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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Dropbox 22.4.24 - Cloud backup and synch...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more
Posterino 3.3.5 - Create posters, collag...
Posterino offers enhanced customization and flexibility including a variety of new, stylish templates featuring grids of identical or odd-sized image boxes. You can customize the size and shape of... Read more
Kodi 17.1. - Powerful media center tool...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Kodi 17.1. - Powerful media center tool...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Bookends 12.8 - Reference management and...
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
Apple iTunes 12.6 - Play Apple Music and...
Apple iTunes lets you organize and stream Apple Music, download and watch video and listen to Podcasts. It can automatically download new music, app, and book purchases across all your devices and... Read more
Default Folder X 5.1.4 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more
Amazon Chime 4.1.5587 - Amazon-based com...
Amazon Chime is a communications service that transforms online meetings with a secure, easy-to-use application that you can trust. Amazon Chime works seamlessly across your devices so that you can... Read more
CrossOver 16.2 - Run Windows apps on you...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Adobe Creative Cloud 4.0.0.185 - Access...
Adobe Creative Cloud costs $19.99/month for a single app, or $49.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more

The best deals on the App Store this wee...
Deals, deals, deals. We're all about a good bargain here on 148Apps, and luckily this was another fine week in App Store discounts. There's a big board game sale happening right now, and a few fine indies are still discounted through the weekend.... | Read more »
The best new games we played this week
It's been quite the week, but now that all of that business is out of the way, it's time to hunker down with some of the excellent games that were released over the past few days. There's a fair few to help you relax in your down time or if you're... | Read more »
Orphan Black: The Game (Games)
Orphan Black: The Game 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dive into a dark and twisted puzzle-adventure that retells the pivotal events of Orphan Black. | Read more »
The Elder Scrolls: Legends is now availa...
| Read more »
Ticket to Earth beginner's guide: H...
Robot Circus launched Ticket to Earth as part of the App Store's indie games event last week. If you're not quite digging the space operatics Mass Effect: Andromeda is serving up, you'll be pleased to know that there's a surprising alternative on... | Read more »
Leap to victory in Nexx Studios new plat...
You’re always a hop, skip, and a jump away from a fiery death in Temple Jump, a new platformer-cum-endless runner from Nexx Studio. It’s out now on both iOS and Android if you’re an adventurer seeking treasure in a crumbling, pixel-laden temple. | Read more »
Failbetter Games details changes coming...
Sunless Sea, Failbetter Games' dark and gloomy sea explorer, sets sail for the iPad tomorrow. Ahead of the game's launch, Failbetter took to Twitter to discuss what will be different in the mobile version of the game. Many of the changes make... | Read more »
Splish, splash! The Pokémon GO Water Fes...
Niantic is back with a new festival for dedicated Pokémon GO collectors. The Water Festival officially kicks off today at 1 P.M. PDT and runs through March 29. Magikarp, Squirtle, Totodile, and their assorted evolved forms will be appearing at... | Read more »
Death Road to Canada (Games)
Death Road to Canada 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: Get it now at the low launch price! Price will go up a dollar every major update. Update news at the bottom of this... | Read more »
Bean's Quest Beginner's Guide:...
Bean's Quest is a new take on both the classic platformer and the endless runner, and it's free on the App Store for the time being. Instead of running constantly, you can't stop jumping. That adds a surprising new level of challenge to the game... | Read more »

Price Scanner via MacPrices.net

Apple Chip Foundry TSMC To Begin A11 System-o...
Digitimes’ Steve Shen is reporting today that according to the Chinese-language Economic Daily News (EDN), chipmaker and major Apple supplier foundery Taiwan Semiconductor Manufacturing Company (TSMC... Read more
MacX MediaTrans 3.5 iOS Data Transfer Spring...
MacXDVD Software has announced general availability of the latest MacX MedTrans 3.5, featuring a new user interface (UI). MacX MediaTrans is ann iPhone manager that enables free data transfer between... Read more
Regular Price $19.95 DupeZap 4 Finder For OS...
Hyperbolic Software has announced the release of DupeZap 4.0.2, their modern duplicate finder developed exclusively for macOS. DupeZap 4 is an utility for Mac owners seeking to reclaim disk space... Read more
B-Eng Releases SSD Health Check for MVNe for...
Fehraltorf, Switzerland based B-Eng has announced the release and immediate availability of SSD Health Check for MVNe for MacBook Pro, their app that delivers important data and insights for MVNe... Read more
Apple iMacs on sale for up to $200 off MSRP,...
B&H Photo has 21″ and 27″ Apple iMacs on sale for up to $200 off MSRP, each including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2099 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac... Read more
Apple Certified Refurbished iMacs available f...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
1.4GHz Mac mini on sale for $419, $80 off MSR...
B&H Photo has the 1.4GHz Mac mini on sale for $80 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $419.88 $80 off MSRP Read more
Apple refurbished Mac minis available for up...
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
Updated iPad Price Trackers
Scan our Apple iPad (and iPod touch) Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the... Read more
12-inch 32GB Space Gray iPad Pro on sale for...
B&H Photo has 12″ Space Gray 32GB WiFi Apple iPad Pros on sale for $50 off MSRP including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50 off... Read more

Jobs Board

Fulltime aan de slag als shopmanager in een h...
Ben jij helemaal gek van Apple -producten en vind je het helemaal super om fulltime shopmanager te zijn in een jonge en hippe elektronicazaak? Wil jij werken in Read more
*Apple* Mobile Master - Best Buy (United Sta...
**492889BR** **Job Title:** Apple Mobile Master **Location Number:** 000886-Norwalk-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** Read more
*Apple* Mobile Master - Best Buy (United Sta...
**492472BR** **Job Title:** Apple Mobile Master **Location Number:** 000470-Seattle-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** Read more
*Apple* Mobile Master - Best Buy (United Sta...
**492562BR** **Job Title:** Apple Mobile Master **Location Number:** 000853-Jackson-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.