AttributedString/Substring扩展:不能通过动态查找属性分配:下标是get-only

Question

如果我有以下内容:

let nsRange = NSRange(location: 5, length: 10)
var attributedString = AttributedString("This is some string that I want to attribute.")
// To attribute a substring I must first convert the range
let start = attributedString.index(attributedString.startIndex, offsetByUnicodeScalars: nsRange.location)
let end = attributedString.index(start, offsetByUnicodeScalars: nsRange.length)
// Now we have a Range<AttributedString.Index>
let range = start..<end
attributedString[range].foregroundColor = .red

^此代码有效^

但是如果我试图通过创建一个下标来删除一些样板文件，如下所示:

extension AttributedString {
subscript(_ range: NSRange) -> AttributedSubstring {
get {
let start = index(startIndex, offsetByUnicodeScalars: range.location)
let end = index(start, offsetByUnicodeScalars: range.length)
return self[start..<end]
}
}
}

attributedString[nsRange].foregroundColor = .red

^这行不通^

我得到一个Cannot assign through subscript: subscript is get-only错误。但我不明白为什么。attributedString[nsRange]返回一个AttributedSubstring,AttributedSubstring可以使用.foregroundColor = .red语法设置属性。它适用于Range<AttributedString.Index>，如str[start..<end].foregroundColor = .red，所以为什么不与我的下标?

Answer 1

AttributedString和AttributedSubstring是结构体。值类型

let range = start..
attributedString[range]。foregroundColor = .red

不起作用。只有当上面的声明是var attributedString- variable，而不是let attributedString- constant时，这才有效。

let substring = attributedString[nsRange]
substring。foregroundColor =r

不起作用。这只适用于var substring- variable

现在和

attributedString[nsRange].foregroundColor = .red

这不起作用，因为下标的内联返回是常量，与let

相同

Answer 2

您只为下标编写了一个getter。如果你想通过它来写(如错误说明)，你还需要一个setter:

set {
let start = index(startIndex, offsetByUnicodeScalars: range.location)
let end = index(start, offsetByUnicodeScalars: range.length)
self[start..<end] = newValue
}

aspi的观点是返回的结构体是一个副本(这在你从文档中给出的例子中也是正确的)。这些例子非常不同:

// This modifies a *copy* of `value`. It does *not* modify `xs`.
// It calls the subscript getter of `xs`.
var value = xs[i]
value.prop = "newThing"
// This modifies `xs`:
// It calls the subscript setter of `xs`.
xs[i].prop = "newThing"