为什么在取消模态视图控制器后从视图中删除CAGradientLayer

Question

I am creating a gradient layer and adding it to my MainViewController self.view.

override func viewDidLoad() {
    super.viewDidLoad()

    gradient = CAGradientLayer()
    gradient.frame = view.bounds
    gradient.colors = colors
    view.layer.insertSublayer(gradient, at: 0)

}

Inside viewDidAppear I am enumerating all the sublayers:

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    for x in (view.layer.sublayers)! {
        print(x)
    }
}

This is what I get when I am running the app (which is normal):

<CAGradientLayer: 0x608000224120>
<_UILabelLayer: 0x60000009a400>
<_UILabelLayer: 0x60800009f180>
<CALayer: 0x600000422d60>
<CALayer: 0x608000223820>

Then from MainViewController I present modally another vc and then dismiss it. After this MainVC viewDidAppear is called again, but this time I get this:

<_UILabelLayer: 0x60000009a400>
<_UILabelLayer: 0x60800009f180>
<CALayer: 0x600000422d60>
<CALayer: 0x608000223820>

The CAGradientLayer was removed from self.view. I can fix this by moving my code from viewDidLoad to viewDidAppear , but I want to understand why is the CAGradientLayer removed, after the controller was dismissed.

Answer 1

I was unable to reproduce your situation in a simple test program.

Here is some debugging advice...

To find out where the gradient layer is being removed from view.layers , define

class MyCAGradientLayer: CAGradientLayer {
    deinit {
        print("there it goes")
    }
}

and replace your CAGradientLayer with MyCAGradientLayer which is created locally in viewDidLoad .

override func viewDidLoad() {
    super.viewDidLoad()

    let gradient = MyCAGradientLayer()
    gradient.frame = view.bounds
    gradient.colors = colors
    view.layer.insertSublayer(gradient, at: 0)
}

After viewDidLoad finishes the local variable gradient will release its reference to the gradient and only one reference to the gradient will exist (the one in the view.layers array).

Set a breakpoint on the print statement to see where it is getting freed.