*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...
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
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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.