ios - Proper way to find the closest beacon -

when didrangebeacons method called, closest beacon beacons.first? or beacon in list of beacons has lowest accuracy value? both correct? thanks

while list of beacons in didrangebeacons:inregion: callback sorted accuracy field (which measures distance in meters), there number of problems relying on this:

• sometimes beacons have accuracy value of -1 if no rssi (signal strength) samples available in past second. gives incorrect results if relay on automatic sorting.

• sometimes beacons drop out of list if aren't detected in last second. common problem in noisy bluetooth environments , infrequently transmitting beacons.

a more robust way know closest beacon implement algorithm ignores accuracy values of -1 , tolerant of temporary dropouts (say 5 seconds) in beacon detections.

or course, beginning. can fancier adding kinds of other filters tune exact use case. without on complicating things, here's swift code use basics:

let expirationtimesecs = 5.0 public var closestbeacon: clbeacon? = nil var trackedbeacons: dictionary<string, clbeacon> var trackedbeacontimes: dictionary<string, nsdate>  override init() {   trackedbeacons = dictionary<string, clbeacon>()   trackedbeacontimes = dictionary<string, nsdate>() }  public func locationmanager(manager: cllocationmanager, didrangebeacons beacons: [clbeacon], inregion region: clbeaconregion) {   let = nsdate()   beacon in beacons {     let key = keyforbeacon(beacon)     if beacon.accuracy < 0 {       nslog("ignoring beacon negative distance")     }     else {       trackedbeacons[key] = beacon       if (trackedbeacontimes[key] != nil) {         trackedbeacontimes[key] =       }       else {         trackedbeacontimes[key] =       }     }   }   purgeexpiredbeacons()   calculateclosestbeacon() }  func calculateclosestbeacon() {   var changed = false     // initialize cloestbeaconcandidate latest tracked instance of current closest beacon     var closestbeaconcandidate: clbeacon?     if closestbeacon != nil {       let closestbeaconkey = keyforbeacon(closestbeacon!)       key in trackedbeacons.keys {         if key == closestbeaconkey {           closestbeaconcandidate = trackedbeacons[key]         }       }     }      key in trackedbeacons.keys {       var closer = false       let beacon = trackedbeacons[key]       if (beacon != closestbeaconcandidate) {         if beacon!.accuracy > 0 {           if closestbeaconcandidate == nil {             closer = true           }           else if beacon!.accuracy < closestbeaconcandidate!.accuracy {             closer = true           }         }         if closer {           closestbeaconcandidate = beacon           changed = true         }       }     }     if (changed) {       closestbeacon = closestbeaconcandidate     } }  func keyforbeacon(beacon: clbeacon) -> string {   return "\(beacon.proximityuuid.uuidstring) \(beacon.major) \(beacon.minor)" }  func purgeexpiredbeacons() {   let = nsdate()   var changed = false   var newtrackedbeacons = dictionary<string, clbeacon>()   var newtrackedbeacontimes = dictionary<string, nsdate>()   key in trackedbeacons.keys {     let beacon = trackedbeacons[key]     let lastseentime = trackedbeacontimes[key]!     if now.timeintervalsincedate(lastseentime) > expirationtimesecs {       nslog("******* expired seeing beacon: \(key) time interval \(now.timeintervalsincedate(lastseentime))")       changed = true     }     else {       newtrackedbeacons[key] = beacon!       newtrackedbeacontimes[key] = lastseentime     }   }   if changed {     trackedbeacons = newtrackedbeacons     trackedbeacontimes = newtrackedbeacontimes   } }