我无法迅速在super.init（）之前或之后初始化事物

Question

I want to be able to initialize the property subviewGroup using self but this gives me the error: Property 'self.gridView' not initialized at super.init call

init(frame: NSRect) {
    super.init(frame: frame)
    subviewGroup = GridViewGroup(rows: 9, columns: 9, gridView: self)
}

/***  Properties  ***/
let subviewGroup: GridViewGroup

Then if I put the super.init() after the initialization like this

init(frame: NSRect) {
    subviewGroup = GridViewGroup(rows: 9, columns: 9, gridView: self)
    super.init(frame: frame)
}

/***  Properties  ***/
let subviewGroup: GridViewGroup

I get the error: 'self' used before super.init call

I know I can use an Optional but since it will never be nil after the init() call it seems unnecessary. Is there a way to do this properly without making subviewGroup an optional?

Here is a working example using an optional

/***  Initializers  ***/
init(frame: NSRect) {
    super.init(frame: frame)
    subviewGroup = GridViewGroup(rows: 9, columns: 9, gridView: self)
}

/***  Properties  ***/
let subviewGroup: GridViewGroup?

Answer 1

This is exactly what implicitly unwrapped optionals are good for. You can easily add them to your example like this:

/***  Initializers  ***/
init(frame: NSRect) {
    super.init(frame: frame)
    subviewGroup = GridViewGroup(rows: 9, columns: 9, gridView: self)
}

/***  Properties  ***/
let subviewGroup: GridViewGroup!

Using an implicitly unwrapped optional allows you to treat the variable as if it weren't an optional and access the underlying directly when you call it.

Using a normal optional your code may look like var nonoptional: GridViewGroup = subviewGroup! while with implicitly unwrapped optionals you can discard the !: var nonoptional: GridViewGroup = subviewGroup .

From Apple's docs:

Implicitly unwrapped optionals are useful when an optional's value is confirmed to exist immediately after the optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of implicitly unwrapped optionals in Swift is during class initialization

Answer 2

Your use case is crying out for an implicitly unwrapped optional; this is a perfect example of when you would use one, and this pattern is used in many places throughout the Cocoa Touch API.

To begin with:

A. The proper order of your initialization should be something like this:

    init(frame: NSRect) {
        subviewGroup = GridViewGroup(rows: 9, columns: 9, gridView: self)
        super.init(frame: frame)
    }

...per the Swift Programming Book:

"A designated initializer must ensure that all of the properties introduced by its class are initialized before it delegates up to a superclass initializer.”

Then secondly:

B. Declare your subviewGroup constant as an implicitly unwrapped optional; doing so gives it a default value of nil , meaning that as far as the compiler is concerned the requirement that it be initialized has been met! This form leaves you with a one-time opportunity (since it is a constant, where you used let ) to assign a non-nil value to it, which you can then do in your initializer without issue as so:

let subviewGroup: GridViewGroup!

init(frame: NSRect) {
    subviewGroup = GridViewGroup(rows: 9, columns: 9, gridView: self)
    super.init(frame: frame)
}

I hope this helps!

Answer 3

The correct solution, as you found, is use optional. But because you know it will never be nil outside init , you can use implicitly unwrapped optional.

let subviewGroup: GridViewGroup!