Display an offline map (on-demand)

Learn how to download and display an offline map for a user-defined geographical area of a web map.

display offline map on demand

Offline maps allow users to continue working when network connectivity is poor or lost. If a web map is enabled for offline use, a user can request that ArcGIS generates an offline map for a specified geographic area of interest.

In this tutorial, you will download an offline map for an area of interest from the web map of the stormwater network within Naperville, IL, USA . You can then use this offline map without a network connection.

Prerequisites

The following are required for this tutorial:

  1. An ArcGIS account to access your API keys. If you don't have an account, sign up for free.
  2. Your system meets the system requirements.

Steps

Open the Xcode project

  1. To start the tutorial, complete the Display a web map tutorial or download and unzip the solution.

  2. Open the .xcodeproj file in Xcode.

  3. If you downloaded the solution project, set your API key.

Get the web map item ID

You can use ArcGIS tools to create and view web maps. Use the Map Viewer to identify the web map item ID. This item ID will be used later in the tutorial.

  1. Go to the Naperville water network in the Map Viewer in ArcGIS Online. This web map displays a stormwater network within Naperville, Illinois, USA.

  2. Make a note of the item ID at the end of the browser's URL.

Display the web map

You can display a web map using the web map's item ID. Create an Map from the web map's PortalItem, and display it in your app's MapView.

  1. In Xcode, in the Project Navigator, click ContentView.swift.

  2. In the editor, modify the map variable. Provide the web map's item ID.

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        @State private var map = Map(
            item: PortalItem(
                portal: .arcGISOnline(connection: .anonymous),
    
                id: PortalItem.ID("5a030a31e42841a89914bd7c5ecf4d8f")!
    
            )
        )
    
  3. Create a private class named Model of type ObservableObject and add a @StateObject variable of the Model to the ContentView. Make the Model the @MainActor. See the programming patterns page for more information on how to manage states.

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    import SwiftUI
    import ArcGIS
    
    @MainActor
    private class Model: ObservableObject {
    
    }
    
    struct ContentView: View {
    
        @StateObject private var model = Model()
    
        @State private var map = Map(
            item: PortalItem(
                portal: .arcGISOnline(connection: .anonymous),
    
                id: PortalItem.ID("5a030a31e42841a89914bd7c5ecf4d8f")!
    
            )
        )
    
  4. In the Model, create a @Published variable named offlineMap of type Map to store the output of the downloaded map.

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    @MainActor
    private class Model: ObservableObject {
    
        @Published private(set) var offlineMap: Map!
    
    }
    
  5. In the ContentView, modify the map view to display the model's offline map or the online map. If offlineMap has no value, then the map view should display the online map.

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    struct ContentView: View {
    
        @StateObject private var model = Model()
    
        @State private var map = Map(
            item: PortalItem(
                portal: .arcGISOnline(connection: .anonymous),
    
                id: PortalItem.ID("5a030a31e42841a89914bd7c5ecf4d8f")!
    
            )
        )
    
        var body: some View {
    
                    MapView(map: model.offlineMap ?? map)
    
        }
    
    }

Specify an area of the web map to take offline

Specify an area of the web map to take offline using either an Envelope or a Polygon. Use views to obtain data about the map view and an overlay to indicate the area on the map to be downloaded.

  1. In the ContentView, wrap the map view inside a MapViewReader and expose the MapViewProxy in its closure. Name it mapView. MapViewProxy provides operations that can be performed on the map view, such as envelope(fromViewRect:). For more information see Perform GeoView operations.

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    struct ContentView: View {
    
        @StateObject private var model = Model()
    
        @State private var map = Map(
            item: PortalItem(
                portal: .arcGISOnline(connection: .anonymous),
    
                id: PortalItem.ID("5a030a31e42841a89914bd7c5ecf4d8f")!
    
            )
        )
    
        var body: some View {
    
                MapViewReader { mapView in
    
                    MapView(map: model.offlineMap ?? map)
    
                }
    
        }
    
    }
  2. Wrap the map view reader inside a GeometryReader and expose the GeometryProxy in its closure. Name it geometry. GeometryProxy provides access to the size and coordinate space (for anchor resolution) of the views enclosed in the geometry reader.

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    struct ContentView: View {
    
        @StateObject private var model = Model()
    
        @State private var map = Map(
            item: PortalItem(
                portal: .arcGISOnline(connection: .anonymous),
    
                id: PortalItem.ID("5a030a31e42841a89914bd7c5ecf4d8f")!
    
            )
        )
    
        var body: some View {
    
            GeometryReader { geometry in
    
                MapViewReader { mapView in
    
                    MapView(map: model.offlineMap ?? map)
    
                }
    
            }
    
        }
    
    }
  3. Add an .overlay modifier to the map view. The overlay contains a red rectangle that encompasses an area to be downloaded.

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        var body: some View {
    
            GeometryReader { geometry in
    
                MapViewReader { mapView in
    
                    MapView(map: model.offlineMap ?? map)
    
                        .overlay {
                            Rectangle()
                                .stroke(.red, lineWidth: 2)
                                .padding(EdgeInsets(top: 60, leading: 20, bottom: 100, trailing: 20))
                                .opacity(model.offlineMap == nil ? 1 : 0)
                        }
    
                }
    
            }
    
        }
    

Download and display the offline map

Generate and download an offline map for a specified area of interest using an asynchronous task. When complete, it will provide the offline map that can be displayed in a map view.

  1. In the Model, create a @Published GenerateOfflineMapJob variable called generateOfflineMapJob and a OfflineMapTask variable called offlineMapTask. These objects will contain the job and task needed to perform the download function. Learn more about Tasks and jobs.

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    @MainActor
    private class Model: ObservableObject {
    
        @Published private(set) var offlineMap: Map!
    
        @Published private(set) var generateOfflineMapJob: GenerateOfflineMapJob!
        private var offlineMapTask: OfflineMapTask!
    
    }
    
  2. Create a @Published Boolean variable called isGenerateDisabled and set it to true. Create a variable called isShowingAlert and set it to false. These variables will be used to determine if the download button is enabled and if the completion alert is present.

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    @MainActor
    private class Model: ObservableObject {
    
        @Published private(set) var offlineMap: Map!
    
        @Published private(set) var generateOfflineMapJob: GenerateOfflineMapJob!
        private var offlineMapTask: OfflineMapTask!
    
        @Published private(set) var isGenerateDisabled = true
        @Published var isShowingAlert = false
    
    }
    
  3. Create a private URL property named temporaryDirectoryURL. This property will generate a unique URL at which to store the offline map on the device.

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    @MainActor
    private class Model: ObservableObject {
    
        @Published private(set) var offlineMap: Map!
    
        @Published private(set) var generateOfflineMapJob: GenerateOfflineMapJob!
        private var offlineMapTask: OfflineMapTask!
    
        @Published private(set) var isGenerateDisabled = true
        @Published var isShowingAlert = false
    
        private let temporaryDirectory: URL = {
            // swiftlint:disable:next force_try
            return try! FileManager.default.url(
                for: .itemReplacementDirectory,
                in: .userDomainMask,
                appropriateFor: FileManager.default.temporaryDirectory,
                create: true
            )
        }()
    
    }
    
  4. Create a deinit function to remove the temporary directory after the app closes.

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    @MainActor
    private class Model: ObservableObject {
    
        @Published private(set) var offlineMap: Map!
    
        @Published private(set) var generateOfflineMapJob: GenerateOfflineMapJob!
        private var offlineMapTask: OfflineMapTask!
    
        @Published private(set) var isGenerateDisabled = true
        @Published var isShowingAlert = false
    
        private let temporaryDirectory: URL = {
            // swiftlint:disable:next force_try
            return try! FileManager.default.url(
                for: .itemReplacementDirectory,
                in: .userDomainMask,
                appropriateFor: FileManager.default.temporaryDirectory,
                create: true
            )
        }()
    
        deinit {
            try? FileManager.default.removeItem(at: temporaryDirectory)
        }
    
    }
    
  5. Create an asynchronous function called initializeOfflineMapTask(onlineMap:). This function loads the online map and creates a OfflineMapTask from it. Set the isGenerateDisabled Boolean to false.

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    @MainActor
    private class Model: ObservableObject {
    
        @Published private(set) var offlineMap: Map!
    
        @Published private(set) var generateOfflineMapJob: GenerateOfflineMapJob!
        private var offlineMapTask: OfflineMapTask!
    
        @Published private(set) var isGenerateDisabled = true
        @Published var isShowingAlert = false
    
        private let temporaryDirectory: URL = {
            // swiftlint:disable:next force_try
            return try! FileManager.default.url(
                for: .itemReplacementDirectory,
                in: .userDomainMask,
                appropriateFor: FileManager.default.temporaryDirectory,
                create: true
            )
        }()
    
        deinit {
            try? FileManager.default.removeItem(at: temporaryDirectory)
        }
    
        func initializeOfflineMapTask(onlineMap: Map) async {
            do {
                try await onlineMap.load()
                offlineMapTask = OfflineMapTask(onlineMap: onlineMap)
                isGenerateDisabled = false
            } catch {
                print(error)
            }
        }
    
    }
    
  6. In the ContentView, call the initializeOfflineMapTask(onlineMap:) function in a .task modifier, passing in the online map. The task's function is called as the map view appears.

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    struct ContentView: View {
    
        @StateObject private var model = Model()
    
        @State private var map = Map(
            item: PortalItem(
                portal: .arcGISOnline(connection: .anonymous),
    
                id: PortalItem.ID("5a030a31e42841a89914bd7c5ecf4d8f")!
    
            )
        )
    
        var body: some View {
    
            GeometryReader { geometry in
    
                MapViewReader { mapView in
    
                    MapView(map: model.offlineMap ?? map)
    
                        .task {
                            await model.initializeOfflineMapTask(onlineMap: map)
                        }
    
                        .overlay {
                            Rectangle()
                                .stroke(.red, lineWidth: 2)
                                .padding(EdgeInsets(top: 60, leading: 20, bottom: 100, trailing: 20))
                                .opacity(model.offlineMap == nil ? 1 : 0)
                        }
    
                }
    
            }
    
        }
    
    }
  7. In the Model, create an asynchronous function called makeGenerateOfflineMapParameters(areaOfInterest:). The function returns GenerateOfflineMapParameters, created using offlineMapTask and the area of interest.

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