如何切换CAGradientLayer？

Question

I initially create a CAGradientLayer on my first scene in viewDidLoad. It's inserted like this:

self.view.layer.insertSublayer(myCAGradientLayer, atIndex: 0)

That works fine. But now I need to change the gradient while the app is running. The user will do something on scene 2 and come back to scene 1, where the gradient changes. I build up a new CAGradientLayer and add to the view the same as above.

But using viewWillAppear doesn't work.

Here is an example:

let gLocations: [Float] = [0.0, 1.0]
let gLayer: CAGradientLayer = CAGradientLayer()
gLayer.colors = [UIColor.yellowColor().CGColor, UIColor.greenColor().CGColor]
gLayer.frame = self.view.bounds
gLayer.locations = gLocations
self.view.layer.insertSublayer(gLayer, atIndex: 0)

Add the above to viewWillAppear. Then add it into a several conditionals using different colors. Only the first conditional chosen appears. Any of the other colors in the other conditionals doesn't appear.

I think part of the issue is that I'm always doing:

self.view.layer.insertSublayer(gLayer, atIndex: 0)

Otherwise, the gradient will appear on top of other UI components. I'm using several different gradients and they all use the above code. But I'm guessing they all can't be inserted at index=0.

Is there some other way to change the gradient?

Answer 1

The ViewWillAppear function should work. It will be called every time the view is about to get active.

override func viewWillAppear(animated: Bool) {
    super.viewWillAppear(animated);
}

Update
The first one is only displayed because your CAGradientLayer color is always set to be gLayer.colors = [UIColor.yellowColor().CGColor, UIColor.greenColor().CGColor] .

Whenever you want to change the color you need to save this in the memory so that you know this when you enter the view again. This is an alternative solution.

Create a NSUserDefault to save which color you want to show.

// Set - set a new color whenever you want this to be displayed when you show the view next time
// forKey is the unique value to access the object you have saved
NSUserDefaults.standardUserDefaults().setObject("red", forKey: "color"

When you show the view do this:

// Get
let color = NSUserDefaults.standardUserDefaults().stringForKey("color")

if color == "red"{
   gLayer.colors = [UIColor.redColor().CGColor, UIColor.blueColor().CGColor]
}
else if...

Answer 2

You can search your layer from sublayers:

myCAGradientLayer.name = "my gradient layer"
        ...

extension UIView {
    func getMyGradient(name:String) -> CAGradientLayer {
        if let layers = self.layer.sublayers {
            for layer in layers {
                if (layer.name == name) {
                    let caGradientLayer = layer as! CAGradientLayer
                    return caGradientLayer
                }
            }
        }
        print("Sorry your gradient doesnt exist in this view..")
        return CAGradientLayer()
    }
}

To retrieve your gradient:

let myCAGradientLayer = self.view.getMyGradient()
if myCAGradientLayer.name == "my gradient layer" {
   // do whatever you want with the same first gradient added 
}

Try to make setNeedsDisplay to your view, to re-render itself if there are problems around display update..