Question

我创建了一个结构体，其中我有一个属性'_photo'，在调用计算属性'photo'时会延迟分配。我一直收到错误

无法在'self'中指定'_photo'

这是代码。我在Swift 1.1（photo1）和swift 1.2（photo2）语法中编写了计算方法。两者都给出了如上所述的编译时错误。

修复该错误需要进行哪些更改？

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

对于struct，如果要改变计算属性中的self属性，则必须将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 { }上的{{1}}功能没有记录。

Answer 2

Structs是值类型，它们是不可变的这意味着你不能设置变量和变异自我如果你需要改变struct，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
  }
}

在你的例子中，我将改变方法而不是属性

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struct，因此是值类型。对于价值类型，只有明确标记为mutating的方法才能修改属性 self的{{1}}，因此在计算属性中不可能。

如果您将PhotoStruct更改为class，那么您的代码会进行编译没有问题。

无法从计算属性中为存储属性赋值

3 个答案: