为什么我的 SwiftUI 结构不能在视图外初始化？

Question

I couldn't get my the below struct to initialise until I moved it into the body of the View. Can someone explain that to me please? Thanks!

This didn't work:

struct ContentView: View {
    var sampleText = "This is sample text"
    var booksArray = ["Book 1", "Book 2", "Book 3"]
    let aBook = Book(passage: sampleText, books: booksArray)
    
    var body: some View {
        ScrollView(.vertical, showsIndicators: false, content: {
            VStack {
                Text(aBook.passage)
                    .frame(width: 350, height: .infinity, alignment: .center)
                    .kerning(1)
            }
        })
    }
}

But this did work:

struct ContentView: View {
    var sampleText = "This is sample text"
    var booksArray = ["Book 1", "Book 2", "Book 3"]
    
    var body: some View {
        let aBook = Book(passage: sampleText, books: booksArray)
        ScrollView(.vertical, showsIndicators: false, content: {
            VStack {
                Text(aBook.passage)
                    .frame(width: 350, height: .infinity, alignment: .center)
                    .kerning(1)
            }
        })
    }
}

Answer 1

The error you got from this code reads:

cannot use instance member 'sampleText' within property initializer; property initializers run before 'self' is available

which in plain English means that when aBook is being assigned, self , which is the ContentView is not yet available, so you cannot use sampleText and booksArray to initialize aBook (because it requires all the properties to be initialized - to hold a value).

You might think, ok then I will use lazy to initialize aBook , which means that the assignment of the property will happen after ContentView has finished the initialization - at first access.

lazy var aBook = Book(passage: sampleText, books: booksArray)

but then a new error appears:

Text(aBook.passage) <= cannot use mutating getter on immutable value: 'self' is immutable

wait, what? But I'm not trying to mutate it you might say.

The reality though is that you do mutate ContentView by accessing aBook . Let's illustrate this:

1. sampleText is assigned a value
2. booksArray is assigned a value
3. aBook is not assigned since is lazy . Waits for access in order to be assigned a value.
4. ContentView is initalized
5. You access aBook in the body
6. aBook tries to assign a value, but because ContentView is a struct (value type - immutable) it needs to make a copy of the whole thing. Which is why you get the error

So, long story short you got a few options:

1. let aBook = Book(passage: sampleText, books: booksArray) in the body (as you already mention)

2. A computed property:

   var aBook: Book { Book(passage: sampleText, books: booksArray) }

or a custom initializer:

init(sampleText: String, booksArray: [String]) {
    self.sampleText = sampleText
    self.booksArray = booksArray
    self.aBook      = Book(passage: sampleText, books: booksArray)
}

or even:

let sampleText = "This is sample text"
let booksArray = ["Book 1", "Book 2", "Book 3"]
let aBook: Book

init() {
    self.aBook = Book(passage: sampleText, books: booksArray)
}

I hope that this makes sense.