SwiftUI 为什么会自动将变量传递给其他视图？

Question

每次更改 ContentView 中的“位置”变量时，我的 MapView“注释”变量如何更新？我用谷歌搜索了 swift 数组是一种值类型，我什至不需要在“注释”上使用绑定并绑定到“位置”，以便“注释”了解“位置”何时发生变化，这是为什么呢？

import SwiftUI
import MapKit

struct ContentView: View {
    @State var centerCoordinate = CLLocationCoordinate2D()
    @State var locations = [MKPointAnnotation]()
    var body: some View {
        ZStack{
            MapView(centerCoordinate: $centerCoordinate, annotations: locations)
            Circle()
                .fill(Color.blue)
                .opacity(0.5)
                .frame(width: 32, height: 32, alignment: .center)
            VStack{
                Spacer()
                HStack{
                    Spacer()
                    Button(action: {
                        let newLocation = MKPointAnnotation()
                        newLocation.coordinate = centerCoordinate
                        locations.append(newLocation)
                    }){
                        Image(systemName: "plus")
                    }
                    .padding()
                    .background(Color.black.opacity(0.75))
                    .foregroundColor(.white)
                    .font(.title)
                    .clipShape(Circle())
                    .padding([.trailing, .bottom])
                }
            }
        }
        .edgesIgnoringSafeArea(.all)
    }
}

struct MapView: UIViewRepresentable{
    @Binding var centerCoordinate: CLLocationCoordinate2D
    var annotations: [MKPointAnnotation]
    
    func makeUIView(context: Context) -> MKMapView {
        let mapView = MKMapView()
        mapView.delegate = context.coordinator
        return mapView
    }
    // Callback function - when anything being sent to UIViewRepresentabel struct is changed
    func updateUIView(_ uiView: UIViewType, context: Context) {
        print("Updating UIView")
        if annotations.count != uiView.annotations.count{
            uiView.removeAnnotations(uiView.annotations)
            uiView.addAnnotations(annotations)
        }
    }
    
    func makeCoordinator() -> Coordinator {
        return Coordinator(self)
    }
    
    class Coordinator: NSObject, MKMapViewDelegate {
        let parent: MapView
        init(_ parent: MapView) {
            self.parent = parent
        }
        
        func mapViewDidChangeVisibleRegion(_ mapView: MKMapView) {
            parent.centerCoordinate = mapView.centerCoordinate
        }
    }
}

extension MKPointAnnotation{
    static var example: MKPointAnnotation{
        let point = MKPointAnnotation()
        point.coordinate = CLLocationCoordinate2D(latitude: 51.5, longitude: -0.13)
        point.title = "London"
        point.subtitle = "The home of 2012 Summer Olympics"
        return point
    }
}

Answer 1

不确定我是否理解你的问题，但让我试试。

首先你的视图中有这个状态：

@State var locations = [MKPointAnnotation]()

接下来，您将其值分配给：

MapView(centerCoordinate: $centerCoordinate, annotations: locations)

这意味着，每次您更改 locations 时，SwiftUI 都会使用新值重新渲染 MapView。

任何UIViewRepresentable 都必须实现函数updateUIView 以便在SwiftUI 重新渲染相应组件时接收这些更改。

所以，每次你的MapView被渲染时，这个函数都会被触发：

func updateUIView(_ uiView: UIViewType, context: Context) {
  print("Updating UIView")
  if annotations.count != uiView.annotations.count{
    uiView.removeAnnotations(uiView.annotations)
    uiView.addAnnotations(annotations)
  }
}

uiView.annotations 和 annotations 相互关联。目前uiView.annotations 发生变化，它只会在视图本身中产生副作用，并且不会更新给定的@State，因为它可能会创建无限渲染循环。