TweetFollow Us on Twitter

Screen Savers in Cocoa

Volume Number: 20 (2004)
Issue Number: 6
Column Tag: Programming

Screen Savers in Cocoa

by Scott Knaster

One of the classic mantra-like goals for computer science over the past 20 years or so has been to "Make simple things simple, and complex things possible". Programming with Cocoa has a sometimes-complex learning curve, but once you've swerved through that curve, there are definitely a bunch of things that are much easier to accomplish than they were in the old pre-OS X world we knew and occasionally loved.

One of the classic mantra-like goals for computer science over the past 20 years or so has been to "Make simple things simple, and complex things possible". Programming with Cocoa has a sometimes-complex learning curve, but once you've swerved through that curve, there are definitely a bunch of things that are much easier to accomplish than they were in the old pre-OS X world we knew and occasionally loved. Writing a screen saver is a perfect example: it should be simple. Most typical OS 9 application programmers never dipped their toes into the slightly wacky world of screen savers, but with Cocoa in OS X, implementing a screen saver is well within everybody's grasp. In this month's column, we'll take a look at how to get your very own screen saver up and, er, saving.

Here's What We're Gonna Do

Let's start by taking a look at the process for creating a screen saver in OS X. Here are the broad steps:

  • Create a new screen saver project in Xcode.
  • Edit our .h file.
  • Override methods and write other code in our .m file.
  • Build the project to create a .saver package.
  • Install the .saver package by putting it into the Library/Screen Savers/ folder.
  • Open System Preferences and see a preview of our screen saver.
  • Enjoy the savings!

We'll go through each of these steps in greater depth now.

Little Help

The basic magic that makes screen savers so easy is the Screen Saver framework in Cocoa. This framework defines the ScreenSaverView class, which is a subclass of NSView. By creating your own subclass of ScreenSaverView and adding some code, you define your screen saver. The Screen Saver framework also defines the class ScreenSaverDefaults, which you can use for handling preferences for your saver. Along with these classes, the framework provides some handy utility functions you can use in your code.

We'll go over some of the most interesting methods and functions in the ScreenSaverView class. You will rarely call methods defined by ScreenSaverView - most of the work is creating your own subclass and override some methods.

initWithFrame:isPreview:

- (id)initWithFrame:(NSRect)frame isPreview:(BOOL)isPreview

You override initWithFrame in your ScreenSaverView subclass. The system calls initWithFrame when the screen saver is about take over the screen or is selected in System Preferences. The frame parameter is the frame rectangle for the view. The isPreview parameter tells whether the screen saver is actually being invoked or is merely being asked to preview itself in System Preferences (as shown in Figure 1).


Figure 1. You can preview the screen saver in a little box in System Preferences. In your code, you can tell whether the screen saver is drawing full-screen or in the preview box.

startAnimation

- (void) startAnimation

The system calls startAnimation right before the screen saver is about to start drawing. You should override startAnimation to set up your screen saver's initial state, such as setting line widths or loading images. You should call the inherited implementation, or bad things might happen, such as incorrect drawing, or all water on earth instantaneously evaporating.

animateOneFrame

- (void) animateOneFrame

This is where your screen saver gets to show off its amazing graphical skills. Mac OS X asks your screen saver to do its drawing by calling animateOneFrame repeatedly. You can actually do your drawing in animateOneFrame or in drawRect, or even a little in both places. If you make any changes here that require further redrawing, your implementation should call setNeedsDisplay:YES, which will cause drawRect to be called.

drawRect

- (void) drawRect:(NSRect)rect 

Override drawRect to draw the screen saver view. You can do your drawing in animateOneFrame, or you can do some or all of it here. rect is the rectangle you're drawing into, which is handy to have when you want to erase the view and start drawing afresh.

stopAnimation

- (void) stopAnimation 

When Mac OS X wants your screen saver to stop doing its thing, it calls stopAnimation. You can override stopAnimation to release resources or do any other cleanup you want before your screen saver goes away.

Saving Time

Now that you're familiar with the cast of characters in ScreenSaverView, let's go ahead and code up our screen saver. To start, we'll open Xcode and create a new project of type Screen Saver (see Figure 2).


Figure 2. Creating a new Screen Saver project gets you started with the Screen Saver framework, including your own subclass of ScreenSaverView.

This proves that Xcode already knows about the screen saver framework, which saves us plenty of work. Our new project already contains a subclass of ScreenSaverView, and we already have the usual .h and .m files. We'll edit the .h file that Xcode gives us until it looks like this:

#import <ScreenSaver/ScreenSaver.h>


@interface SaveyerView : ScreenSaverView 
{
   NSBezierPath *path;
}


@end

The header file is pretty darn basic. All we do here is create a subclass of ScreenSaverView and add an NSBezierPath object to keep track of what we're drawing.

Now let's get into the implementation files and see what we can find. When we told Xcode to create a new ScreenSaver project, it start us off with some code, including the implementation for initWithFrame:isPreview:, the designated initializer. In this case, we're able to use the supplied code for initWithFrame without any changes:

- (id)initWithFrame:(NSRect)frame isPreview:(BOOL)isPreview
{
    self = [super initWithFrame:frame isPreview:isPreview];
    if (self) {
        [self setAnimationTimeInterval:1/30.0];
            // Draw 30 frames per second
    }
    return self;

The code here starts by calling the inherited implementation. After that, we use setAnimationTimeInterval to tell Mac OS X that we want our screen saver to draw 30 frames per second. As I mentioned, this is the default code that Xcode writes for this method. You can modify it if you want to perform some other task when the screen saver starts up. For example, if your screen saver has user-settable options, you can handle them here.

Next, we'll take a look at our startAnimation method, which the system calls right before asking our screen saver to start drawing. Our implementation of startAnimation begins by calling the inherited implementation. Then, we create our Bezier path and choose a nifty line join style:

- (void)startAnimation
{
   NSPoint x;

   [super startAnimation];
	
   path = [[NSBezierPath alloc] init];
      // We'll use a Bezier path for drawing

   [path setLineJoinStyle: NSRoundLineJoinStyle];	
      // Just for fun, connect the lines with
      // a round joint

When the system asks our screen saver to get ready to draw, we can call the view's isPreview method to see if we're being asked to draw on the full screen or in the little preview box in System Preferences (as shown back in Figure 1).

We can use the result of isPreview to make decisions about just what to draw. In our screen saver, we'll make the lines skinny for the preview, and fatter for the real, full-screen version:

   if ([self isPreview])
      // When drawing a preview, make the lines
      // much thinner than when saving screens.
   {
      [path setLineWidth: 0.0];
      // This is the thinnest possible line width
   }
   else
   {
      [path setLineWidth: 10.18];
      // This line width was chosen at random.
      // OK, actually, it's my son's birthdate.
   }

Our last task here is to get the Bezier path started. We'll do that by picking a random starting point and moving the path there:

   x = SSRandomPointForSizeWithinRect 
         (NSMakeSize (0,0), [self bounds]);
      // Call utility function to get a random point

   [path moveToPoint:x];
      // Start the path at the random point
}

We get a random point by calling SSRandomPointForSizeWithinRect, a handy function provided by the screen saver framework for just this purpose. Hooray for handy functions! Then, we simply move the path pen to that random point to start it out.

Everything that starts must end, and the next method we define is stopAnimation, which is called when the system doesn't need the screen saver to draw any more. Here's our implementation of stopAnimation:

- (void)stopAnimation
{
   [super stopAnimation];
	
   [path release];
         // Release the path

   path = nil;
         // Tell our screen saver view that there's no path
}

The standard stopAnimation provided by Xcode simply calls the inherited implementation. In our version, we keep that super call, and add code to release the Bezier path object and set the path instance variable to nil.

Every time the system wants our screen saver to draw another piece, it calls our animateOneFrame method. Let's take a look at that. First, we'll call that convenient SSRandomPointForSizeWithinRect utility function to get another random point:

- (void)animateOneFrame
{
    NSPoint x;
	
   x = SSRandomPointForSizeWithinRect 
         (NSMakeSize (0,0), [self bounds]);
            // Get a random point to extend the Bezier path

We want our screen saver to draw a bunch of lines on the screen, and every so often, we want it to erase the lines and start over. Let's say we want 50 lines at a time, in honor of our 50 states. If we haven't reached 50 yet, we add the new random point to the path:

if ([path elementCount] < 50)
      // Draw 50 lines before erasing
      {
         [path lineToPoint: x];
            // If we don't have 50 yet, add the 
            // new point to the line
      }

Once we have 50 points in the path, we want to reset the path by callously discarding all points and then start building it up again:

      else
      {
         [path removeAllPoints];
         [path moveToPoint:x];
            // If we do have 50, clean out the path
            // and get ready to start over
      }

We finish by telling the system that we've messed with the path and it needs to be redrawn by calling the screen saver view's drawRect method. Alternatively, we could do the actual drawing right here in animateOneFrame:

   [self setNeedsDisplay:YES];
      // Tell the system that something has changed
      // and drawRect should be called
}

The actual drawing happens in drawRect, which we'll look at NeXT. We start by calling the inherited drawRect, which by default erases the background to black.

- (void)drawRect:(NSRect)rect
{
   NSColor *color;

   [super drawRect:rect];

We then choose a pretty color, and call set to make sure that the drawing happens in that color. Then we call stroke on the Bezier path object to actually draw the thing:

   color = [NSColor colorWithCalibratedRed:(0.0) 
                  green:(1.0) blue:(1.0) alpha:(1.0)];

   [color set];
      // Set the color to teal. Go Sharks!
	
   [path stroke];
      // Draw the Bezier path 
}

The last method we implement is our version of dealloc. The view's Bezier path is the only allocated object we have to worry about, so our method looks like this:

- (void) dealloc
{
   [path release];
      // Release the Bezier path

   [super dealloc];
}

Put Me In, Coach

When we have all the source code done, we build our project. If everything builds OK, a file with the suffix .saver ends up in the project's build folder. To install the screen saver, start by quitting System Preferences if it's running. Then move or copy the .saver file into the /Library/Screen Savers directory. You can put it in ~/Library/Screen Savers if you want to keep it all to yourself and prevent other users from seeing it.

Once our screen saver is in the folder, you can start System Preferences, click Desktop & Screen Saver, click the Screen Saver tab, and select our screen saver in the list. You should see the skinny lines in the preview mode. Then click Test, and observe the big teal lines with their round elbows. There you go! You can get this month's code at http://www.papercar.com/mt/Jun04.zip

If you're interested in making your own screen savers, there are lots of directions you can go from here. Add user-settable options by overriding the hasConfigureSheet and configureSheet methods. Use random colors. Do some much fancier drawing in your animateOneFrame method - for example, draw shapes, use curveToPoint instead of lineToPoint, or load images from disk. Whatever you do, have fun, and remember: the screen you save may be your own.


Scott Knaster writes books, including the recently published Mac Toys and the brand-new Hacking iPod and iTunes, both from Wiley Publishing. Scott can't read and listen to vocal music at the same time. Scott writes these little bios in the third person. Write to Scott at scottk@mactech.com.

 
AAPL
$96.52
Apple Inc.
-0.67
MSFT
$44.60
Microsoft Corpora
-0.28
GOOG
$595.03
Google Inc.
-0.95

MacTech Search:
Community Search:

Software Updates via MacUpdate

Airfoil 4.8.7 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
Microsoft Remote Desktop 8.0.8 - Connect...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more
xACT 2.30 - Audio compression toolkit. (...
xACT stands for X Aaudio Compression Toolkit, an application that encodes and decodes FLAC, SHN, Monkey’s Audio, TTA, Wavpack, and Apple Lossless files. It also can encode these formats to MP3, AAC... Read more
Firefox 31.0 - Fast, safe Web browser. (...
Firefox for Mac offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals... Read more
Little Snitch 3.3.3 - Alerts you to outg...
Little Snitch gives you control over your private outgoing data. Track background activityAs soon as your computer connects to the Internet, applications often have permission to send any... Read more
Thunderbird 31.0 - Email client from Moz...
As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements... Read more
Together 3.2 - Store and organize all of...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Smart storage. With simple drag-and-drop functionality,... Read more
Cyberduck 4.5 - FTP and SFTP browser. (F...
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
iExplorer 3.4 - View and transfer all th...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
Airmail 1.4 - Powerful, minimal email cl...
Airmail is a powerful, minimal mail client.It was designed to retain the same experience with a single or multiple accounts and provide a quick, modern and easy-to-use user experience. Airmail... Read more

Latest Forum Discussions

See All

Revolution 60 Review
Revolution 60 Review By Jordan Minor on July 24th, 2014 Our Rating: :: LASS EFFECTUniversal App - Designed for iPhone and iPad Revolution 60 is a bold, cinematic action game with ambition to spare.   | Read more »
Matter (Photography)
Matter 1.0.1 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0.1 (iTunes) Description: Add stunning 3D effects to your photos with real-time shadows and reflections. Export your creations as photos or video loops... | Read more »
Fanatic Earth Review
Fanatic Earth Review By Brittany Vincent on July 24th, 2014 Our Rating: :: BY-THE-NUMBERSUniversal App - Designed for iPhone and iPad Kemco’s stable of mobile RPGs grows, but in Fanatic Earth’s situation it’s a case of quantity... | Read more »
Together for iOS (Productivity)
Together for iOS 1.0 Device: iOS Universal Category: Productivity Price: $9.99, Version: 1.0 (iTunes) Description: Together is an app for keeping things in one place. Notes, documents, images, movies, sounds, web pages and bookmarks... | Read more »
The Phantom PI Mission Apparition (Game...
The Phantom PI Mission Apparition 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: ** Release sale! 50% off for a limited time! ** The Phantom PI Mission Apparition is a spooky, puzzly, rock’... | Read more »
The Great Prank War (Games)
The Great Prank War 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: Help Mordecai, Rigby, Muscle Man and Skips take the park back from Gene and his goons with a plethora of prank-related... | Read more »
Teenage Mutant Ninja Turtles (Games)
Teenage Mutant Ninja Turtles 1.0.0 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.0 (iTunes) Description: Download the all new Teenage Mutant Ninja Turtles Official Movie Game! | Read more »
Dream Revenant (Games)
Dream Revenant 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: EXCLUSIVE LAUNCH PRICE ! Dream Revenant is at $1.99 for a limited time ! | Read more »
Traps n' Gemstones (Games)
Traps n' Gemstones 1.00 Device: iOS Universal Category: Games Price: $2.99, Version: 1.00 (iTunes) Description: LAUNCH SALE! 40% off, JULY ONLY! TRAPS N' GEMSTONES is an adventurous platform game, among gamers typically known as the... | Read more »
Soccer Physics (Games)
Soccer Physics 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: One-button soccer game! So dumb it's fun. "Soccer Physics is probably the funniest football game you'll play on iOS" —... | Read more »

Price Scanner via MacPrices.net

What Should Apple’s Next MacBook Priority Be;...
Stabley Times’ Phil Moore says that after expanding its iMac lineup with a new low end model, Apple’s next Mac hardware decision will be how it wants to approach expanding its MacBook lineup as well... Read more
ArtRage For iPhone Painting App Free During C...
ArtRage for iPhone is currently being offered for free (regularly $1.99) during Comic-Con San Diego #SDCC, July 24-27, in celebration of the upcoming ArtRage 4.5 and other 64-bit versions of the... Read more
With The Apple/IBM Alliance, Is The iPad Now...
Almost since the iPad was rolled out in 2010, and especially after Apple made a 128 GB storage configuration available in 2012, there’s been debate over whether the iPad is a serious tool for... Read more
MacBook Airs on sale starting at $799, free s...
B&H Photo has the new 2014 MacBook Airs on sale for up to $100 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only. They also include free copies of Parallels... Read more
Apple 27″ Thunderbolt Display (refurbished) a...
The Apple Store has Apple Certified Refurbished 27″ Thunderbolt Displays available for $799 including free shipping. That’s $200 off the cost of new models. Read more
WaterField Designs Unveils Cycling Ride Pouch...
High end computer case and bag maker WaterField Designs of San Francisco now enters the cycling market with the introduction of the Cycling Ride Pouch – an upscale toolkit with a scratch-free iPhone... Read more
Kingston Digital Ships Large Capacity Near 1T...
Kingston Digital, Inc., the Flash memory affiliate of Kingston Technology Company, Inc.,has announced its latest addition to the SSDNow V300 series, the V310. The Kingston SSDNow V310 solid-state... Read more
Apple’s Fiscal Third Quarter Results; Record...
Apple has announced financial results for its fiscal 2014 third quarter ended June 28, 2014, racking up quarterly revenue of $37.4 billion and quarterly net profit of $7.7 billion, or $1.28 per... Read more
15-inch 2.0GHz MacBook Pro Retina on sale for...
B&H Photo has the 15″ 2.0GHz Retina MacBook Pro on sale for $1829 including free shipping plus NY sales tax only. Their price is $170 off MSRP. B&H will also include free copies of Parallels... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished Mac minis for up to $150 off the cost of new models. Apple’s one-year warranty is included with each mini, and shipping is free: - 2.5GHz Mac... Read more

Jobs Board

Sr Software Lead Engineer, *Apple* Online S...
Sr Software Lead Engineer, Apple Online Store Publishing Systems Keywords: Company: Apple Job Code: E3PCAK8MgYYkw Location (City or ZIP): Santa Clara Status: Full Read more
Senior Interaction Designer, *Apple* Online...
**Job Summary** Apple is looking for a hands on Senior…will be a key player in designing for the Apple Online Store. The ideal designer will have a Read more
*Apple* Sales Chat Rep - Apple (United State...
…is looking for motivated, outgoing, and tech savvy individuals who want to offer Apple Customers an unparalleled customer experience over chat. At Apple , we believe Read more
Mac Expert - *Apple* Online Store Mexico -...
…MUST be fluent in English and Spanish to be considered for this position At Apple , we believe that hard work, a fun environment, creativity and innovation fuel the Read more
*Apple* Industrial Design CAD Sculptor - App...
**Job Summary** The Apple Industrial Design team is looking for a CAD sculptor/Digital 3D modeler to create high quality CAD models used in the industrial design process Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.