英文:
@State variable $ sign usage
问题
Learning SwiftUI and having some difficulty in understanding @State. For example in the code below, why don't we use State variable (that is with $ sign) with if statement? Why only as Toggle argument? How do we differentiate both the states?
学习SwiftUI并且在理解@State时遇到一些困难。例如，在下面的代码中，为什么我们不在if语句中使用State变量（带有$符号）？为什么只在Toggle参数中使用？我们如何区分这两种状态？
英文:
Learning SwiftUI and having some difficulty in understanding @State. For example in the code below, why don't we use State variable (that is with $ sign) with if statement? Why only as Toggle argument? How do we differentiate both the states?
import SwiftUI
struct ContentView: View {
@State private var isFrown = true
var body: some View {
VStack
{
Text ("Check toggle state")
Toggle(isOn: $isFrown) {
Text("")
.padding()
if isFrown { //why not $isFrown here
Text("🤨")
}
else {
Text("😃")
}
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
答案1
得分: 3
@State 是一个属性包装器结构，它只是包装任何值，以确保您的视图在该值更改时刷新或重绘。
(https://developer.apple.com/documentation/swiftui/state)
使用 $ 美元符号可以访问状态结构的 projectedValue，在这种情况下，它提供了一个 Binding (https://developer.apple.com/documentation/swiftui/binding)，最好通过文档解释：
使用绑定来创建视图与其底层模型之间的双向连接。例如，您可以在 Toggle 和 State 的 Bool 属性之间创建一个绑定。与切换控件交互会更改 Bool 的值，而更改 Bool 的值会导致切换更新其呈现的状态。
英文:
@State is a property wrapper struct that just wraps any value to make sure your view will refresh or redraw whenever that value changes.
(https://developer.apple.com/documentation/swiftui/state)
Using a $ dollar sign provides access to the projectedValue of the state struct, which in this case gives a Binding (https://developer.apple.com/documentation/swiftui/binding), best explained by the documentation:
> Use a binding to create a two-way connection between a view and its
> underlying model. For example, you can create a binding between a
> Toggle and a Bool property of a State. Interacting with the toggle
> control changes the value of the Bool, and mutating the value of the
> Bool causes the toggle to update its presented state.
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