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我是初学iOS开发人员（这实际上是我第一次尝试iOS应用程序）。我制作的应用程序应该在地图上显示污染等级。我想要的是设置一个滑块并根据滑块设置在一定时间内显示污染等级。使用滑块过滤NSFetchResultsController并使用NSPredicate
我最初的，显然是错误的做法，是改变fetchResultsController获取请求谓词，只要滑块发生变化并更改func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>)中的显示结果。这种方法的问题在于，更改提取请求似乎没有调用controllerDidChangeContent函数。看起来这只是在将新元素添加到数据库时触发的。什么是使这种动态改变获取请求的标准解决方案？
我出现在做上述工作的类下面。
CoreDataViewController
import UIKit
import CoreData
class CoreDataViewController: UIViewController, NSFetchedResultsControllerDelegate {
// MARK: Properties
var fetchedResultsController : NSFetchedResultsController<NSFetchRequestResult>? {
didSet {
// Whenever the frc changes, execute the search and
// reload the data
print("fetchedResultsController did set")
fetchedResultsController?.delegate = self
executeSearch()
}
}
// MARK: Initializers
init?(_ coder: NSCoder, fetchedResultsController fc : NSFetchedResultsController<NSFetchRequestResult>) {
fetchedResultsController = fc
super.init(coder: coder)
}
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
}
}
extension CoreDataViewController {
func executeSearch() {
if let fc = fetchedResultsController {
do {
try fc.performFetch()
} catch let e as NSError {
print("Error while trying to perform a search: \n\(e)\n\(fetchedResultsController)")
}
}
}
}
的MapViewController（延伸CoreDataViewController）
import UIKit
import GoogleMaps
import CoreData
class MapViewController: CoreDataViewController {
// var mapView: GMSMapView!
@IBOutlet weak var testButton: UIBarButtonItem!
@IBOutlet weak var timestampSlider: UISlider!
override func loadView() {
// Get the stack
let delegate = UIApplication.shared.delegate as! AppDelegate
let stack = delegate.stack
// Create a fetchrequest
let fr = NSFetchRequest<NSFetchRequestResult>(entityName: "Measurement")
fr.sortDescriptors = [NSSortDescriptor(key: "fromDatetime", ascending: true)]
fr.predicate = NSPredicate(format: "fromDatetime = %@", argumentArray: [2])
// Create the FetchedResultController
fetchedResultsController = NSFetchedResultsController(fetchRequest: fr, managedObjectContext: stack.context,
sectionNameKeyPath: nil, cacheName: nil)
let measurements = fetchedResultsController?.fetchedObjects as! [Measurement]
// Create a GMSCameraPosition that tells the map to display the
// coordinate latitude: 50.0646501, longitude: 19.9449799 at zoom level 6.
let camera = GMSCameraPosition.camera(withLatitude: 50.0646501, longitude: 19.9449799, zoom: 14.0)
let mapView = GMSMapView.map(withFrame: CGRect.zero, camera: camera)
view = mapView
_ = measurements.flatMap({(m: Measurement) -> Void in
print(m.fromDatetime)
let marker = GMSMarker()
marker.position = CLLocationCoordinate2D(latitude: m.latitude, longitude: m.longitude)
marker.title = "Measurement"
marker.snippet = String(format: "pm10: %f, pm25: %f", m.pm10, m.pm25)
marker.map = mapView
})
}
override func viewWillAppear(_ animated: Bool) {
self.navigationController?.isToolbarHidden = true
}
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
@IBAction func testButtonAction(_ sender: Any) {
// Get the stack
let delegate = UIApplication.shared.delegate as! AppDelegate
let stack = delegate.stack
_ = Measurement(sensorId: 0, fromDatetime: 2, pm10: 0.6, pm25: 0.3, airQualityIndex: 0.8, latitude: 50.0626402, longitude: 19.9385001, source: "Airly", windDeg: 0.9, windSpeed: 23.0, insertInto: stack.context)
}
@IBAction func timestampSliderAction(_ sender: Any) {
if self.timestampSlider == nil {
return
}
let sliderValue = floor(self.timestampSlider.value)
print(sliderValue)
let delegate = UIApplication.shared.delegate as! AppDelegate
let stack = delegate.stack
let fr = NSFetchRequest<NSFetchRequestResult>(entityName: "Measurement")
fr.sortDescriptors = [NSSortDescriptor(key: "fromDatetime", ascending: true)]
fr.predicate = NSPredicate(format: "fromDatetime = %@", argumentArray: [sliderValue])
// Create the FetchedResultController
fetchedResultsController = NSFetchedResultsController(fetchRequest: fr, managedObjectContext: stack.context,
sectionNameKeyPath: nil, cacheName: nil)
// alternative solution without creating new NSFetchedResultsController
// fetchedResultsController?.fetchRequest.predicate = NSPredicate(format: "fromDatetime = %@", argumentArray: [sliderValue])
// executeSearch()
}
func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
print("controllerDidChangeContent called")
let measurements = fetchedResultsController?.fetchedObjects as! [Measurement]
let mapView = view as! GMSMapView
_ = measurements.flatMap({(m: Measurement) -> Void in
let marker = GMSMarker()
marker.position = CLLocationCoordinate2D(latitude: m.latitude, longitude: m.longitude)
marker.title = "Measurement"
marker.snippet = String(format: "pm10: %f, pm25: %f", m.pm10, m.pm25)
marker.map = mapView
})
}
}