具有静态间距的 UIStackView 子类

Question

I am creating a subclass of UIStackView, and I want spacing to be static and non-overwritable. I thought this would be simply done by overriding the spacing property:

class MyStackView: UIStackView {
    override var spacing: CGFloat {
        get { return 8.0 }
        set {}
    }
}

This, however, does not work. The getter is never called and no spacing is displayed. The only solution I found for this is the following piece of code:

class MyStackView: UIStackView {
    override init(frame: CGRect) {
        super.init(frame: frame)
        super.spacing = 8.0
    }

    override var spacing: CGFloat {
        get { return 8.0 }
        set {}
    }
}

It actually doesn't matter what I return in get , it's never called by anything anyway.

Can anybody explain this behavious]r and how to avoid it?

Answer 1

In my opinion here what's happening. You want to achieve two things:

1. Spacing at MyStackView should be 8.0 by default
2. That spacing value should not be changeable from outside

To achieve the first thing you need to override the initializers and set your default value inside on them.

class MyStackView: UIStackView {

    private let defaultSpacing: CGFloat = 8.0

    override init(frame: CGRect) {
        super.init(frame: frame)
        super.spacing = defaultSpacing
    }

    required init(coder: NSCoder) {
        super.init(coder: coder)
        super.spacing = defaultSpacing
    }

}

For the second thing, you need to override the getter and setter of spacing to get the control on any changing from the outside.

class MyStackView: UIStackView {

    ...

    override var spacing: CGFloat {
        get { return defaultSpacing }
        set { print("MyStackView spacing it always \(defaultSpacing)") }
    }

}

Why can't you simply override a variable and get the same result?

UIStackView.spacing property is computed, it's not a container. We might not care how does original getter works because we know that our spacing is always the same. We know its value and can always return it at overriden getter.

But we also don't know how does original setter works and what changes does it make in UIStackView . Since that, we can't simply override getter . We have to explicitly call super.spacing = defaultValue once, to trigger original setter and set the desired value.