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State

A property wrapper type that can read and write a value managed by SwiftUI. https://developer.apple.com/documentation/swiftui/state

@State

In a View's struct, you can't change a value of properties directly, whereas a wrapped property using @State attribute can be changed its value by SwiftUI. State properties change, then SwiftUI re-renders the View that refers to these.

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ObservableObject

A type of object with a publisher that emits before the object has changed. https://developer.apple.com/documentation/combine/observableobject

This protocol lets you sync any views referring to a property of a class conformed to it.

@ObservedObject

Well, we have a class declared as below:

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// 具象型 A
struct A;

// A より先に <A> がない。
// また、A は具象型である。
// よって Single も具象型になる。
struct Single(A);

// T よりも先に <T> が使用されている。
// よって型パラメータ T はジェネリック型である。