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
$100.95
Apple Inc.
-0.01
MSFT
$47.02
Microsoft Corpora
-0.50
GOOG
$587.45
Google Inc.
-8.63

MacTech Search:
Community Search:

Software Updates via MacUpdate

Monosnap 2.2.2 - Versatile screenshot ut...
Monosnap allows you to save screenshots easily, conveniently, and quickly, sharing them with friends and colleagues at once. It's the ideal choice for anyone who is looking for a smart and fast... Read more
Tunnelblick 3.4beta36 - GUI for OpenVPN...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
SoftRAID 5.0.4 - High-quality RAID manag...
SoftRAID allows you to create and manage disk arrays to increase performance and reliability. SoftRAID's intuitive interface and powerful feature set makes this utility a must have for any Mac OS X... Read more
Audio Hijack Pro 2.11.3 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Airfoil 4.8.9 - 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
WhatRoute 1.13.0 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the... Read more
Chromium 37.0.2062.122 - 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. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Attachment Tamer 3.1.14b9 - Take control...
Attachment Tamer gives you control over attachment handling in Apple Mail. It fixes the most annoying Apple Mail flaws, ensures compatibility with other email software, and allows you to set up how... Read more
Duplicate Annihilator 5.0 - Find and del...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
jAlbum Pro 12.2 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more

Latest Forum Discussions

See All

Kitty Powers’ Matchmaker – Tips, Tricks,...
Hey There, Kittens: | Read more »
Major New Update for CSR Racing Adds Fer...
Major New Update for CSR Racing Adds Ferrari and Multiplaye​r Posted by Jessica Fisher on September 22nd, 2014 [ permalink ] | Read more »
Fruit Ninja Will be Reborn With a Massiv...
Fruit Ninja Will be Reborn With a Massive Update and Origins Animation Series Posted by Jessica Fisher on September 22nd, 2014 [ permalink ] Halfbrick Studios is rebuilding | Read more »
CloudMagic Updated for iOS 8 – Adds Inte...
CloudMagic Updated for iOS 8 – Adds Interactive Notifications, Share Extension, and More Posted by Jessica Fisher on September 22nd, 2014 [ | Read more »
Starbase Annex (Games)
Starbase Annex 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: "it’s really very clever... a little bit of Hearthstone and a dash of Eclipse" - PocketTactics.com From the creator of Starbase... | Read more »
Star Nomad Review
Star Nomad Review By Jordan Minor on September 22nd, 2014 Our Rating: :: BUMPY RIDEiPad Only App - Designed for the iPad Star Nomad will only appeal to the most hardcore space explorers.   | Read more »
Partyrs Review
Partyrs Review By Jordan Minor on September 22nd, 2014 Our Rating: :: LIFE OF THE PARTYUniversal App - Designed for iPhone and iPad Partyrs makes party planning as fun as partying itself.   | Read more »
View Source – HTML, JavaScript and CSS...
View Source – HTML, JavaScript and CSS 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: View Source is an app plus an iOS 8 Safari extension that makes it easy to do one key web developer... | Read more »
Avenged Sevenfold’s Hail To The King: De...
Avenged Sevenfold’s Hail To The King: Deathbat is Coming to iOS on October 16th Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Just in time for Halloween, on October 16 Avenged Sevenfold will be launching | Read more »
Talisman Has Gone Universal – Can Now be...
Talisman Has Gone Universal – Can Now be Played on the iPhone Posted by Jessica Fisher on September 19th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Apple restocks some refurbished 2014 MacBook...
The Apple Store has restocked some Apple Certified Refurbished 2014 MacBook Airs, with prices starting at $769. An Apple one-year warranty is included with each MacBook, and shipping is free. These... Read more
13-inch 128GB MacBook Air on sale for $949, s...
B&H Photo has the new 2014 13″ 1.4GHz/128GB MacBook Air on sale for $949.99 including free shipping plus NY tax only. Their price is $50 off MSRP. B&H will also include free copies of... Read more
Apple offering free $25 iTunes Gift Card with...
The Apple Store is offering a free $25 iTunes Gift Card with the purchase of a $99 Apple TV for a limited time. Shipping is free. Read more
Apple refurbished iPod touch available for up...
The Apple Store has Apple Certified Refurbished 5th generation iPod touches available starting at $149. Apple’s one-year warranty is included with each model, and shipping is free. Most colors are... Read more
iFixIt Tears Down iPhone 6; Awards Respectabl...
iFixit notes that even the smaller 4.7″ iPhone 6 is a giant among iPhones; so big that Apple couldn’t fit it into the familiar iPhone form factor. In a welcome reversal of a recent trend to more or... Read more
Phone 6 Guide – Tips Book For Both iPhone 6...
iOS Guides has announced its latest eBook: iPhone 6 Guide. Brought to you by the expert team at iOS Guides, and written by best-selling technology author Tom Rudderham, iPhone 6 Guide is packed with... Read more
How to Upgrade iPhone iPad to iOS 8 without D...
PhoneClean, a iPhone cleaner utility offered by iMobie Inc., reveals a solution for upgrading iPhone and iPad to iOS 8 without deleting photos, apps, the new U2 album or anything. Thanks to more than... Read more
Inpaint 6 – Photo Retouching Tool Gets Faster...
TeoreX has announced Inpaint 6, a simple retouching tool for end users that helps remove scratches, watermarks, and timestamps as well as more complex objects like strangers, unwanted elements and... Read more
Worldwide PC Monitor Market Sees Growth in To...
Worldwide PC monitor shipments totaled 32.5 million units in the second quarter of 2014 (2Q14), a year-over-year decline of -2.9%, according to the International Data Corporation (IDC) Worldwide... Read more
Updated Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more

Jobs Board

Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of 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
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.