按计算结果排序 NSFetchRequest

Posted 2023-03-05

【中文标题】按计算结果排序 NSFetchRequest

【英文标题】：Sort NSFetchRequest by calculation result

【发布时间】：2016-01-27 20:23:18

【问题描述】：

所以我有一个具有纬度和经度值的点数据集，我想检索它们并按到用户当前位置的距离对它们进行排序。目前，我有以下内容：

mainMoc.performBlockAndWait 
            // Fetching data from CoreData
            let fetchRequest = NSFetchRequest()
            fetchRequest.predicate = NSPredicate(format: "pointLatitude BETWEEN %f,%f AND pointLongitude BETWEEN %f,%f", (latitude-0.03), (latitude+0.03), (longitude-0.03), (longitude+0.03))
            let entity = NSEntityDescription.entityForName("PointPrimary", inManagedObjectContext: self.mainMoc)
            fetchRequest.entity = entity

            let sortDescriptor = NSSortDescriptor(key: "pointTitle", ascending: false)
            fetchRequest.sortDescriptors = [sortDescriptor]

            do 
                points = try self.mainMoc.executeFetchRequest(fetchRequest) as! [PointPrimary]

             catch 
                let jsonError = error as NSError
                NSLog("\(jsonError), \(jsonError.localizedDescription)")
                abort()
            
        

所以目前我只根据标题对其进行排序。但是，如果我想计算距离以说出 CLLocationCoordinate2D 并以此为基础对 fetchRequest 结果进行排序，我将如何进行？

非常感谢！

【参考方案1】：

NSFetchRequest 不能使用模型中定义（并存储在数据库中）以外的属性作为排序描述符，因此您必须求助于内存排序。在您的 PointPrimary 类上定义一个 distance 方法，该方法执行适当的计算并执行以下操作：

let sortedPoints = points.sortedArrayUsingDescriptors([NSSortDescriptor(key: "distance", ascending: true)])

【参考方案2】：

这应该可行。基本上你会使用带有自定义比较器的NSSortDescriptor。

诀窍是使用"self" 作为NSSortDescriptor 的键，它将获取的对象传递给比较器。

var userLocation : CLLocation // get that from somewhere
var distanceCompare : NSComparator = 
    (obj1: AnyObject!, obj2: AnyObject!) -> NSComparisonResult in
    let lng1 = obj1.valueForKey("pointLongitude") as! CLLocationDegrees
    let lat1 = obj1.valueForKey("pointLatitude") as! CLLocationDegrees
    let p1Location : CLLocation = CLLocation(latitude: lat1, longitude: lng1)
    let p1DistanceToUserLocation = userLocation.distanceFromLocation(p1Location)

    let lng2 = obj2.valueForKey("pointLongitude") as! CLLocationDegrees
    let lat2 = obj2.valueForKey("pointLatitude") as! CLLocationDegrees
    let p2Location : CLLocation = CLLocation(latitude: lat1, longitude: lng1)
    let p2DistanceToUserLocation = userLocation.distanceFromLocation(p2Location)

    if (p1DistanceToUserLocation > p2DistanceToUserLocation) 
        return .OrderedDescending
     else if (p1DistanceToUserLocation < p2DistanceToUserLocation) 
        return .OrderedAscending
     else 
        return .OrderedSame
    


var distanceSortDescriptor = NSSortDescriptor(key: "self", ascending: true, comparator: distanceCompare)
fetchRequest.sortDescriptors = [distanceSortDescriptor]