在 Xcode11 Beta 4 中将 String(format: , args) 与 SwiftUI 一起使用时出错 [英] Error using String(format: , args) with SwiftUI in Xcode11 Beta 4

问题描述

升级到 Xcode 11 Beta 4 后，我在使用 String(format: , args) 和 @State 属性时开始看到错误.请参阅下面的代码.第二个 Text 行抛出错误:

After upgrading to Xcode 11 Beta 4, I starting seeing an error when using String(format: , args) with @State property. See code below. Second Text line throws an error:

没有更多上下文的表达式类型字符串"是不明确的

Expression type 'String' is ambiguous without more context

而 Texts 1、3 和 4 工作得很好.

while Texts 1, 3, and 4 work just fine.

struct ContentView : View {
    @State var selection = 2

    var body: some View {
        VStack {
            Text("My selection \(selection)") // works
            Text("My selection \(String(format: "%02d", selection))") // error
            Text("My selection \(String(format: "%02d", Int(selection)))") // works
            Text("My selection \(String(format: "%02d", $selection.binding.value))") // works
        }
    }
}

我知道这是 Beta 版软件，但很好奇是否有人能看到这种行为的原因，或者这只是一个错误.如果这不能解释，我会提交雷达.

I realize this is Beta software, but was curious if anyone can see a reason for this behavior or is this simply a bug. If this can't be explained, I'll file a radar.

推荐答案

在 beta 4 中，属性包装器实现略有变化.在 beta 3 中，您的 View 被编译器重写为:

In beta 4, the property wrapper implementation changed slightly. In beta 3, your View was rewritten by the compiler as:

internal struct ContentView : View {
  @State internal var selection: Int { get nonmutating set }
  internal var $selection: Binding<Int> { get }
  @_hasInitialValue private var $$selection: State<Int>
  internal var body: some View { get }
  internal init(selection: Int = 2)
  internal init()
  internal typealias Body = some View
}

在 Beta 4 上，它会这样做:

while on Beta 4, it does this:

internal struct ContentView : View {
  @State @_projectedValueProperty($selection) internal var selection: Int { get nonmutating set }
  internal var $selection: Binding<Int> { get }
  @_hasInitialValue private var _selection: State<Int>
  internal var body: some View { get }
  internal init(selection: Int = 2)
  internal init()
  internal typealias Body = some View
}

现在我在猜测:这个变化让编译器更难推断你的变量的类型?请注意，另一个有效的替代方法是通过将 selection 强制转换为 Int 来帮助编译器:

Now I am guessing: this change makes it more difficult for the compiler to infer the type of your variable? Note that another alternative that does work, is helping the compiler a little, by casting selection as Int:

Text("My selection \(String(format: "%02d", selection as Int))")

这篇关于在 Xcode11 Beta 4 中将 String(format: , args) 与 SwiftUI 一起使用时出错的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！