Draw triangle cutout for selection indication in UIView

Question

I'm creating a view that serves as a category selector for my app. I'd like it to have a cutout triangle as the selection indication, as in this image:

I'm not sure how to draw the triangle so that it is a cutout, revealing the main view underneath. The main view underneath will most likely have a custom, possibly non-repeating (I haven't decided yet) image as its background. In addition, I would like the triangle to animate to a new location when the selection changes, which further complicates things a bit.

I realize a subview would make the animation easier, but would complicate the drawing; direct drawing would probably make the animation a bit harder. And I'm not too familiar with Quartz, so I'm not sure how to go with the direct drawing route.

Thanks in advance!

Update: I've looked at Matt Gallagher's post on drawing shapes with holes, but it doesn't really answer my question. Is there a way for me to "see" what's underneath a certain path within my shape, and copy that? …And then support animating it?

Update 2: I've done a partial job by simply drawing an additional path. The result looks like this: http://dl.dropbox.com/u/7828009/Category%20Selector.mov

The code:

CGRect cellRect = [self rectForCategoryNumber:(selectedCategoryIndex + 1)];
[[UIColor scrollViewTexturedBackgroundColor] setFill];
CGContextMoveToPoint(currentContext, self.frame.size.width, (cellRect.origin.y + cellRect.size.height * 0.15));
CGContextAddLineToPoint(currentContext, self.frame.size.width, (cellRect.origin.y + cellRect.size.height * 0.65));
CGContextAddLineToPoint(currentContext, self.frame.size.width * 0.8, (cellRect.origin.y + cellRect.size.height * 0.4));
CGContextClosePath(currentContext);
CGContextFillPath(currentContext);
[[UIColor darkGrayColor] setStroke];
CGContextSetLineCap(currentContext, kCGLineCapRound);
CGContextMoveToPoint(currentContext, self.frame.size.width, (cellRect.origin.y + cellRect.size.height * 0.15));
CGContextAddLineToPoint(currentContext, self.frame.size.width * 0.8, (cellRect.origin.y + cellRect.size.height * 0.4));
CGContextSetLineWidth(currentContext, 2.5);
CGContextStrokePath(currentContext);
[[UIColor lightGrayColor] setStroke];
CGContextMoveToPoint(currentContext,self.frame.size.width * 0.8, (cellRect.origin.y + cellRect.size.height * 0.4));
CGContextAddLineToPoint(currentContext, self.frame.size.width, (cellRect.origin.y + cellRect.size.height * 0.65));
CGContextSetLineWidth(currentContext, 1.0);
CGContextStrokePath(currentContext);

Obviously, in this case it only works because I'm using the same fill color in both cases; I'd like to eliminate this dependency if possible. Also, of course I'd like to animate the position of that triangle.

Update 3: What I tried to do to animate:

static CALayer *previousLayer = nil;
static CGMutablePathRef previousPath = nil;
// … Get context, etc.
CAShapeLayer *shapeLayer = [[CAShapeLayer alloc] init];
shapeLayer.path = shapePath;
[shapeLayer setFillColor:[[UIColor redColor] CGColor]];
[shapeLayer setStrokeColor:[[UIColor blackColor] CGColor]];
[shapeLayer setBounds:self.bounds];
[shapeLayer setAnchorPoint:self.bounds.origin];
[shapeLayer setPosition:self.bounds.origin];
if (previousPath) {     // Animate change
    CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"changePath"];
    animation.duration = 0.5;
    animation.fromValue = (id)previousPath;
    animation.toValue = (id)shapePath;
    [shapeLayer addAnimation:animation forKey:@"animatePath"];
    previousPath = shapePath;
}

if (previousLayer)
    [previousLayer removeFromSuperlayer];
previousLayer = shapeLayer;
[self.layer addSublayer:shapeLayer];

Answer 1

Have you looked at the CAShapeLayer? You can create a path for your selection pane that defines the whole outline including the exclusion of the triangle for each state you need to mask. You can then stroke the layer if you want the outline you're showing in your image by setting the lineWidth and strokeColor properties. That should be able to give you what you need. The path property in the CAShapeLayer is animatable which means that all you have to do is set the path property and it will animate (assuming your layers are sublayers of the view and not the root layer).

Update With Code

This code:

- (void)viewDidLoad
{
  [super viewDidLoad];

  [[self view] setBackgroundColor:[UIColor blueColor]];
  
  CGMutablePathRef path = CGPathCreateMutable();
  CGPathMoveToPoint(path,NULL,0.0,0.0);
  CGPathAddLineToPoint(path, NULL, 160.0f, 0.0f);  
  CGPathAddLineToPoint(path, NULL, 160.0f, 100.0f);
  CGPathAddLineToPoint(path, NULL, 110.0f, 150.0f);
  CGPathAddLineToPoint(path, NULL, 160.0f, 200.0f);
  CGPathAddLineToPoint(path, NULL, 160.0f, 480.0f);
  CGPathAddLineToPoint(path, NULL, 0.0f, 480.0f);
  CGPathAddLineToPoint(path, NULL, 0.0f, 0.0f);
  

  CAShapeLayer *shapeLayer = [CAShapeLayer layer];
  [shapeLayer setPath:path];
  [shapeLayer setFillColor:[[UIColor redColor] CGColor]];
  [shapeLayer setStrokeColor:[[UIColor blackColor] CGColor]];
  [shapeLayer setBounds:CGRectMake(0.0f, 0.0f, 160.0f, 480)];
  [shapeLayer setAnchorPoint:CGPointMake(0.0f, 0.0f)];
  [shapeLayer setPosition:CGPointMake(0.0f, 0.0f)];
  [[[self view] layer] addSublayer:shapeLayer];

  CGPathRelease(path);
  
}

results in this display:

And you can download the sample project here:

https://dzwonsemrish7.cloudfront.net/items/180s2a200N2i3b0y1g37/ArrowIndicator.zip