Question

升级到Xcode 11 Beta 4之后，在将String(format: , args)与@State属性一起使用时，我开始看到错误。请参见下面的代码。 Text第二行引发错误：

表达式类型'String'不明确，没有更多上下文

同时Text的1、3和4工作正常。

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
        }
    }
}

我意识到这是Beta版软件，但很好奇是否有人可以看到此行为的原因，或者仅仅是一个错误。如果无法解释，我将提交雷达报告。

Answer 1

在beta 4中，属性包装器的实现略有变化。在beta 3中，编译器将您的View重写为：

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，它执行以下操作：

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
}

现在我正在猜测：此更改使编译器更难推断变量的类型？请注意，另一个可行的替代方法是通过强制转换selection as Int：

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

Answer 2

更新（Xcode 11.2）

我也收到错误消息：

struct ContentView : View { @State var selection = 2 var body: some View { VStack { Text(String(format: "%d", selection)) // does not work } } }

使用以下代码：

通过添加wrappedValue前缀然后在String(format:, args:)中访问struct ContentView : View { @State var selection = 2 var body: some View { VStack { Text(String(format: "%d", $selection.wrappedValue)) // works } } }来解决：

        SocketConfig socketConfig = SocketConfig.custom().setSoKeepAlive(true).setSoTimeout(360000000).build(); //We need to set socket keep alive
    RequestConfig requestConfig = RequestConfig.custom().setConnectTimeout(360000000).build();
    CloseableHttpClient client = HttpClientBuilder.create().setDefaultRequestConfig(requestConfig).
            setDefaultSocketConfig(socketConfig).build();

在Xcode11 Beta 4中将String（format：，args）与SwiftUI一起使用时出错

2 个答案: