在 Combine 的 `assign(to:on:)` 主题中分配给 @State 不会导致视图更新
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【中文标题】在 Combine 的 `assign(to:on:)` 主题中分配给 @State 不会导致视图更新【英文标题】：Assigning to @State in Combine's `assign(to:on:)` subject does not result in view update 【发布时间】：2020-03-26 06:34:44 【问题描述】：我有一个地图视图，它有一个按钮，当按下该按钮时，应该将地图居中放在用户的当前位置上。我正在尝试使用 Swift 的 Combine 框架来实现这一点。我尝试通过添加一个名为mapCenter 的@State 属性并在Combine 的assign(to:on:) 主题中分配给该属性来解决此问题，如下所示：
struct MapWithButtonView: View
// What the current map view center should be.
@State var mapCenter = CLLocationCoordinate2D(latitude: 42.35843, longitude: -71.05977)
// A subject whose `send(_:)` method is being called from within the CenterButton view to center the map on the user's location.
private var centerButtonTappedPublisher = PassthroughSubject<Bool, Never>()
// A publisher that turns a "center button tapped" event into a coordinate.
private var centerButtonTappedCoordinatePublisher: AnyPublisher<CLLocationCoordinate2D?, Never>
centerButtonTappedPublisher
.map _ in LocationManager.default.currentUserCoordinate
.eraseToAnyPublisher()
private var coordinatePublisher: AnyPublisher<CLLocationCoordinate2D, Never>
Publishers.Merge(LocationManager.default.$initialUserCoordinate, centerButtonTappedCoordinatePublisher)
.replaceNil(with: CLLocationCoordinate2D(latitude: 42.35843, longitude: -71.05977))
.eraseToAnyPublisher()
private var cancellableSet: Set<AnyCancellable> = []
init()
// This does not result in an update to the view... why not?
coordinatePublisher
.receive(on: RunLoop.main)
.handleEvents(receiveSubscription: (subscription) in
print("Receive subscription")
, receiveOutput: output in
print("Received output: \(String(describing: output))")
, receiveCompletion: _ in
print("Receive completion")
, receiveCancel:
print("Receive cancel")
, receiveRequest: demand in
print("Receive request: \(demand)")
)
.assign(to: \.mapCenter, on: self)
.store(in: &cancellableSet)
var body: some View
ZStack
MapView(coordinate: mapCenter)
.edgesIgnoringSafeArea(.all)
CenterButton(buttonTappedPublisher: centerButtonTappedPublisher)
MapView 是一个 UIViewRepresentable 视图，如下所示：
struct MapView: UIViewRepresentable
// The center of the map.
var coordinate: CLLocationCoordinate2D
func makeUIView(context: Context) -> MKMapView
let mapView = MKMapView(frame: .zero)
mapView.showsUserLocation = true
return mapView
func updateUIView(_ view: MKMapView, context: Context)
let span = MKCoordinateSpan(latitudeDelta: 0.02, longitudeDelta: 0.02)
let region = MKCoordinateRegion(center: coordinate, span: span)
view.setRegion(region, animated: true)
CenterButton 是一个简单的按钮，如下所示：
struct CenterButton: View
var buttonTappedPublisher: PassthroughSubject<Bool, Never>
var body: some View
Button(action:
self.buttonTappedPublisher.send(true)
)
Image(systemName: "location.fill")
.imageScale(.large)
.accessibility(label: Text("Center map"))
而LocationManager 是一个ObservableObject，它发布了用户的当前和初始位置：
class LocationManager: NSObject, ObservableObject
// The first location reported by the CLLocationManager.
@Published var initialUserCoordinate: CLLocationCoordinate2D?
// The latest location reported by the CLLocationManager.
@Published var currentUserCoordinate: CLLocationCoordinate2D?
private let locationManager = CLLocationManager()
static let `default` = LocationManager()
private override init()
super.init()
locationManager.delegate = self
locationManager.desiredAccuracy = kCLLocationAccuracyBest
locationManager.pausesLocationUpdatesAutomatically = true
locationManager.activityType = .other
locationManager.requestWhenInUseAuthorization()
extension LocationManager: CLLocationManagerDelegate
func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus)
switch status
case .authorizedAlways, .authorizedWhenInUse:
NSLog("Location authorization status changed to '\(status == .authorizedAlways ? "authorizedAlways" : "authorizedWhenInUse")'")
enableLocationServices()
case .denied, .restricted:
NSLog("Location authorization status changed to '\(status == .denied ? "denied" : "restricted")'")
disableLocationServices()
case .notDetermined:
NSLog("Location authorization status changed to 'notDetermined'")
default:
NSLog("Location authorization status changed to unknown status '\(status)'")
func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation])
// We are only interested in the user's most recent location.
guard let location = locations.last else return
// Use the location to update the location manager's published state.
let coordinate = location.coordinate
if initialUserCoordinate == nil
initialUserCoordinate = coordinate
currentUserCoordinate = coordinate
func locationManager(_ manager: CLLocationManager, didFailWithError error: Error)
NSLog("Location manager failed with error: \(error)")
// MARK: Helpers.
private func enableLocationServices()
locationManager.startUpdatingLocation()
private func disableLocationServices()
locationManager.stopUpdatingLocation()
不幸的是，上述方法不起作用。当CenterButton 被点击时，视图永远不会更新。我最终通过使用具有@Published var mapCenter 属性的ObservableObject 视图模型对象解决了这个问题，但是我不知道为什么我使用@State 的初始解决方案不起作用。像我上面所做的那样更新@State 有什么问题？
请注意，如果尝试重现此问题，您需要在 Info.plist 文件中添加具有诸如“此应用需要访问您的位置”之类的值的 NSLocationWhenInUseUsageDescription 键，以便能够授予位置权限.
【问题讨论】：
你检查过这个吗？ ***.com/questions/57799548/… 向我们展示您的地图视图代码.... @Chris 我已经添加了MapView 和CenterButton 代码:)
可能你的locationmanager有缺陷......我们不知道，因为你没有向我们展示所有相关代码，只是一些代码......
@Chris，我很抱歉遗漏了代码。我包含了我认为是问题所在的代码。我不希望像你这样的读者不必阅读一堆其他代码。但显然这是必要的，所以我道歉。我现在已经包含了所有代码。什么都没有。
【参考方案1】：
新答案：
好的，我是一个组合新手，但它并没有让我离开，所以我尝试了又尝试......现在它可以工作了，即使在模拟器中也是如此。
问题是，您在 struct/Contentview 中完成了所有操作，而不是在发布您想要更改的值的单独类中进行组合。
看看这个：
class Model : ObservableObject
@Published var mapCenter : CLLocationCoordinate2D = CLLocationCoordinate2D(latitude: 0, longitude: 0)
// A subject whose `send(_:)` method is being called from within the CenterButton view to center the map on the user's location.
public var centerButtonTappedPublisher = PassthroughSubject<Bool, Never>()
// A publisher that turns a "center button tapped" event into a coordinate.
private var centerButtonTappedCoordinatePublisher: AnyPublisher<CLLocationCoordinate2D?, Never>
centerButtonTappedPublisher
.map _ in
print ("new loc in pub: ", LocationManager.default.currentUserCoordinate)
return LocationManager.default.currentUserCoordinate
.eraseToAnyPublisher()
private var coordinatePublisher: AnyPublisher<CLLocationCoordinate2D, Never>
Publishers.Merge(LocationManager.default.$initialUserCoordinate, centerButtonTappedCoordinatePublisher)
.replaceNil(with: CLLocationCoordinate2D(latitude: 2.0, longitude: 2.0))
.eraseToAnyPublisher()
private var cancellableSet: Set<AnyCancellable> = []
var cancellable: AnyCancellable?
init()
// This does not result in an update to the view... why not?
coordinatePublisher
.receive(on: RunLoop.main)
// .handleEvents(receiveSubscription: (subscription) in
// print("Receive subscription")
// , receiveOutput: output in
// print("Received output: \(String(describing: output))")
//
// , receiveCompletion: _ in
// print("Receive completion")
// , receiveCancel:
// print("Receive cancel")
// , receiveRequest: demand in
// print("Receive request: \(demand)")
// )
.assign(to: \.mapCenter, on: self)
.store(in: &cancellableSet)
print(cancellableSet)
self.cancellable = self.coordinatePublisher.receive(on: DispatchQueue.main)
.assign(to: \.mapCenter, on: self)
struct ContentView: View
@ObservedObject var model = Model()
var body: some View
VStack
MapView(coordinate: model.mapCenter)
.edgesIgnoringSafeArea(.all)
CenterButton(buttonTappedPublisher: model.centerButtonTappedPublisher)
struct MapView: UIViewRepresentable
// The center of the map.
var coordinate: CLLocationCoordinate2D
let mapView = MKMapView(frame: .zero)
func makeUIView(context: Context) -> MKMapView
mapView.showsUserLocation = true
return mapView
func updateUIView(_ view: MKMapView, context: Context)
let span = MKCoordinateSpan(latitudeDelta: 0.02, longitudeDelta: 0.02)
let region = MKCoordinateRegion(center: coordinate, span: span)
print("map new coordinate", coordinate)
view.setRegion(region, animated: true)
class LocationManager: NSObject, ObservableObject
// The first location reported by the CLLocationManager.
@Published var initialUserCoordinate: CLLocationCoordinate2D?
// The latest location reported by the CLLocationManager.
@Published var currentUserCoordinate: CLLocationCoordinate2D?
private let locationManager = CLLocationManager()
static let `default` = LocationManager()
private override init()
super.init()
locationManager.delegate = self
locationManager.desiredAccuracy = kCLLocationAccuracyBest
locationManager.pausesLocationUpdatesAutomatically = true
locationManager.activityType = .other
locationManager.requestWhenInUseAuthorization()
enableLocationServices()
extension LocationManager: CLLocationManagerDelegate
func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus)
switch status
case .authorizedAlways, .authorizedWhenInUse:
NSLog("Location authorization status changed to '\(status == .authorizedAlways ? "authorizedAlways" : "authorizedWhenInUse")'")
()
case .denied, .restricted:
NSLog("Location authorization status changed to '\(status == .denied ? "denied" : "restricted")'")
disableLocationServices()
case .notDetermined:
NSLog("Location authorization status changed to 'notDetermined'")
default:
NSLog("Location authorization status changed to unknown status '\(status)'")
func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation])
// We are only interested in the user's most recent location.
guard let location = locations.last else return
// Use the location to update the location manager's published state.
let coordinate = location.coordinate
if initialUserCoordinate == nil
initialUserCoordinate = coordinate
currentUserCoordinate = coordinate
func locationManager(_ manager: CLLocationManager, didFailWithError error: Error)
NSLog("Location manager failed with error: \(error)")
// MARK: Helpers.
public func enableLocationServices()
locationManager.startUpdatingLocation()
private func disableLocationServices()
locationManager.stopUpdatingLocation()
struct CenterButton: View
var buttonTappedPublisher: PassthroughSubject<Bool, Never>
var body: some View
Button(action:
self.buttonTappedPublisher.send(true)
)
Image(systemName: "location.fill")
.imageScale(.large)
.accessibility(label: Text("Center map"))
struct ContentView_Previews: PreviewProvider
static var previews: some View
ContentView()
旧答案：
好的，因为你没有给我们一个可复制的可重现的例子，我做了一个简单的例子。随意复制并使用它来解决您的问题。
struct MapView: UIViewRepresentable
var coordinate: CLLocationCoordinate2D
func makeUIView(context: Context) -> MKMapView
let mapView = MKMapView(frame: .zero)
mapView.showsUserLocation = true
return mapView
func updateUIView(_ view: MKMapView, context: Context)
let span = MKCoordinateSpan(latitudeDelta: 1.02, longitudeDelta: 1.02)
let region = MKCoordinateRegion(center: coordinate, span: span)
view.setRegion(region, animated: true)
struct ContentView: View
@State var coordinate = CLLocationCoordinate2D(latitude: 0, longitude: 0)
var body: some View
VStack
Button(action:
self.coordinate.latitude += 10
self.coordinate.longitude = 30
)
Text("new coordinate")
MapView(coordinate: coordinate)
【讨论】：
谢谢@Chris！我已经更新了我的问题以提供完整的、可重复的示例。我再次为最初遗漏它而道歉。 嘿@Chris，感谢您花时间解决这个问题！我很高兴您至少在此过程中学到了很多关于 Combine 的知识 :) 您更新的解决方案确实有效！ “问题是，你在 struct/Contentview 中完成了所有工作，而不是在一个单独的类中进行组合，它发布了你想要更改的值” - 你知道为什么这需要在一个单独的类中发生吗？我想我只是不明白为什么直接分配给assign(to:on:) 中的@State 属性不起作用。【参考方案2】：
是的，您的代码有效。试试这个：
var body: some View
ZStack
MapView(coordinate: mapCenter)
.edgesIgnoringSafeArea(.all)
CenterButton(buttonTappedPublisher: centerButtonTappedPublisher)
.onAppear()
Timer.scheduledTimer(withTimeInterval: 1, repeats: true) (timer) in
self.mapCenter.latitude += 0.1
您会看到，地图在不断移动。 也许您在模拟器中尝试过您的代码？那里的用户位置永远不会改变，所以如果你再次点击你的按钮什么也不会发生......在真实设备上尝试并开始移动；）
【讨论】：
嘿@Chris！我的代码可以在真实设备上运行吗？我在模拟器上试了一下，是的。然而，在模拟器的Debug -> Location 菜单中，我选择了“City Run”，它模拟用户在加利福尼亚州库比蒂诺四处走动，因此理论上点击“中心地图”按钮应该会导致地图重新居中，即使在模拟器上也是如此.你确定我做的事情正确吗？我目前没有开发者帐号，所以无法在真机上试用代码。
我不知道 - 没有在设备上测试。仅使用我的代码就可以了-这就是我的意思。您的代码中的问题是地图中心永远不会改变，尽管当前用户位置发生了变化.....所以它永远不会更新地图视图
问题一定出在您的发布者链中...输出显示正确的值（坐标），但 mapcenter 没有获得新的/更新的值...。
我更新了我的答案，它现在可以工作了；）我学到了很多关于结合的知识；）【参考方案3】：
您不需要CLLocationManager，只需在UIViewRepresentable 中访问当前用户注释，例如map.userLocation.location.coordinate。
https://developer.apple.com/documentation/mapkit/mkuserlocation/1452415-location?language=objc
【讨论】：
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