无法从计算属性中为存储的属性赋值

Question

我创建了一个结构，其中我有一个属性“_photo”，当调用计算属性“照片”时，该属性被延迟分配。我不断收到错误

无法分配给“self”中的“_photo”

这里是代码。我用 Swift 1.1 (photo1) 和 swift 1.2 (photo2) 语法编写了计算方法。两者都给出与上述相同的编译时错误。

需要进行哪些更改才能修复该错误？

import UIKit

public struct PhotoStruct {
    var _photo:UIImage?
    var urlString:String?

    init(image:UIImage?, url:String?){
        self._photo = image
        self.urlString = url

    }

    init(url:String?){
        self.urlString = url
    }


    var photo1:UIImage? {

        if let theURL = self._photo {
            return self._photo
        }else{
            if let urlString = self.urlString{
                if let url = NSURL(string: urlString as String){
                    if let imageData :NSData = NSData(contentsOfURL: url){
                        if let image:UIImage = UIImage(data:imageData){

                            self._photo = image //** Cannot assign error is here**
                        }
                    }
                }
            }
            return self._photo
        }
    }

    var photo2:UIImage? {

        if let theURL = self._photo {
            return self._photo
        }else{
            if let  urlString = self.urlString,
                    url = NSURL(string: urlString as String),
                    imageData :NSData = NSData(contentsOfURL: url),
                    image:UIImage = UIImage(data:imageData){

                            self._photo = image //** Cannot assign error is here**
            }
            return self._photo
        }
    }



}

Answer 1

至于struct，如果你想改变计算属性中的self属性，你必须将getter显式声明为mutating get { .. }

public struct PhotoStruct {
    var _photo:UIImage?
    var urlString:String?

    init(image:UIImage?, url:String?){
        self._photo = image
        self.urlString = url
    }

    init(url:String?){
        self.urlString = url
    }

    var photo1:UIImage? {
        mutating get {
    //  ^^^^^^^^^^^^
            // you can set `self._photo = image` here
        }
    }

    var photo2:UIImage? {
        mutating get {
    //  ^^^^^^^^^^^^
            // you can set `self._photo = image` here
        }
    }
}

当然，结构本身必须是可变的：

var pVar:PhotoStruct = PhotoStruct(image: nil, url: nil)
pVar.photo1 // no problem
let pLet:PhotoStruct = PhotoStruct(image: nil, url: nil)
pLet.photo1 // < [!] error: immutable value of type 'PhotoStruct' only has mutating members named 'photo1'

一个警告：

据我所知，mutating get { } 功能在 the language reference 上没有记录。

Answer 2

Structs 是值类型，它们是不可变的。
这意味着您不能设置变量和改变自我。
如果你需要改变结构，你必须使mutating func

public struct PhotoStruct {
    var _photo:UIImage?
    var urlString:String?

   mutating func loadPhoto() -> UIImage {
    .. Code here
   _photo = UIImage(data:imageData)
   // Here you mutate a struct, but it's ok because we make method as mutating
  }
}

在您的示例中，我将创建一个变异方法而不是属性

mutating func photo () -> UIImage? {

    if let photo = self._photo {
        return photo
    } else {
        if let urlString = self.urlString,
        url = NSURL(string: urlString),
        imageData = NSData(contentsOfURL: url),
        image = UIImage(data:imageData) {
            self._photo = image
        }
        return self._photo
    }
}

Answer 3

PhotoStruct 是一个struct，因此是一个值类型。对于值类型， 只有明确标记为mutating 的方法才能修改属性 self，所以这在计算属性中是不可能的。

如果您将PhotoStruct 更改为class，那么您的代码将编译 没有问题。