Swift无法在属性初始化程序中使用实例成员

Question

I want to do something like this in a class, but Swift doesn't allows it:

 let minDelay = Float(0.05) //Like #define minDelay 0.05 in Objective-C

 private var delay = minDelay

I get an error "Cannot use instance member minDelay within property initializer". What is the best way to correct this without initializing delay var in init or something?

Answer 1

You could use a static variable (which means there is a single instance that belongs to the type):

class MyClass {

    static let minDelay: Float = 0.05
    // You can write `Self.minDelay` starting in Swift 5.1
    private var delay = MyClass.minDelay 

}

There are a bunch of ways to approach this, but this is probably the closest to the #define you mentioned. You could also define minDelay outside of the class entirely, but I don't think that makes sense since it is only relevant to this class.

Answer 2

Swift's property initializers can't reference other properties.

struct S {
    let a = 0
    let b = a // ❌
}

error: cannot use instance member a within property initializer; property initializers run before self is available

This is one approach to trying to prevent circular definitions like this:

struct S {
    let a = b
    let b = a //❓what would these values even be?
}

Some languages like Java take a more tolerant approach, by letting a member reference any members above it (ie on a line above it), forming a directed acyclic graph of interconnected member definitions.

Swift takes a stricter approach, and bans it outright. To get around this, you can:

1. Move your minDelay variable to a different place.

   • Make it a static member
   • Make it a static member of a different type (eg a FooConstants case-less enum).
   • Move it to a global variable (don't do this)

2. Make it a lazy var , as you said

3. Set its value in an initializer, where the order of assignments is explicitly expressed.