函数产生预期类型“String”，你的意思是用“（）”调用它吗？ [英] Function produces expected type "String", did you mean to call it with "()"

问题描述

let documentUrl: NSURL? = {
    return NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first
}

var test: String = {
    return "String"
}

这是一段非常简单的代码，但是有错误来自Xcode，让我在计算属性的末尾添加（），如下所示：

This is a very simple piece of code, but there is an error from Xcode, asked me to add () at the end of the computed property, like this:

let documentUrl: NSURL? = {
        return NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first
    }()

var test: String = {
            return "String"
        }()

谁能告诉我原因？

推荐答案

您写道：

var test: String = {
    return "String"
}

那是 not 计算属性。您正在将变量 test 初始化为函数体（匿名函数，有时称为闭包）。那不是你的意思。您希望调用函数并将变量 test 设置为结果。括号使这种情况发生;这就是你调用一个函数的方式。因此：

That is not a computed property. You are initializing your variable test to a function body (an anonymous function, sometimes called a closure). That isn't what you mean to do. You want to call the function and set the variable test to the result. The parentheses make that happen; that is how you call a function. Thus:

var test: String = {
    return "String"
}()

因此，您可以一次性定义函数并调用它，并将结果指定为<的初始值 test 。

Thus you define the function and call it, all in one move, and assign the result as the initial value of test.

如果你想要一个计算属性，你应该这样写：

If you wanted a computed property, you should have written it like this:

var test: String {
    return "String"
}

请注意，没有等号。更全面的形式是：

Notice there is no equal sign. A fuller form would be:

var test: String {
    get {
        return "String"
    }
}

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