TriangleView.m

@interface TriangleView()
@property (nonatomic, strong, readonly) CAShapeLayer *shapeLayer;
@end

@implementation TriangleView

static NSString *const kAniamtionKey = @"path";
static CGFloat const ANIMATION_DURATION = 0.25f;

- (id)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if (self) {
        self.backgroundColor = [UIColor clearColor];
        _shapeLayer = [CAShapeLayer layer];
        [self.layer addSublayer:self.shapeLayer];
        [self.shapeLayer setFillColor:[UIColor yellowColor].CGColor];
        [self.shapeLayer setStrokeColor:[UIColor blackColor].CGColor];
        self.shapeLayer.path = [self trianglePathWithPoint:CGPointMake(0, self.frame.size.height)];
    }
    return self;
}

- (CGPathRef)trianglePathWithPoint:(CGPoint)point {
    UIBezierPath *path = [UIBezierPath bezierPath];
    
    [path moveToPoint:CGPointZero];
    [path addLineToPoint:CGPointMake(self.frame.size.width, 0)];
    [path addLineToPoint:point];
    [path closePath];
    
    return path.CGPath;
}

- (void)triangleAnimation:(CGPoint)point {
    CGPathRef oldPath = self.shapeLayer.path;
    CGPathRef newPath = [self trianglePathWithPoint:point];
    CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:kAniamtionKey];
    [animation setFromValue:(__bridge id)oldPath];
    [animation setToValue:(__bridge id)newPath];
    [animation setDuration:ANIMATION_DURATION];
    [animation setFillMode:kCAFillModeBackwards];
    [animation setTimingFunction:[CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseOut]];
    [self.shapeLayer addAnimation:animation forKey:kAniamtionKey];
    self.shapeLayer.path = newPath;
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    CGPoint point = [[touches anyObject] locationInView:self];
    [self triangleAnimation:point];
}

@end