SwiftUI：避免使用 MKMapView+UIViewRepresentable 在 TabView 中重新创建/重新渲染视图

Question

I have a TabView with three tabs, one of which contains a map view that's implemented like this:

struct MapView: UIViewRepresentable {
    let region: MKCoordinateRegion
    let animatedRegion: Bool

    func makeUIView(context: Context) -> MKMapView {
        let mapView = MKMapView(frame: .zero)
        mapView.delegate = context.coordinator
        mapView.showsUserLocation = true
        mapView.setRegion(region, animated: animatedRegion)
        return mapView
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func updateUIView(_ mapView: MKMapView, context: Context) {

    }

    class Coordinator: NSObject, MKMapViewDelegate {
        var control: MapView

        init(_ control: MapView) {
            self.control = control
        }
    }
}

The tab view is implemented like this:

TabView(selection: $selection) {
    MapView(/* params */)
        .tabItem {
            Image(systemName: "1.square.fill")
            Text("map")
        }.tag(1)
    Text("Screen #2")
        .tabItem {
            Image(systemName: "2.square.fill")
            Text("2")
        }.tag(2)
    Text("Screen #3")
        .tabItem {
            Image(systemName: "3.square.fill")
            Text("3")
        }.tag(3)
}

The problem is that the makeUIView(:context) method is executed every time I switch back to the map tab from one of the other two tabs. It appears that the underlying MKMapView instance is deallocated when I switch to another tab, then it's recreated when I switch back. In UIKit it doesn't rerender the whole view like that. Am I doing something wrong, or is there anything I can do to make sure that the underlying MKMapView instance is retained when I switch back so that it doesn't have to recreate it every single time?

Answer 1

You need to store instance of MKMapView for each tab and destroy before you leaving the view because SwiftUI will destroy MapView when it need to render for example the binding vars have changed.

struct MapView: UIViewRepresentable {
    @Binding var currentTab: Int
    static private var mapViews = [Int: MKMapView]()

    let region: MKCoordinateRegion
    let animatedRegion: Bool

    func makeUIView(context: Context) -> MKMapView {
        guard MapView.mapViews[currentTab] != nil else { return MapView.mapViews[currentTab]! }

        let mapView = MKMapView(frame: .zero)
        mapView.delegate = context.coordinator
        mapView.showsUserLocation = true
        mapView.setRegion(region, animated: animatedRegion)

        MapView.mapViews[currentTab] = mapView

        return mapView
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func updateUIView(_ mapView: MKMapView, context: Context) {

    }

    class Coordinator: NSObject, MKMapViewDelegate {
        var control: MapView

        init(_ control: MapView) {
            self.control = control
        }
    }
}

Answer 2

use EnvironmentObject. always update it and pass it in TabView {MapView(coordinates: Object.coordinates)}