Sprite Kit Tutorial: Making a Universal App: Part 2

Learn how to make a universal app that works on the iPhone, iPad, and retina display in this Sprite Kit tutorial! By Nicholas Waynik.

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Whack that laugh off this mole's face!

Whack that laugh off this mole's face!

Note from Ray: Tutorial Team member Nick Waynik has ported this tutorial from Cocos2D to Sprite Kit as part of the iOS 7 Feast. We hope you enjoy!

This article is the second part of a two-part series on how to create a mole whacking game with Sprite Kit. This series brings together a lot of concepts from other Sprite Kit tutorials on this site, and introduces some new concepts along the way as well.

In the first part of the series, we created the basics of the game – cute little moles popping out of holes. We spent a lot of time thinking about how to organize the art and coordinates so that the game would look good on the iPhone 3.5-inch, iPhone 4-inch, iPad, and iPad Retina – and be efficient too!

In this article, we’ll add some cute animations to the mole as he laughs and gets whacked, add gameplay so you can do the whacking and earn points, and of course add some gratuitous sound effects as usual.

If you don’t have it already, grab a copy of the project where we left things off in the last Sprite Kit tutorial.

Defining Animations: Practicalities

To make the game a little more fun, we’re going to give the mole two animations. First, he’ll laugh a little when he pops out of the hole (to make you really want to whack him!), then if you do manage to whack him he’ll make a “just got whacked” face.

But before we begin, let’s talk about the practicalities of defining our animations in code.

Our mole’s laugh animation is going to be these images in this order: mole_laugh1.png, mole_laugh2.png mole_laugh3.png, mole_laugh2.png, mole_laugh3.png, mole_laugh1.png.

So we could hard-code a bunch of lines to set up our animation, like this:

[animFrames addObject:
    [SKTexture textureWithImageNamed:@"mole_laugh1.png"]];
[animFrames addObject:
    [SKTexture textureWithImageNamed:@"mole_laugh2.png"]];
[animFrames addObject:
    [SKTexture textureWithImageNamed:@"mole_laugh3.png"]];
[animFrames addObject:
    [SKTexture textureWithImageNamed:@"mole_laugh2.png"]];
// And so on...

But that would make our code kind of ugly. To make things a bit cleaner, instead of defining the images in the animation in code, we’ll bring them out to a property list instead.

Property Lists

If you haven’t used property lists before, they are special files you can create in XCode to contain data like arrays, dictionaries, strings, numbers, and so on in a hierarchial format. It’s extremely easy to create these, and just as easy to read them from code.

Let’s see what I mean by trying this out in XCode. Right click on WhackAMole, choose “New File…”, choose “iOS\Resource\Property List”, and click “Next”. Name the new file “laughAnim.plist”, and click Create. At this point the property list editor for laughAnim.plist should be visible, as shown below:

XCode's Property List Editor

Every property list has a root element. This is usually either an array or a dictionary. This property list is going to contain an array of image names that make up the laugh animation, so click on the second column for the root element (Type, currently set to Dictionary), and change it to Array.

Next, click the small plus sign button to the right of the word Root – this adds a new entry to the array. By default, the type of the entry is a String – which is exactly what we want. Change the value to “mole_laugh1.png” for the first entry in the animation.

Click the + button to add a new row, and repeat to add all of the frames of the animation, as shown below:

Setting up Laugh Animation in Property List Editor

Next, repeat the process for the animation to play when the mole is hit. Follow the same steps as above to create a new property list named hitAnim.plist, and set it up as shown below:

Setting up Hit Animation in Property List Editor

Now, time to add the code to load these animations. Start by opening up MyScene.h and add property for each animation action, as shown below:

// Inside @interface MyScene
@property (strong, nonatomic) SKAction *laughAnimation;
@property (strong, nonatomic) SKAction *hitAnimation;

These will be used to keep a handy reference to each SKAction so it can be easily found and reused in the code.

Next add a method to create a SKAction based on the images defined in the property list, as follow:

- (SKAction *)animationFromPlist:(NSString *)animPlist
    NSString *plistPath = [[NSBundle mainBundle] pathForResource:animPlist ofType:@"plist"]; // 1
    NSArray *animImages = [NSArray arrayWithContentsOfFile:plistPath]; // 2
    NSMutableArray *animFrames = [NSMutableArray array]; // 3
    for (NSString *imageName in animImages) { // 4
        [animFrames addObject:[SKTexture textureWithImageNamed:imageName]]; // 5
    float framesOverOneSecond = 1.0f/(float)[animFrames count];
    return [SKAction animateWithTextures:animFrames timePerFrame:framesOverOneSecond resize:NO restore:YES]; // 6

This is important to understand, so let’s go through it line by line.

  1. The property list is included in the project file, so it’s in the app’s “main bundle”. This helper method gives a full path to a file in the main bundle, which you’ll need to read in the property list.
  2. To read a property list, it’s as easy as calling a method on NSArray called arrayWithContentsOfFile and passing in the path to the property list. It will return an NSArray with the contents (a list of strings for the image names in the animation, in this case). Note this works because we set the root element to be an NSArray. If we had set it to a NSDictionary, we could use [NSDictionary dictionaryWithContentsOfFile…] instead.
  3. Creates an empty array that will store the animation frames.
  4. Loops through each image name in the array read from the property list.
  5. Gets the texture for each image and adds it to the array.
  6. Returns a SKAction based on the array of textures.

Next, add the code to the end of your init method to call this helper function for each animation:

self.laughAnimation = [self animationFromPlist:@"laughAnim"];
self.hitAnimation = [self animationFromPlist:@"hitAnim"];

One last step – let’s use the animations (just the laugh one for now). Modify the popMole method to read as the following:

- (void)popMole:(SKSpriteNode *)mole
    SKAction *easeMoveUp = [SKAction moveToY:mole.position.y + mole.size.height duration:0.2f];
	easeMoveUp.timingMode = SKActionTimingEaseInEaseOut;
	SKAction *easeMoveDown = [SKAction moveToY:mole.position.y duration:0.2f];
	easeMoveDown.timingMode = SKActionTimingEaseInEaseOut;
    SKAction *sequence = [SKAction sequence:@[easeMoveUp, self.laughAnimation, easeMoveDown]];
    [mole runAction:sequence];

The only difference here is that instead of delaying a second before popping down, it runs the laughAnimation action instead. The laughAnimation action uses textures from the laughAnim.plist, and sets restore to YES so that when the animation is done, it reverts back to the normal mole face.

Compile and run your code, and now when the moles pop out, they laugh at you!

Mole with Laugh Animation

Time to wipe that smile off their faces and start whacking!

Nicholas Waynik


Nicholas Waynik


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