List KML contents

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// Copyright 2018 Esri.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import UIKit
import ArcGIS

class ListKMLContentsSceneViewController: UIViewController {
    @IBOutlet weak var sceneView: AGSSceneView? {
        didSet {
            sceneView?.scene = scene
        }
    }

    private let scene: AGSScene = {
        // initialize scene with labeled imagery
        let scene = AGSScene(basemapStyle: .arcGISImagery)

        // create an elevation source and add it to the scene's surface
        let elevationSourceURL = URL(string: "https://elevation3d.arcgis.com/arcgis/rest/services/WorldElevation3D/Terrain3D/ImageServer")!
        let elevationSource = AGSArcGISTiledElevationSource(url: elevationSourceURL)
        scene.baseSurface?.elevationSources.append(elevationSource)

        return scene
    }()

    var kmlDataset: AGSKMLDataset? {
        didSet {
            guard kmlDataset != oldValue,
                let kmlDataset = kmlDataset else {
                    return
            }

            // clear any existing layers
            scene.operationalLayers.removeAllObjects()

            // create a layer to display the dataset
            let kmlLayer = AGSKMLLayer(kmlDataset: kmlDataset)
            // add the kml layer to the scene
            scene.operationalLayers.add(kmlLayer)
        }
    }
    var node: AGSKMLNode? {
        didSet {
            guard kmlDataset != oldValue,
                let node = node else {
                return
            }
            // set the title so the name will appear in the navigation bar
            title = node.name

            // set the viewpoint based on the new node
            setSceneViewpoint(for: node)
        }
    }

    /// A queue for calculating the node viewpoint in the background.
    private let dispatchQueue = DispatchQueue(label: "node viewpoint getter", qos: .userInitiated, attributes: .concurrent, autoreleaseFrequency: .workItem, target: nil)

    // MARK: - Viewpoint

    /// Sets the viewpoint of the scene based on the node, or hides the node
    /// if a viewpoint cannot be determined.
    private func setSceneViewpoint(for node: AGSKMLNode) {
        dispatchQueue.async {
            // get the viewpoint asynchronously so not to block the main thread
            let nodeViewpoint = self.viewpoint(for: node)

            // run UI updates on the main thread
            DispatchQueue.main.async {
                // load the view before setting a viewpoint
                self.loadViewIfNeeded()

                guard let sceneView = self.sceneView else {
                    return
                }

                if let nodeViewpoint = nodeViewpoint,
                    !nodeViewpoint.targetGeometry.isEmpty {
                    // reveal the view in case it was previously hidden
                    sceneView.isHidden = false

                    // set the viewpoint for the node
                    sceneView.setViewpoint(nodeViewpoint)
                } else {
                    // this node has no viewpoint so hide the scene view, showing the info text
                    sceneView.isHidden = true
                }
            }
        }
    }

    /// Returns the elevation of the scene's surface corresponding to the point's x/y.
    private func sceneSurfaceElevation(for point: AGSPoint) -> Double? {
        guard let surface = scene.baseSurface else {
            return nil
        }

        var surfaceElevation: Double?
        let group = DispatchGroup()
        group.enter()
        // we want to return the elevation synchronously, so run the task in the background and wait
        surface.elevation(for: point) { (elevation, error) in
            if error == nil {
                surfaceElevation = elevation
            }
            group.leave()
        }
        // wait, but not longer than three seconds
        _ = group.wait(timeout: .now() + 3)
        return surfaceElevation
    }

    /// Returns the viewpoint showing the node, converting it from the node's AGSKMLViewPoint if possible.
    private func viewpoint(for node: AGSKMLNode) -> AGSViewpoint? {
        if let kmlViewpoint = node.viewpoint {
            return viewpointFromKMLViewpoint(kmlViewpoint)
        } else if let extent = node.extent {
            // the node does not have a predefined viewpoint, so create a viewpoint based on its extent
            return viewpointFromKMLNodeExtent(extent)
        }
        // the node doesn't have a predefined viewpoint or geometry
        return nil
    }

    // Converts the KML viewpoint to a viewpoint for the scene.
    // The KML viewpoint may not correspond to the node's geometry.
    private func viewpointFromKMLViewpoint(_ kmlViewpoint: AGSKMLViewpoint) -> AGSViewpoint? {
        switch kmlViewpoint.type {
        case .lookAt:
            var lookAtPoint = kmlViewpoint.location
            if kmlViewpoint.altitudeMode != .absolute {
                // if the elevation is relative, account for the surface's elevation
                let elevation = sceneSurfaceElevation(for: lookAtPoint) ?? 0
                lookAtPoint = AGSPoint(x: lookAtPoint.x, y: lookAtPoint.y, z: lookAtPoint.z + elevation, spatialReference: lookAtPoint.spatialReference)
            }
            let camera = AGSCamera(lookAt: lookAtPoint,
                                   distance: kmlViewpoint.range,
                                   heading: kmlViewpoint.heading,
                                   pitch: kmlViewpoint.pitch,
                                   roll: kmlViewpoint.roll)
            // only the camera parameter is used by the scene
            return AGSViewpoint(center: kmlViewpoint.location, scale: 1, camera: camera)
        case .camera:
            // convert the KML viewpoint to a camera
            let camera = AGSCamera(location: kmlViewpoint.location,
                                   heading: kmlViewpoint.heading,
                                   pitch: kmlViewpoint.pitch,
                                   roll: kmlViewpoint.roll)
            // only the camera parameter is used by the scene
            return AGSViewpoint(center: kmlViewpoint.location, scale: 1, camera: camera)
        case .unknown:
            fallthrough
        @unknown default:
            print("Unexpected AGSKMLViewpointType \(kmlViewpoint.type)")
            return nil
        }
    }

    /// Creates a default viewpoint framing the node based on its extent.
    private func viewpointFromKMLNodeExtent(_ extent: AGSEnvelope) -> AGSViewpoint? {
        // some nodes do not include a geometry, so check that the extent isn't empty
        guard !extent.isEmpty else {
            return nil
        }

        let extentCenter = extent.center
        // take the scene's elevation into account
        let elevation = sceneSurfaceElevation(for: extentCenter) ?? 0

        // It's possible for `isEmpty` to be false but for width/height to still be zero.
        if extent.width == 0,
            extent.height == 0 {
            let lookAtPoint = AGSPoint(
                x: extentCenter.x,
                y: extentCenter.y,
                z: extentCenter.z + elevation,
                spatialReference: extent.spatialReference
            )
            // Defaults based on Google Earth.
            let camera = AGSCamera(lookAt: lookAtPoint, distance: 1000, heading: 0, pitch: 45, roll: 0)
            // only the camera parameter is used by the scene
            return AGSViewpoint(targetExtent: extent, camera: camera)
        } else {
            // expand the extent to give some margins when framing the node
            let bufferRadius = max(extent.width, extent.height) / 20
            let bufferedExtent = AGSEnvelope(
                xMin: extent.xMin - bufferRadius,
                yMin: extent.yMin - bufferRadius,
                zMin: extent.zMin - bufferRadius + elevation,
                xMax: extent.xMax + bufferRadius,
                yMax: extent.yMax + bufferRadius,
                zMax: extent.zMax + bufferRadius + elevation,
                spatialReference: .wgs84()
            )
            return AGSViewpoint(targetExtent: bufferedExtent)
        }
    }
}