Swift初始化悖论 [英] Swift initialization paradox

问题描述

在UIView的子类中，我声明了一个属性.此属性是UIPanGestureRecognizer的实例，不是可选的或隐式解包的.这是代码:

In a subclass of UIView I declared a property. This property is an instance of UIPanGestureRecognizer and is not optional or implicitly unwrapped. Here is the code:

class DraggableView: UIView {

    let panGestureRecognizer: UIPanGestureRecognizer

    override init() {

        panGestureRecognizer = UIPanGestureRecognizer(target: self, action: "dragged:")

        super.init()

        backgroundColor = UIColor.greenColor()

    }

    required init(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}

在panGestureRecognizer Xcode的初始化行上，出现错误:"super.init调用前已使用自我".如果将super.init()行移到panGestureRecognizer初始化行的上方，则Xcode会出现另一个错误:属性self.panGestureRecognizer在调用super.init时未初始化".最后一次尝试是在init方法之外的声明中初始化panGestureRecognizer.像这样:

Here on the the initialization line of panGestureRecognizer Xcode gives an error: "self used before super.init call". If I move super.init() line above the panGestureRecognizer initialization line, Xcode gives another error: "Property self.panGestureRecognizer not initialized at super.init call". The last attempt made was to initialize panGestureRecognizer in its declaration outside of the init method. Like this:

let panGestureRecognizer = UIPanGestureRecognizer(target: self, action: "dragged:")

此处Xcode给出了另一个错误:类型DraggableView -> () -> DraggableView!不符合协议AnyObject"

Here Xcode gives yet another error: "Type DraggableView -> () -> DraggableView! does not conform to protocol AnyObject"

有没有一种方法可以在不使用可选选项或隐式展开的可选选项的情况下完成这项工作?

Is there a way to make this work without using optionals or implicitly unwrapped optionals?

推荐答案

问题是self在super.init()发生之前不存在.

The problem is that self doesn't exist before super.init() has happened.

您可以分两步创建它-在声明时进行初始化:

You can create it in two steps - initialise at declaration time:

let panGestureRecognizer = UIPanGestureRecognizer()

然后在super.init()之后，添加目标:

Then after super.init(), add the target:

panGestureRecognizer.addTarget(self, action: "dragged:")

或者，您也可以将其设置为lazy var，但我不喜欢使事物可变，除非它们应该变化.

Alternatively you could make it a lazy var but I don't like making things variable unless they're supposed to vary.

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