Swift:如何在不调用didSet函数的情况下更改属性的值 [英] Swift: how to change a property's value without calling its didSet function

问题描述

如何在Swift中设置属性的值，而无需在初始化上下文之外调用didSet()函数?下面的代码是在类的noside()函数

How can you set a property's value in Swift, without calling its didSet() function outside of an initialization context? The code below was a failed experiment to achieve this within the classes' noside() function

class Test
{
    var toggle : Bool = 0
    var hoodwink : Int = 0 {
        didSet(hoodwink)
        {
            toggle = !toggle
        }
    }

// failed attempt to set without a side effect

    func noside(newValue : Int)
    {
        hoodwink = newValue
        println("hoodwink: \(hoodwink) state: \(toggle)")
    }

    func withside(newValue : Int)
    {
        self.hoodwink = newValue
        println("hoodwink: \(hoodwink) state: \(toggle)")
    }
}

在Objective-C中使用自动合成的属性是很简单的:

It is quite trivial to do in Objective-C with auto-synthesized properties:

有副作用(如果出现在二传手中):

With side effect (if present in setter):

self.hoodwink = newValue;

没有副作用:

_hoodwink = newValue;

推荐答案

您在Objective-C中为避免副作用"所做的操作正在访问属性的后备存储-它的实例变量，默认情况下带有下划线前缀(您可以使用@synthesize指令更改此变量).

What you do in Objective-C to "avoid side effects" is accessing the backing store of the property - its instance variable, which is prefixed with underscore by default (you can change this using the @synthesize directive).

但是，看来Swift语言设计人员特别注意使不可能访问属性的后备变量:根据书，

However, it looks like Swift language designers took specific care to make it impossible to access the backing variables for properties: according to the book,

如果您有使用Objective-C的经验，您可能知道它提供了两种将值和引用存储为类实例的一部分的方法.除了属性之外，您还可以将实例变量用作存储在属性中的值的后备存储.

If you have experience with Objective-C, you may know that it provides two ways to store values and references as part of a class instance. In addition to properties, you can use instance variables as a backing store for the values stored in a property.

Swift将这些概念统一为一个属性声明. Swift属性没有相应的实例变量，并且不能直接访问该属性的后备存储. (强调是我的)

Swift unifies these concepts into a single property declaration. A Swift property does not have a corresponding instance variable, and the backing store for a property is not accessed directly. (emphasis is mine)

当然，这仅适用于使用常规语言".意味着，与使用反射相反:它可能会提供一种绕过此限制的方法，但会降低可读性.

Of course this applies only to using the "regular language" means, as opposed to using reflection: it might provide a way around this restriction, at the expense of readability.

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