self 在 swift 项目中不可用

Question

我正在尝试运行在 swift 中进行图像比较。但是我收到一条错误消息，指出 Cannot use instance member 'featureprintObservationForImage' within property initializer; property initializers run before 'self' is available。错误出现在 let 行的第一行。

为什么 self 在 ViewController 中不可用？

import UIKit
import Vision

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    
    func featureprintObservationForImage(atURL url: URL) -> VNFeaturePrintObservation? {
        let requestHandler = VNImageRequestHandler(url: url, options: [:])
        let request = VNGenerateImageFeaturePrintRequest()
        do {
          try requestHandler.perform([request])
          return request.results?.first as? VNFeaturePrintObservation
        } catch {
          print("Vision error: \(error)")
          return nil
        }
      }
    
    let apple1 = featureprintObservationForImage(atURL:Bundle.main.url(forResource:"apple1", withExtension: "jpg")!)
    let apple2 = featureprintObservationForImage(atURL: Bundle.main.url(forResource:"apple2", withExtension: "jpg")!)
    let pear = featureprintObservationForImage(atURL: Bundle.main.url(forResource:"pear", withExtension: "jpg")!)
    var distance = Float(0)
    try apple1!.computeDistance(&distance, to: apple2!)
    var distance2 = Float(0)
    try apple1!.computeDistance(&distance2, to: pear!)
    
}

Answer 1

featureprintObservationForImage 函数右大括号后的所有代码都不在任何函数或闭包中。你不能那样做。（该代码不仅仅是变量声明。您有函数调用，这在函数之外是不合法的。）

您可以创建称为计算属性的东西，您可以在其中提供一个闭包，每次您从属性.

读取值时都会调用该闭包

Answer 2

您不能在变量声明中访问 self。

import UIKit
import Vision

class ViewController: UIViewController {
    
    var testVeriable : Int = 0//hear you cannot access `self`
    
    func test(){
        self.testVeriable = 1// In function you can access class variable or function using `self` 
    }
}

self 在 swift 项目中不可用

self is not available in swift project

xcode

swift

swift5