SwiftUI:强制更新

Question

通常情况下，我们不能讨论 Apple 预发布的东西，但我已经看到很多关于 SwiftUI 的讨论，所以我怀疑这没问题；就这一次。

我正在学习其中一个教程(我这样做)。

我在“与 UIKit 交互”教程中的可滑动屏幕下方添加了一对按钮:https://developer.apple.com/tutorials/swiftui/interfacing-with-uikit

这些是“下一个”和“上一个”按钮。在一端或另一端时，相应的按钮会隐藏。我有那个工作正常。

我遇到的问题是访问由 PageViewController 表示的 UIPageViewController 实例。

我更改了 currentPage 属性(通过使 PageViewController 成为 UIPageViewController 的委托(delegate))，但我需要强制 UIPageViewController 以编程方式更改。

我知道我可以通过重绘 PageView 主体来“强制”显示，反射(reflect)一个新的 currentPage，但我不确定该怎么做。

struct PageView<Page: View>: View {
    var viewControllers: [UIHostingController<Page>]
    @State var currentPage = 0

    init(_ views: [Page]) {
        self.viewControllers = views.map { UIHostingController(rootView: $0) }
    }

    var body: some View {
        VStack {
            PageViewController(controllers: viewControllers, currentPage: $currentPage)

            HStack(alignment: .center) {
                Spacer()

                if 0 < currentPage {
                    Button(action: {
                        self.prevPage()
                    }) {
                        Text("Prev")
                    }

                    Spacer()
                }

                Text(verbatim: "Page \(currentPage)")

                if currentPage < viewControllers.count - 1 {
                    Spacer()

                    Button(action: {
                        self.nextPage()
                    }) {
                        Text("Next")
                    }
                }

                Spacer()
            }
        }
    }

    func nextPage() {
        if currentPage < viewControllers.count - 1 {
            currentPage += 1
        }
    }

    func prevPage() {
        if 0 < currentPage {
            currentPage -= 1
        }
    }
}

我知道答案应该很明显，但我很难弄清楚如何以编程方式刷新 VStack 或主体。

Answer 1

2021 SWIFT 1 和 2:

重要的事情!如果你搜索这个 hack，可能你做错了什么!请在阅读 hack 解决方案之前阅读此 block !!!!!!!!!!!!

Your UI wasn't updated automatically because of you miss something important.

• Your ViewModel must be a class wrapped into ObservableObject/ObservedObject
• Any field in ViewModel must be a STRUCT. NOT A CLASS!!!! Swift UI does not work with classes!
• Must be used modifiers correctly (state, observable/observedObject, published, binding, etc)
• If you need a class property in your View Model (for some reason) - you need to mark it as ObservableObject/Observed object and assign them into View's object !!!!!!!! inside init() of View. !!!!!!!
• Sometimes is needed to use hacks. But this is really-really-really exclusive situation! In most cases this wrong way! One more time: Please, use structs instead of classes!

Your UI will be refreshed automatically if all of written above was used correctly.

正确用法示例:

struct SomeView : View {
    @ObservedObject var model : SomeViewModel
    @ObservedObject var someClassValue: MyClass
    
    init(model: SomeViewModel) {
        self.model = model
    
        //as this is class we must do it observable and assign into view manually
        self.someClassValue = model.someClassValue
    }

    var body: some View {
         //here we can use model.someStructValue directly

         // or we can use local someClassValue taken from VIEW, BUT NOT value from model

    }

}

class SomeViewModel : ObservableObject {
    @Published var someStructValue: Bool = false
    var someClassValue: MyClass = MyClass() //myClass : ObservableObject

}

以及主题问题的答案。

(黑客解决方案 - 最好不要使用它)

方式一:在 View 内部声明:

@State var updater: Bool = false

所有你需要更新的是 toogle() 它:updater.toogle()

---

方式二:从ViewModel刷新

Works on SwiftUI 2

public class ViewModelSample : ObservableObject
    func updateView(){
        self.objectWillChange.send()
    }
}

---

方式三:从ViewModel刷新:

works on SwiftUI 1

import Combine
import SwiftUI

class ViewModelSample: ObservableObject {
    private let objectWillChange = ObservableObjectPublisher()

    func updateView(){
        objectWillChange.send()
    }
}