Swift:访问结构字典中的属性

Question

如果我制作一个结构数组，一切都按预期工作，但是如果我制作一个结构字典，它们的属性将不再可访问，是否有额外的步骤来访问它们？请参阅下面的简单示例:

struct SceneStruct {
   var number: Int

   init(number: Int) {
       self.number = number
   }
}
let aStruct = SceneStruct(number: 0)  //shows in assistant editor as:  {number 0}
let bStruct = SceneStruct(number: 26) //shows in assistant editor as: {number 26}

//EXPECTED BEHAVIOR:
let arr = [aStruct,bStruct] //shows in assistant editor as: [{number 0}, {number 26}]
var i = arr[0] //shows in assistant editor as: {number 0}
var j = i.number //shows in assistant editor as: 0

//BROKEN BEHAVIOR:
let dict = ["one": aStruct, "two": bStruct] //shows in assistant editor as: ["one": {number 0}, "two": {number 26}]
var x = dict["one"] //shows in assistant editor as: {{number 0}}
var y = x.number // Fails: "error: 'SceneStruct?' does not have a member named 'number'"

看起来对象以某种方式包装在字典中，但就我的生活而言，我找不到有关此行为的任何信息。

Answer 1

对于数组，下标运算符返回值。

var i = arr[0] // <- i is of type SceneStruct

对于字典，下标运算符返回一个可选值。

var x = dict["one"] // <- x is of type SceneStruct? (Optional< SceneStruct>)

原因是arr[100]因为数组越界而抛出异常。而 dict["invalid"] 返回 nil 查找失败。

您可以通过几种方式解决此差异。

用默认值求值

var defaultValue = SceneStruct(number: 0)
var x = dict["one"] ?? defaultValue // provide a default value if dict["one"] is nil.
var y = x.number

条件评估

if let x = dict["one"] { // conditionally set x
    var y = x.number
} else {
    // dict["one"] is nil
}

强制展开

var x = dict["one"]! // force unwrap dict["one"], this will throw an exception if dict["one"] is nil. 
var y = x.number