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

Navicat Premium Essentials 12.1.25 - Pro...
Navicat Premium Essentials is a compact version of Navicat which provides basic and necessary features you will need to perform simple administration on a database. It supports the latest features... Read more
Sketch 58 - Design app for UX/UI for iOS...
Sketch is an innovative and fresh look at vector drawing. Its intentionally minimalist design is based upon a drawing space of unlimited size and layers, free of palettes, panels, menus, windows, and... Read more
ClipGrab 3.8.5 - Download videos from Yo...
ClipGrab is a free downloader and converter for YouTube, Vimeo, Facebook and many other online video sites. It converts downloaded videos to MPEG4, MP3 or other formats in just one easy step Version... Read more
Dash 4.6.6 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
FotoMagico 5.6.8 - Powerful slideshow cr...
FotoMagico lets you create professional slideshows from your photos and music with just a few, simple mouse clicks. It sports a very clean and intuitive yet powerful user interface. High image... Read more
Civilization VI 1.2.4 - Next iteration o...
Sid Meier’s Civilization VI is the next entry in the popular Civilization franchise. Originally created by legendary game designer Sid Meier, Civilization is a strategy game in which you attempt to... Read more
Skype 8.52.0.138 - 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
Bookends 13.2.6 - 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
BusyContacts 1.4.0 - 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
Chromium 77.0.3865.75 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 77.0.3865.75: A list of changes is available... Read more

Latest Forum Discussions

See All

Yoozoo Games launches Saint Seiya Awaken...
If you’re into your anime, you’ve probably seen or heard of Saint Seiya. Based on a shonen manga by Masami Kurumada, the series was massively popular in the 1980s – especially in its native Japan. Since then, it’s grown into a franchise of all... | Read more »
Five Nights at Freddy's AR: Special...
Five Nights at Freddy's AR: Special Delivery is a terrifying new nightmare from developer Illumix. Last week, FNAF fans were sent into a frenzy by a short teaser for what we now know to be Special Delivery. Those in the comments were quick to... | Read more »
Rush Rally 3's new live events are...
Last week, Rush Rally 3 got updated with live events, and it’s one of the best things to happen to racing games on mobile. Prior to this update, the game already had multiplayer, but live events are more convenient in the sense that it’s somewhat... | Read more »
Why your free-to-play racer sucks
It’s been this way for a while now, but playing Hot Wheels Infinite Loop really highlights a big issue with free-to-play mobile racing games: They suck. It doesn’t matter if you’re trying going for realism, cart racing, or arcade nonsense, they’re... | Read more »
Steam Link Spotlight - The Banner Saga 3
Steam Link Spotlight is a new feature where we take a look at PC games that play exceptionally well using the Steam Link app. Our last entry talked about Terry Cavanaugh’s incredible Dicey Dungeons. Read about how it’s a great mobile experience... | Read more »
PSA: GRIS has some issues
You may or may not have seen that Devolver Digital just released GRIS on the App Store, but we wanted to do a quick public service announcement to say that you might not want to hop on buying it just yet. The puzzle platformer has come to small... | Read more »
Combo Quest (Games)
Combo Quest 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Combo Quest is an epic, time tap role-playing adventure. In this unique masterpiece, you are a knight on a heroic quest to retrieve... | Read more »
Hero Emblems (Games)
Hero Emblems 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: ** 25% OFF for a limited time to celebrate the release ** ** Note for iPhone 6 user: If it doesn't run fullscreen on your device... | Read more »
Puzzle Blitz (Games)
Puzzle Blitz 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Puzzle Blitz is a frantic puzzle solving race against the clock! Solve as many puzzles as you can, before time runs out! You have... | Read more »
Sky Patrol (Games)
Sky Patrol 1.0.1 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.1 (iTunes) Description: 'Strategic Twist On The Classic Shooter Genre' - Indie Game Mag... | Read more »

Price Scanner via MacPrices.net

Preorder your Apple Watch Series 5 today at A...
Amazon has Apple Watch Series 5 GPS models available for preorder and on sale today for $15 off Apple’s MSRP. Shipping is free and starts on September 20th: – 40mm Apple Watch Series 5 GPS: $384.99 $... Read more
21″ iMacs on sale for $100 off Apple’s MSRP,...
B&H Photo has new 21″ Apple iMacs on sale for $100 off MSRP with models available starting at $999. These are the same iMacs offered by Apple in their retail and online stores. Overnight shipping... Read more
2018 4 and 6-Core Mac minis on sale today for...
Apple resellers are offering new 2018 4-Core and 6-Core Mac minis for $100-$150 off MSRP for a limited time. B&H Photo has the new 2018 4-Core and 6-Core Mac minis on sale for up to $150 off... Read more
Save $150-$250 on 10.2″ WiFi + Cellular iPads...
Verizon is offering $150-$250 discounts on Apple’s new 10.2″ WiFi + Cellular iPad with service. Buy the iPad itself and save $150. Save $250 on the purchase of an iPad along with an iPhone. The fine... Read more
Apple continues to offer 13″ 2.3GHz Dual-Core...
Apple has Certified Refurbished 2017 13″ 2.3GHz Dual-Core non-Touch Bar MacBook Pros available starting at $1019. An standard Apple one-year warranty is included with each model, outer cases are new... Read more
Apple restocks 2018 MacBook Airs, Certified R...
Apple has restocked Certified Refurbished 2018 13″ MacBook Airs starting at only $849. Each MacBook features a new outer case, comes with a standard Apple one-year warranty, and is shipped free. The... Read more
Sunday Sale! 2019 27″ 5K 6-Core iMacs for $20...
B&H Photo has the new 2019 27″ 5K 6-Core iMacs on stock today and on sale for up to $250 off Apple’s MSRP. Overnight shipping is free to many locations in the US. These are the same iMacs sold by... Read more
Weekend Sale! 2019 13″ MacBook Airs for $200...
Amazon has new 2019 13″ MacBook Airs on sale for $200 off Apple’s MSRP, with prices starting at $899, each including free shipping. Be sure to select Amazon as the seller during checkout, rather than... Read more
2019 15″ MacBook Pros now on sale for $350-$4...
B&H Photo has Apple’s 2019 15″ 6-Core and 8-Core MacBook Pros on sale today for $350-$400 off MSRP, starting at $2049, with free overnight shipping available to many addresses in the US: – 2019... Read more
Buy one Apple Watch Series 5 at Verizon, get...
Buy one Apple Watch Series 5 at Verizon, and get a second Watch for 50% off. Plus save $10 on your first month of service. The fine print: “Buy Apple Watch, get another up to 50% off on us. Plus $10... Read more

Jobs Board

*Apple* Mobility Pro-Store 149 - Best Buy (U...
**731985BR** **Job Title:** Apple Mobility Pro-Store 149 **Job Category:** Store Associates **Location Number:** 000149-Towson-Store **Job Description:** At Best Read more
Student Employment (Blue *Apple* Cafe) Spri...
Student Employment (Blue Apple Cafe) Spring 2019 Penn State University Campus/Location: Penn State Brandywine Campus City: Media, PA Date Announced: 12/20/2018 Date Read more
Windows/ *Apple* Technical Support Engineer...
Windows/ Apple Technical Support Engineer McLean , VA , US Apply + Be you + Be Booz Allen + Be empowered + Learn More Job Description Location: McLean, VA, US Job Read more
*Apple* Mobile Master - Best Buy (United Sta...
**725617BR** **Job Title:** Apple Mobile Master **Job Category:** Store Associates **Location Number:** 001095-Chesterfield-Store **Job Description:** **What does a Read more
Geek Squad *Apple* Master Consultation Agen...
**732415BR** **Job Title:** Geek Squad Apple Master Consultation Agent **Job Category:** Services/Installation/Repair **Location Number:** 000425-Hickory-Store **Job Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.