如何避免在Swift中拼写复杂类型？

huangapple
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英文:

How to avoid spelling a complex type in Swift?

问题

这是使用SwiftUI时出现的问题，但实际上是有关Swift类型的基本问题。尝试创建如下的视图：

  1. struct BorderedRectangle1: View {
  2.   let gesture: Gesture
  4.   init(color: Color, border: Bool) {
  5.     self.gesture = TapGesture().modifiers(.option).onEnded {
  6.       // do something
  7.     }.simultaneously(with: TapGesture().modifiers(.command).onEnded {
  8.       // do something else
  9.     })
  10.   }
  12.   var body: some View {
  13.     Rectangle()
  14.       .gesture(gesture)
  15.   }
  16. }

但这在let gesture: Gesture上不能编译，出现错误 Use of protocol 'Gesture' as a type must be written 'any Gesture'. Replace 'Gesture' with 'any Gesture'

通过将 let gesture: Gesture 替换为 let gesture: any Gesture，可以消除上述错误，但在body中的 Rectangle() 上会出现另一个错误：Type 'any Gesture' cannot conform to 'Gesture'.

我可以通过将gesture属性更改为计算属性来“修复”它：

  1.   var gesture: some Gesture {
  2.     TapGesture().modifiers(.option).onEnded {
  3.       // do something
  4.     }.simultaneously(with: TapGesture().modifiers(.command).onEnded {
  5.       // do something else
  6.     })  
  7.   }

我明白编译器必须能够在声明和使用位置都能够推断出类型。那么如何在保持gesture的声明为存储属性且不拼写显式类型的情况下使其工作呢？

  1. let gesture: ?? // <= 在这里填入什么？
英文:

This is coming up while using SwiftUI bit it really is a fundamental Swift question about types. Trying to have a view like so:

  1. struct BorderedRectangle1: View {
  2.   let gesture: Gesture
  4.   init(color: Color, border: Bool) {
  5.     self.gesture = TapGesture().modifiers(.option).onEnded {
  6.       // do something
  7.     }.simultaneously(with: TapGesture().modifiers(.command).onEnded {
  8.       // do something else
  9.     })
  10.   }
  12.   var body: some View {
  13.     Rectangle()
  14.       .gesture(gesture)
  15.   }
  16. }

but this doesn't compile with Use of protocol 'Gesture' as a type must be written 'any Gesture'. Replace 'Gesture' with 'any Gesture'

Replacing let gesture: Gesture => let gesture: any Gesture eliminates the above error but another error appears on Rectangle() in the body: Type 'any Gesture' cannot conform to 'Gesture'.

I was able to "fix" it by making the gesture property computed:

  1.   var gesture: some Gesture {
  2.     TapGesture().modifiers(.option).onEnded {
  3.       // do something
  4.     }.simultaneously(with: TapGesture().modifiers(.command).onEnded {
  5.       // do something else
  6.     })  }

I get it that the compiler must be able to infer a type in both the declaration and use sites. So how to make it work while keeping the declaration of gesture as a stored property and without spelling the explicit type which is SimultaneousGesture&lt;_EndedGesture&lt;_ModifiersGesture&lt;TapGesture&gt;&gt;, _EndedGesture&lt;_ModifiersGesture&lt;TapGesture&gt;&gt;&gt;

  1. let gesture: ?? // &lt;= what goes in here?

答案1

得分: 0

你需要声明一个泛型类型，该类型将是存储属性的类型。该泛型类型必须符合 Gesture。

这里是一个编译通过的代码示例：

  1. // 创建一个符合 Gesture 的泛型类型
  2. struct BorderedRectangle1<G: Gesture>: View {
  3.     
  4.     // 属性的类型是 G
  5.     let gesture: G
  6.     
  7.     init(color: Color, border: Bool) {
  8.         self.gesture = TapGesture().modifiers(.option).onEnded {
  9.             // 做一些事情
  10.         }.simultaneously(with: TapGesture().modifiers(.command).onEnded {
  11.             // 做另一些事情
  12.         }) as! G        // 确保 gesture 的类型是 G
  13.     }
  14.     
  15.     var body: some View {
  16.         Rectangle()
  17.             .gesture(gesture)
  18.     }
  19. }

如果你需要将手势传递给 View，以下是如何实现的示例：

  1. // 创建一个符合 Gesture 的泛型类型
  2. struct BorderedRectangle1<G: Gesture>: View {
  3.     
  4.     // 属性的类型是 G
  5.     let gesture: G
  6.     
  7.     // 参数的类型是 G
  8.     init(gesture: G) {
  9.         self.gesture = gesture
  10.     }
  11.     
  12.     var body: some View {
  13.         Rectangle()
  14.             .gesture(gesture)
  15.     }
  16. }
英文:

You need to declare a generic type, which will be the type of the stored property. That generic type must conform to Gesture.

Here's a code that compiles:

  2. // Create a generic type that conforms to Gesture
  3. struct BorderedRectangle1&lt;G: Gesture&gt;: View {
  4.     
  5.     // The property is of type G
  6.     let gesture: G
  7.     
  8.     init(color: Color, border: Bool) {
  9.         self.gesture = TapGesture().modifiers(.option).onEnded {
  10.             // do something
  11.         }.simultaneously(with: TapGesture().modifiers(.command).onEnded {
  12.             // do something else
  13.         }) as! G        // Ensure the gesture is of type G
  14.     }
  15.     
  16.     var body: some View {
  17.         Rectangle()
  18.             .gesture(gesture)
  19.     }
  20. }

If you need to pass a gesture to the View, here's how it works:

  1. // Create a generic type that conforms to Gesture
  2. struct BorderedRectangle1&lt;G: Gesture&gt;: View {
  3.     
  4.     // The property is of type G
  5.     let gesture: G
  6.     
  7.     // The parameter is of type G
  8.     init(gesture: G) {
  9.         self.gesture = gesture
  10.     }
  11.     
  12.     var body: some View {
  13.         Rectangle()
  14.             .gesture(gesture)
  15.     }
  16. }

huangapple
  • 本文由 发表于 2023年5月29日 22:26:15
  • 转载请务必保留本文链接：https://go.coder-hub.com/76358160.html
  • swift
  • swiftui
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