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

Google Earth 7.1.7.2602 - View and contr...
Google Earth gives you a wealth of imagery and geographic information. Explore destinations like Maui and Paris, or browse content from Wikipedia, National Geographic, and more. Google Earth combines... Read more
ClamXav 2.10 - Virus checker based on Cl...
ClamXav is a popular virus checker for OS X. I have been working on ClamXav for more than 10 years now, and over those years, I have invested a huge amount of my own time and energy into bringing... Read more
Tweetbot 2.4.4 - Popular Twitter client.
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Sierra Cache Cleaner 11.0.1 - Clear cach...
Sierra Cache Cleaner is an award-winning general purpose tool for macOS X. SCC makes system maintenance simple with an easy point-and-click interface to many macOS X functions. Novice and expert... Read more
Things 2.8.8 - Elegant personal task man...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
Remotix 4.1 - Access all your computers...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more
Airfoil 5.1.2 - 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
Firefox 49.0.1 - Fast, safe Web browser.
Firefox 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 and casual... Read more
Default Folder X 5.0.7 - 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
Safari Technology Preview 10.1 - The new...
Safari Technology Preview contains the most recent additions and improvements to WebKit and the latest advances in Safari web technologies. And once installed, you will receive notifications of... Read more

3 apps to boost your focus
As someone who works from home, my workspace is a minefield of distraction. Cats, tasty snacks, the wind blowing past my window, that cleaning that I suddenly can’t put off any longer. If I let distraction takes its course, I find that soon half... | Read more »
4 games like Burly Men at Sea to inspire...
Burly Men at Sea is out today and it looks a treat. It tells the tale of three Scandinavian fishermen who leave the humdrum of their daily lives to go exploring. It’s a beautiful folksy story that unfurls as you interact with the environment... | Read more »
3 reasons you need to play Kingdom: New...
Developed by a tag team of indie developers - Thomas "Noio" van den Berg and Marco "Licorice" Bancale - Kingdom is a vibrant medieval fantasy adventure that casts players as a king or queen who must expand their empire by exploring the vasts lands... | Read more »
JoyCity have launched a brand new King o...
Great news for all of you Game of Dice fans out there - JoyCity have just released a brand new limited edition pack with a really cool twist. The premise of Game of Dice is fairly straightforward, asking you to roll dice to navigate your way around... | Read more »
Burly Men at Sea (Games)
Burly Men at Sea 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Burly Men at Sea is a folktale about a trio of large, bearded fishermen who step away from the ordinary to seek adventure. | Read more »
3 tips for catching the gnarliest waves...
Like a wave breaking on the shore, Tidal Rider swept its way onto the App Store charts this week settling firmly in the top 10. It’s a one-touch high score-chaser in which you pull surfing stunts while dodging seagulls and collecting coins. The... | Read more »
The beginner's guide to destroying...
Age of Heroes: Conquest is 5th Planet Games’ all new turn-based multiplayer RPG, full of fantasy exploration, guild building, and treasure hunting. It’s pretty user-friendly as far as these games go, but when you really get down to it, you’ll find... | Read more »
Infinite Tanks (Games)
Infinite Tanks 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: | Read more »
Agatha Christie - The ABC Murders (FULL)...
Agatha Christie - The ABC Murders (FULL) 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: Agatha Christie: The ABC Murders Your weapon is your knowledge. Your wits will be put to the ultimate... | Read more »
HeadlessD (Games)
HeadlessD 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: HeadlessD is hand-painted dungeon crawler with intuitive touch controls and NO in-app purchases. | Read more »

Price Scanner via MacPrices.net

Apple refurbished 2015 13-inch MacBook Airs a...
Apple has Certified Refurbished 2015 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2015 13″ 1.6GHz/4GB/128GB MacBook... Read more
MacBook Airs on sale for up to $100 off MSRP
B&H Photo has 13″ and 11″ MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $799 $100 MSRP - 11″ 1.6GHz/256GB... Read more
Apple refurbished 12-inch 128GB iPad Pros ava...
Apple has Certified Refurbished 12″ Apple iPad Pros available for up to $160 off the cost of new iPads. An Apple one-year warranty is included with each model, and shipping is free: - 32GB 12″ iPad... Read more
Phone2Action Unveils New Voter Turnout Techno...
Phone2Action, a leading digital advocacy platform, today launched its Tech to Vote Civic Action Center digital advocacy and communications platform on National Voter Registration Day September 27.... Read more
Apple & Deloitte Team Up to Help Business...
Apple and international professional services firm Deloitte have announced a partnership to help companies quickly and easily transform their workflow dynamics by maximizing the power, ease-of-use,... Read more
Chop Commute – See Traffic and Drive Times on...
Shrewsbury, Massachusetts based Indie developer, InchWest has released Chop Commute 1.61, a Mac app that takes the guesswork out of daily commute by showing real-time traffic and drive times right on... Read more
12-inch 32GB WiFi iPad Pros on sale for $50 o...
B&H Photo has 12″ 32GB WiFi Apple iPad Pros on sale for $50 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50 off MSRP -... Read more
Recent price drops on refurbished iPad minis...
Apple recently dropped prices on several Certified Refurbished iPad mini 4s and 2s as well as iPad Air 2s. An Apple one-year warranty is included with each model, and shipping is free: - 16GB iPad... Read more
Apple refurbished Mac minis available startin...
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
13-inch 2.5GHz MacBook Pro available for $928...
Overstock has the 13″ 2.5GHz MacBook Pro available for $927.99 including free shipping. Their price is $171 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
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
*Apple* Wireless Lead - T-ROC - The Retail O...
…wealth of knowledge in wireless sales and activations to the Beautiful and NEW APPLE Experience store within MACYS.. THIS role, APPLE Wireless Lead, isbrandnewas Read more
Lead *Apple* Advocate - T-ROC - The Retail...
…Company, is proud of its unprecedented relationship with our partner and client, APPLE ,in bringing amazing" APPLE ADVOCATES"to "non" Apple store locations. Read more
*Apple* Advocate - T-ROC - The Retail Outsou...
…Company, is proud of its unprecedented relationship with our partner and client, APPLE ,in bringing amazing" APPLE ADVOCATES"to "non" Apple store locations. Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.