How to make two lines in UIBezierPath to animate at different speed - xcode

Question

Say the following scenario:
I have drawn a quadrilateral shape, which is a mask for a UIView. I denote the shape layer as maskLayer. maskLayer crops the bottom of the UIView asymmetrically.
But then I want to fully reveal my UIView in an animation. The animation should be left side of maskLayer drops down to the bottom of UIView, and .2 sec later my right side of maskLayer also drops down to the bottom of UIView, thus fully reveal the entity of UIView.
My approach is to drop down left line first, then right one as the following code:
//this quadrilateral will put down left corner to the bottom of screen
var path2 = UIBezierPath()
path2.moveToPoint(CGPointZero)
path2.addLineToPoint(CGPoint(x: 0, y: frame.height))
path2.addLineToPoint(CGPoint(x: frame.width, y: frame.height / goldRatio / goldRatio))
path2.addLineToPoint(CGPoint(x: frame.width, y: 0))
path2.closePath()
//this rectangle path will put down both corner to the bottom of screen
//thus fix the view to its original shape
var path3 = UIBezierPath()
path3.moveToPoint(CGPointZero)
path3.addLineToPoint(CGPoint(x: 0, y: frame.height))
path3.addLineToPoint(CGPoint(x: frame.width, y: frame.height))
path3.addLineToPoint(CGPoint(x: frame.width, y: 0))
path3.closePath()
I have spent 2 hours trying to figure it out to no avail. May you please give me some instructions about how to achieve just that.
The initial state is like the following:
The end state is like the following:
I truly appreciate your help!

Answer 1

Easiest way to do this... Cheat.
Don't try to animate the path of the shape it really doesn't need it.
What you should do is something like this.
Create your final state view. Rectangular, at the bottom of the screen with the UI on it etc...
This final state view will not move. It will always be here.
Now create another view and insert it as a sub view underneath the final state view. On this you can add a shape layer with the angular corner cut off.
Now all you need to do is animate the position of this angular view downward until it is completely below the final state view.
If they are the same colour the this will give the effect of animating the path of the shape.
To get the different speeds you could have a rectangluar shape layer rotated to 45 degrees. Then animate it to 0 degrees as the view slides down?
In fact, you could do this with a single shape layer that is rotated and moved.

Answer 2

To do this sort of animation, you would generally use a CADisplayLink (sort of like a timer, but linked to updates of the display rather than some arbitrary interval) to repeatedly change the path associated with desired shape. I think this is easiest if you use a CAShapeLayer and just change the path property of this shape.
To make this work, you need a function that represents the path at a given point of time (or easier, a path at an instant a certain percentageComplete along the animation duration). Since you have a regular shape (constantly the same number of points), you can simply interpolate between some array of startPoints and endPoints.
So, create the shape layer, capture the start time, start the display link, and then for every "tick" of the display link, calculate what percentage of the total animationDuration has passed, and update the shape layer's path accordingly:
class ViewController: UIViewController {
let animationDuration = 2.0
var displayLink: CADisplayLink!
var startTime: CFAbsoluteTime!
var shapeLayer: CAShapeLayer!
let goldRatio: CGFloat = 1.6180339887
var startPoints:[CGPoint]!
var endPoints:[CGPoint]!
override func viewDidLoad() {
super.viewDidLoad()
self.startAnimation()
}
func startAnimation() {
startPoints = [
CGPoint(x: 0, y: view.bounds.size.height),
CGPoint(x: 0, y: view.bounds.size.height - view.bounds.size.height / goldRatio),
CGPoint(x: view.bounds.size.width, y: 0),
CGPoint(x: view.bounds.size.width, y: view.bounds.size.height)
]
endPoints = [
CGPoint(x: 0, y: view.bounds.size.height),
CGPoint(x: 0, y: view.bounds.size.height * 0.75),
CGPoint(x: view.bounds.size.width, y: view.bounds.size.height * 0.75),
CGPoint(x: view.bounds.size.width, y: view.bounds.size.height)
]
assert(startPoints.count == endPoints.count, "Point counts don't match")
createShape()
startDisplayLink()
}
func startDisplayLink() {
displayLink = CADisplayLink(target: self, selector: "handleDisplayLink:")
startTime = CFAbsoluteTimeGetCurrent()
displayLink.addToRunLoop(NSRunLoop.mainRunLoop(), forMode: NSRunLoopCommonModes)
}
func stopDisplayLink() {
displayLink.invalidate()
displayLink = nil
}
func handleDisplayLink(displayLink: CADisplayLink) {
var percent = CGFloat((CFAbsoluteTimeGetCurrent() - startTime) / animationDuration)
if percent >= 1.0 {
percent = 1.0
stopDisplayLink()
}
updatePathBasedUponPercentComplete(percent)
}
func createShape() {
shapeLayer = CAShapeLayer()
shapeLayer.fillColor = UIColor.blueColor().CGColor
shapeLayer.strokeColor = UIColor.clearColor().CGColor
updatePathBasedUponPercentComplete(0.0)
view.layer.addSublayer(shapeLayer)
}
func updatePathBasedUponPercentComplete(percentComplete: CGFloat) {
shapeLayer.path = pathBasedUponPercentComplete(percentComplete, frame: view.frame).CGPath
}
func pathBasedUponPercentComplete(percentComplete: CGFloat, frame: CGRect) -> UIBezierPath {
var path = UIBezierPath()
for i in 0 ..< startPoints.count {
let point = CGPoint(
x: startPoints[i].x + (endPoints[i].x - startPoints[i].x) * percentComplete,
y: startPoints[i].y + (endPoints[i].y - startPoints[i].y) * percentComplete
)
if i == 0 {
path.moveToPoint(point)
} else {
path.addLineToPoint(point)
}
}
path.closePath()
return path
}
}