Display drawing status

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// Copyright 2016 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 MapViewDrawStatusViewController: UIViewController {
    @IBOutlet private weak var mapView: AGSMapView!
    @IBOutlet private weak var activityIndicatorView: UIView!

    private var map: AGSMap?
    /// The observation of the map view's draw status.
    private var drawStatusObservation: NSKeyValueObservation?

    override func viewDidLoad() {
        super.viewDidLoad()

        // Add the source code button item to the right of navigation bar.
        (self.navigationItem.rightBarButtonItem as! SourceCodeBarButtonItem).filenames = ["MapViewDrawStatusViewController"]

        // Instantiate the map with topographic basemap.
        map = AGSMap(basemapStyle: .arcGISTopographic)

        // Add a feature layer.
        let featureTable = AGSServiceFeatureTable(url: URL(string: "https://sampleserver6.arcgisonline.com/arcgis/rest/services/DamageAssessment/FeatureServer/0")!)
        let featureLayer = AGSFeatureLayer(featureTable: featureTable)
        map?.operationalLayers.add(featureLayer)

        // Assign the map to mapView.
        mapView.map = map
        mapView.setViewpoint(AGSViewpoint(targetExtent: AGSEnvelope(xMin: -13639984, yMin: 4537387, xMax: -13606734, yMax: 4558866, spatialReference: .webMercator())))
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        drawStatusObservation = mapView.observe(\.drawStatus, options: .initial) { [weak self] (mapView, _) in
            DispatchQueue.main.async {
                self?.activityIndicatorView.isHidden = mapView.drawStatus == .completed
            }
        }
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        drawStatusObservation = nil
    }
}