Swift struct初始化，第一个已经提供了所有初始值仍然无法初始化；第二个能够初始化
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SwiftUI doesn’t allow you to change @State in the initializer but you can initialize it.
Remove the default value and use _fullText to set @State directly instead of going through the property wrapper accessor.
@State var fullText: String // No default value of “”
init(letter: String) {
_fullText = State(initialValue: list[letter]!)
}
`