Star and text

Drawing the basic shape

The first step is to draw the ’star’ shape. I decided to create a re-usable path, although at the minute I’m actually only drawing it once, so the benefits are limited. Doing this involves using the CGPath... versions of the Core Graphics functions, that add themselves to a mutable path objects, rather than being drawn directly on the currently active graphics context.

    CGMutablePathRef r = CGPathCreateMutable();
    // Always have to move to a start point
    CGPathMoveToPoint(r, NULL, x1, y1); 
    CGPathAddLineToPoint(r, NULL, x2, y2);
    CGPathCloseSubpath(r);

Once you’ve got created the path you can merrily set various stroke and fill colours and draw it over-and-over to your hearts content.

The star drawing is parameterised in a few ways: we can set the inner radius (r2), outer radius (r1) and the number of points, which must be divisible by 3. We draw the points in groups of three, the first on the outer radius, then the inner, then the last on the outer. In that way the sections tesselate into a complete circle.

Using it with CALayer

So now we’ve got some code to draw the shape we want, we need a way to get it onto the screen, using CALayers. There are 3 main ways of providing content for layers:

• Use a delegate that implements drawInContext (and don’t forget to call setNeedsDisplay at least once to cause it be drawn!)
• Set the content to a CGImageRef. Meh… that’s going to mean it’s a bitmap, with all the associated aliasing/scaling issues.
• Subclassing. Nah.

We’ll use the first approach; we can create a simple NSObject-derived class that can manage the layer hierarchy (more of that later) and implement the required selector on it. In that function we can switch on the layer we’re being asked to draw, and do the appropriate handling:

- (void)drawLayer:(CALayer *)theLayer
        inContext:(CGContextRef)context 
{
    if (theLayer == _textLayer) {
        // ...
    } else if (theLayer == _starLayer) {
        // ...
    }
}

Animating

We’re only going to be doing fairly basic rotation animation here, so we can use CABasicAnimation. We use the key-value coding support to specify the transform.rotation property as the target. This is an alias for rotation around the Z axis, which is pointing “out” of the screen. We rotate from 0 to 2*pi radians, repeating essentially indefinitely by specifying a large repeatCount.

    CABasicAnimation *animation = 
        [CABasicAnimation animationWithKeyPath:@"transform.rotation"];
    animation.duration=8.0;
    animation.repeatCount=HUGE_VALF;
    animation.autoreverses=NO;
    animation.fromValue=[NSNumber numberWithFloat:0.0];
    animation.toValue=[NSNumber numberWithFloat:TWOPI];
    [_starLayer addAnimation:animation forKey:@"rotation"];

Drawing the text

This was an interesting one. I originally started looking at a CATextLayer-based solution, but was surprised to find that it’s quite difficult to get vertical alignment within a rectangle. Instead, I decided to use the NSString UIKit additions that provide enough drawing and – importantly – measuring functions for us to work out exactly where we need to place the text.

One important thing to note here is that we’re potentially mixing Core Graphics and UIKit functions here. They have different expectations about how to get hold of the required graphics context; with Core Graphics it’s always passed explicitly, whereas UIKit will grab the current context. This means that if you try and call drawInRect within your drawLayer function, you’ll see errors like “Invalid context: 0×0″ on the console, and no output.

The solution is simple when you know how: tell UIKit about your explicit context, like this:

- (void)drawLayer:(CALayer *)theLayer
        inContext:(CGContextRef)context 
{
    // ...
    // Let UIKit know about this context
    UIGraphicsPushContext(context);
    // Because this function uses it internally...
    [myString drawInRect:r 
                     withFont:font
            lineBreakMode:UILineBreakModeClip 
                    alignment:UITextAlignmentCenter];
    UIGraphicsPopContext();
}

By measuring the text before we draw, we can align it centrally vertically and fill the space horizontally, letting iOS worry about the horizontal alignment. sizeWithFont takes into account a bounding rectangle and our desired breaking/clipping options:

        CGSize sz = [s sizeWithFont:font 
                 constrainedToSize:theLayer.bounds.size 
                     lineBreakMode:UILineBreakModeClip];

Setting up a layer hierarchy

Given that I wanted some parts of the thing to rotate, and others to be static, I needed to create multiple layers and put them together. This is very easily achieved by adding layers to the subLayers collection of our root layer, then we return the root layer and add that to the view.

Layer arrangement

    _star = [[StarLayer alloc] initWithRect:CGRectMake(100, 100, 100, 100)];
    [[self.view.layer] addSublayer:_star.root];

Next Steps

Here’s a red star and random text sitting a bit incongruously in a prototype of a spelling app I’m writing.

It would be good to create a whole load of stars (probably not with text in) and shoot them across the screen in a star-burst by generating a random direction/speed vector and animating their speeds, opacity and scale to make them fade out.

You can check out the code here.

_toplayer = [CALayer layer];

Splitting images

Now we’ve got the layers, we need something to but into them. I didn’t want to create lots of pre-processed images for the top and bottom sections of each number, so instead I wrote a routine to split the images in code, drawing each half into an image context and grabbing a UIImage from it. This is then stored in an array for later use. We can get any of the elements from the array and set it as the content of the layer as required.

// The size of each part is half the height of the whole image
CGSize size = CGSizeMake([img size].width, [img size].height/2);
 
// Create image-based graphics context for top half
UIGraphicsBeginImageContext(size);
 
// Draw into context, bottom half is cropped off
[img drawAtPoint:CGPointMake(0.0,0.0)];
// Grab the current contents of the context as a UIImage 
// and add it to our array
UIImage *top = [UIGraphicsGetImageFromCurrentImageContext() retain];			
[_imagesTop addObject:(id)top];
 
// Now draw the image starting half way down, to get the bottom half
[img drawAtPoint:CGPointMake(0.0,-[img size].height/2)];
// And store that image in the array too
UIImage *bottom = [UIGraphicsGetImageFromCurrentImageContext() retain];			
[_imagesBottom addObject:(id)bottom];
 
UIGraphicsEndImageContext();

Now our _imagesTop and _imagesBottom arrays contains all the images we need, let’s get ‘em moving.

Applying animation

The axes and anchor point of a CALayer

_fliplayer.anchorPoint = CGPointMake(0.5, 1.0);

I use the CATransform3DMakeRotation function to create a transform around the X axis using the vector [1,0,0]. For the first part of the motion, we go from 0 radians to pi/2 radians (0 to 90 degrees), using an explicit CABasicAnimation object:

CABasicAnimation *topAnim = [CABasicAnimation animationWithKeyPath:@"transform"];
topAnim.duration=0.25;
topAnim.repeatCount=1;
topAnim.fromValue= [NSValue valueWithCATransform3D:CATransform3DMakeRotation(0.0, 1, 0, 0)];
float f = -M_PI/2;
topAnim.toValue=[NSValue valueWithCATransform3D:CATransform3DMakeRotation(f, 1, 0, 0)];
topAnim.delegate = self;
topAnim.removedOnCompletion = FALSE;
[_fliplayer addAnimation:topAnim forKey:[NSString stringWithUTF8String:k_TopAnimKey]];

Drawing layer content

Unfortunately a CALayer doesn’t have a “back”, so creating the flip layer is not as easy as it could be. We need to add a step in the process to change the image displayed by the flip layer half way through its fall. We can do this in a variety of ways, I’ve chosen to set a delegate object to perform the drawing of the layer contents:

[_fliplayer setDelegate:_layerHelper];

This means our delegate object (_layerHelper) has its drawLayer method called whenever the layer needs to display its contents. Be wary though, this may not be as often as you think! Core graphics aggressively caches the contents of the layer, and will only call your delegate when absolutely necessary. You can force it to happen by calling setNeedsDisplay, which invalidates the contents of the layer:

[_fliplayer setNeedsDisplay];

I encapsulated this in the helper object itself, by providing a custom implementation of the isTop property setter that calls setNeedsDisplay. At least that way callers don’t have to be aware of this requirement.

The drawLayer method itself simply draws one of two images into the context, depending on whether it’s in ‘top’ mode. A slight further complication is that when drawing the bottom part, the image needs to be inverted to display upside down. This is achieved by applying a translation and scale to the context before drawing the image:

CGContextSaveGState(context);
CGContextTranslateCTM(context, 0.0f, r.size.height);
CGContextScaleCTM(context, 1.0f, -1.0f);
 
CGContextDrawImage(context, r, [_imgTop CGImage]);
 
CGContextRestoreGState(context);

Avoiding implicit animation

I’ve tried to avoid using implicit animation here, instead using explicit construction of CABasicAnimation objects, because we need some control of when things happen, and the order in which they happen. Normally, it would be easier to just set some property of the view and let the OS manage animating it. Many properties have default transition that are applied whenever they’re changed, so it takes some additional effort to make them change immediately.

In our example we change the contents of the top half and show the flip layer, but we need that to happen immediately so we disable actions within an explicit transaction that we then commit. This gives us the control we need, and avoids the default behaviour of the transition occurring when the message loop next runs.

[CATransaction begin];
[CATransaction setValue:(id)kCFBooleanTrue
				forKey:kCATransactionDisableActions];
_toplayer.contents = (id)[[_imagesTop objectAtIndex:[self getNextIndex]] CGImage];
_fliplayer.hidden = NO;
[CATransaction commit];

Getting some perspective

Although the images I’ve used to illustrate this post are isometric, what we really want is a front-on perspective view, so that the falling piece seems to stick out. For this we have to set a sublayerTransform on the view’s inherent layer, by setting an individual element on a transform:

CATransform3D aTransform = CATransform3DIdentity;
float zDistance = 1000;
aTransform.m34 = 1.0 / -zDistance;	
[self layer].sublayerTransform = aTransform;

This gives us a much more realistic look.

Putting it together

Wow, so now we’ve got some appropriately sliced images, some layers and some animations. Let’s put them together to get the final effect.

The animation states (last is same as first)

- (void)animationDidStop:(CAAnimation *)oldAnimation finished:(BOOL)flag
{
	[self changeState];
}

• TopToMiddle: show flip layer, and start rotation through 90 degrees
• MiddleToBottom: change image on flip layer, start rotation through further 90 degrees
• ResetForNext: change displayed top and bottom layer contents and hide flip layer

Let’s see how it looks in action:

Well, I’ve done it: I’ve got my first game live on the AppStore. It’s been an interesting journey. I’m terribly bad at getting my hands on devkits and SDKs, having a play with them and then not doing anything constructive. This dates way back to things like the Playstation NetYaroze, which was pretty expensive, and with which I failed to produce anything concrete. So this time around all the pieces were in place: shiny new “gaming” kit, interesting SDK, low cost of entry. I was determined to create!

I’ll be making a series of posts on the process and details of creating it, in the interest of sharing the fun. In the meantime, you can find out more about the game here.

One of the things that seemed a little odd about the SDK is it’s use of CAF-format audio files, detailed here. I got hold of a few very nice audio samples from the freesound site, but needed to convert them from WAVs to CAFs.

I thought ffmpeg might be up to the job, but the version I had didn’t list it as an avaliable output format using ffmpeg -formats. However after a bit of digging I discovered that it is supported by libsndfile, so set about installing it using MacPorts:

sudo port install libsndfile

Then I used the included libsndfile-convert app to convert my file:

libsndfile-convert file.wav file.caf

The output format is inferred from the file extension, so you don’t have to specify it. However, when I rebuilt and ran my iPhone app using the new file, it didn’t play back. I suspected there may be something wrong with the format of the file, so I took a look to see what file reported. For the original WAV file I got

file.wav: RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit, mono 44100 Hz

Unfortunately file doesn’t work on .CAF files, but you can open them using QuickTime Player, and using the Movie Inspector window you can see that the file has the following format:

16-bit Integer (Big Endian), Mono, 44.100 kHz

So it looks like the problem may be libsndfile-convert changing the endian-ness of the file contents, from the x86-style little-endian to Motorola-ish (i.e. pre-x86 Mac) big-endian, which is a bit of a pain. According to the docs, the libsndfile API supports endian-ness manipulation, so it’s probably just the case that the helper app is doing the wrong thing automatically. I’ll look at putting together a small command line app to use the API directly and enable me to batch process .WAV files correctly.

More by luck than judgment I picked up the machine a couple of days after the launch of Leopard, so got that included, and it’s a good job because one of the killer features for me was bootcamp, and this comes as standard with the new OS. Don’t worry, I won’t be going into excruciating detail about the other new features – but mostly because I haven’t used previous versions of the OS enough to be able to make a worthwhile comparison.

Despite that, I still think that it’s a stark reminder of how good an OS can be; a solid foundation combined with a good consistent and cutting edge UI experience.

But I’m off to boot back into Windows for a quick bit of HL2 EP2…