swiftui - SwiftUI 中的记忆化？

Question

我来自 React，在那里我使用 useMemo 来确保某些计算不会过于频繁地执行。我如何在 SwiftUI 中做类似的事情？

考虑这个例子:

struct MyView: View {
    var records: [Record]

    var body: some View {
        Text("expensive summary: \(self.expensiveSummary)")
    }

    var expensiveSummary: String {
        // based on records, return string
        // containing a summary of records

        return ""
    }
}

有什么方法可以确保我的 expensiveSummary 仅在我的记录数组更改时被调用？

Answer 1

我认为我们可以创建一个包装器 View 作为 Equatable。(也许，我们可以使用 EquatableView 或 .equatable() 修饰符吗？)

struct UseMemo<KeyType: Equatable, ViewType: View>: View, Equatable {
    let key: KeyType
    let content: (KeyType) -> ViewType

    var body: some View {
        content(key)
    }
    
    static func ==(lhs: UseMemo, rhs: UseMemo) -> Bool {
        return lhs.key == rhs.key
    }
}

struct MyView: View {
    @State var records: [Int] = (Array<Int>)(1...10000)
    @State var other = false

    var body: some View {
        VStack {
            Button("Update other", action: { self.other.toggle() })
            Text("Other: \(String(other))") // This will be called everytime

            Button("Update records", action: { self.records.append(1) })
            
            UseMemo(key: records) { keys in
                // This is not called even if updated "other"
                // This is called when updates "records"
                Text("expensive summary: \(expensiveSummary)")
            }
            
            // This will be called everytime
            Text("expensive summary: \(expensiveSummary)")
        }
    }
    
    var expensiveSummary: String {
        String(records.randomElement()!)
    }
}

以下内容对我有帮助。

• SwiftUI — Why should you write as many custom views for better performance
• SwiftUI: prevent View from refreshing when presenting a sheet