Is is possible to animate text color changing only in a part of text in iOS? - ios

Question

I wonder if it even possible in iOS to animate changing color in only a part of the text, preferably not char by char, but pixel by pixel, like on this picture?
I know how to change text color in static with NSAttributedString and I know how to animate the whole text with CADisplayLink, but this makes me worry.
Maybe I can dive into CoreText, but I'm still not sure it is possible even with it. Any thoughts?
UPD I decided to add a video with my first results to make the question more clear:
my efforts for now (the label is overlapping)

Answer 1

You can quite easily achieve this using CoreAnimation possibilities.
I've added a simple demo, you play with it here (just build the project and tap anywhere to see the animation).
The logic is the following:
Create a custom subclass of UIView.
When some text is set, create two similar CATextLayers, each with the same text and frame.
Set different foregroundColor and mask for those layers. The mask of the left layer will be the left part of the view, and the mask of the right layer will be the right part.
Animate foregroundColor for those layers (simultaneously).
The code of a custom view:
class CustomTextLabel: UIView {
override init(frame: CGRect) {
super.init(frame: frame)
backgroundColor = .green
}
required init?(coder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
private var textLayer1: CATextLayer?
private var textLayer2: CATextLayer?
func setText(_ text: String, fontSize: CGFloat) {
// create 2 layers with the same text and size, we'll set the colors for them later
textLayer1 = createTextLayer(text, fontSize: fontSize)
textLayer2 = createTextLayer(text, fontSize: fontSize)
// estimate the frame size needed for the text layer with such text and font size
let textSize = textLayer1!.preferredFrameSize()
let w = frame.width, h = frame.height
// calculate the frame such that both layers will be in center of view
let centeredTextFrame = CGRect(x: (w-textSize.width)/2, y: (h-textSize.height)/2, width: textSize.width, height: textSize.height)
textLayer1!.frame = centeredTextFrame
textLayer2!.frame = centeredTextFrame
// set up default color for the text
textLayer1!.foregroundColor = UIColor.yellow.cgColor
textLayer2!.foregroundColor = UIColor.yellow.cgColor
// set background transparent, that's very important
textLayer1!.backgroundColor = UIColor.clear.cgColor
textLayer2!.backgroundColor = UIColor.clear.cgColor
// set up masks, such that each layer's text is visible only in its part
textLayer1!.mask = createMaskLayer(CGRect(x: 0, y: 0, width: textSize.width/2, height: textSize.height))
textLayer2!.mask = createMaskLayer(CGRect(x: textSize.width/2, y: 0, width: textSize.width/2, height: textSize.height))
layer.addSublayer(textLayer1!)
layer.addSublayer(textLayer2!)
}
private var finishColor1: UIColor = .black, finishColor2: UIColor = .black
func animateText(leftPartColor1: UIColor, leftPartColor2: UIColor, rightPartColor1: UIColor, rightPartColor2: UIColor) {
finishColor1 = leftPartColor2
finishColor2 = rightPartColor2
if let layer1 = textLayer1, let layer2 = textLayer2 {
CATransaction.begin()
let animation1 = CABasicAnimation(keyPath: "foregroundColor")
animation1.fromValue = leftPartColor1.cgColor
animation1.toValue = leftPartColor2.cgColor
animation1.duration = 3.0
layer1.add(animation1, forKey: "animation1")
let animation2 = CABasicAnimation(keyPath: "foregroundColor")
animation2.fromValue = rightPartColor1.cgColor
animation2.toValue = rightPartColor2.cgColor
animation2.duration = 3.0
layer2.add(animation2, forKey: "animation2")
CATransaction.setCompletionBlock {
self.textLayer1?.foregroundColor = self.finishColor1.cgColor
self.textLayer2?.foregroundColor = self.finishColor2.cgColor
}
CATransaction.commit()
}
}
private func createTextLayer(_ text: String, fontSize: CGFloat) -> CATextLayer {
let textLayer = CATextLayer()
textLayer.string = text
textLayer.fontSize = fontSize // TODO: also set font name
textLayer.contentsScale = UIScreen.main.scale
return textLayer
}
private func createMaskLayer(_ holeRect: CGRect) -> CAShapeLayer {
let layer = CAShapeLayer()
let path = CGMutablePath()
path.addRect(holeRect)
path.addRect(bounds)
layer.path = path
layer.fillRule = CAShapeLayerFillRule.evenOdd
layer.opacity = 1
return layer
}
}
The calls of a custom view:
class ViewController: UIViewController {
var customLabel: CustomTextLabel!
override func viewDidLoad() {
super.viewDidLoad()
let viewW = view.frame.width, viewH = view.frame.height
let labelW: CGFloat = 200, labelH: CGFloat = 50
customLabel = CustomTextLabel(frame: CGRect(x: (viewW-labelW)/2, y: (viewH-labelH)/2, width: labelW, height: labelH))
customLabel.setText("Optimizing...", fontSize: 20)
view.addSubview(customLabel)
let tapRecogniner = UITapGestureRecognizer(target: self, action: #selector(onTap))
view.addGestureRecognizer(tapRecogniner)
}
#objc func onTap() {
customLabel.animateText(leftPartColor1: UIColor.blue,
leftPartColor2: UIColor.red,
rightPartColor1: UIColor.white,
rightPartColor2: UIColor.black)
}
}

Answer 2

Thanks to Olha's (#OlhaPavliuk) answer, I used two CATextLayer shapes and two CAShapeLayer masks for text layers. In draw method I just change masks frames to calculated size (bounds.width * progress value), and also change the second mask origin to a new start (bounds.width - bounds.width * progress value).
Also, it was very important to set layer.fillRule = CAShapeLayerFillRule.evenOdd while creating a mask, so that both layers became visible.
It turned out that I actually didn't need any animation code involved, because changing frames looks just ok.
In motion: https://giphy.com/gifs/LMbmlMoxY9oaWhXfO1
Full code: https://gist.github.com/joliejuly/a792c2ab8d97d304d731a4a5202f741a