问题描述
class X {
static let global: [String:String] = [
"x":"x data",
"y":"y data",
"z":"z data"
]
func test(){
let type = "x"
var data:String = X.global[type]!
}
}
我收到错误:可选类型字符串?"的值未解包
.
为什么我需要在 X.global[type]
之后使用 !
?我的字典中没有使用任何可选的?
Why do I need to use !
after X.global[type]
? I'm not using any optional in my dictionary?
已编辑:
即使该类型可能不存在 X.global[type]
,强制解包仍然会在运行时崩溃.更好的方法可能是:
Even if X.global[type]
may not exist for the type, force unwrapping will still crash on runtime. A better approach may be:
if let valExist = X.global[type] {
}
但是 Xcode 通过暗示可选类型给了我错误的想法.
but Xcode is giving me the wrong idea by hinting about optional type.
推荐答案
字典访问器返回其值类型的可选,因为它不知道"运行时字典中是否存在某个键.如果存在,则返回关联的值,如果不存在,则返回 nil
.
Dictionary accessor returns optional of its value type because it does not "know" run-time whether certain key is there in the dictionary or not. If it's present, then the associated value is returned, but if it's not then you get nil
.
来自 文档:
您还可以使用下标语法从字典中检索特定键的值.因为可以请求不存在值的键,所以字典的下标返回字典值类型的可选值.如果字典包含请求键的值,则下标返回一个可选值,其中包含该键的现有值.否则下标返回nil...
为了正确处理这种情况,您需要解开返回的可选项.
In order to handle the situation properly you need to unwrap the returned optional.
有几种方法:
选项 1:
func test(){
let type = "x"
if var data = X.global[type] {
// Do something with data
}
}
选项 2:
func test(){
let type = "x"
guard var data = X.global[type] else {
// Handle missing value for "type", then either "return" or "break"
}
// Do something with data
}
选项 3:
func test(){
let type = "x"
var data = X.global[type] ?? "Default value for missing keys"
}
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