How to optimize code for drawing map overlay

Question

I draw path on MKMapView based on coordinates stored in SQLite on iPhone.
But now I stored 14000 coordinates (just lat/lng) in database and now when I want to display overlay path I get application crash.
My question is is there any way to optimize this code to be faster? This is in view did load:

// ar is NSMutableArray and it is populate from database for a few seconds but code bellow cost me app crash
    for(Path* p in ar)
        {
            self.routeLine = nil;
            self.routeLineView = nil;

            // while we create the route points, we will also be calculating the bounding box of our route
            // so we can easily zoom in on it.
            MKMapPoint northEastPoint;
            MKMapPoint southWestPoint;

            // create a c array of points.
            MKMapPoint* pointArr = malloc(sizeof(CLLocationCoordinate2D) * ar.count);

            for(int idx = 0; idx < ar.count; idx++)
            {
                Path *m_p = [ar objectAtIndex:idx];

                CLLocationDegrees latitude  = m_p.Latitude;
                CLLocationDegrees longitude = m_p.Longitude;

                // create our coordinate and add it to the correct spot in the array
                CLLocationCoordinate2D coordinate = CLLocationCoordinate2DMake(latitude, longitude);


                MKMapPoint point = MKMapPointForCoordinate(coordinate);

                // adjust the bounding box
                // if it is the first point, just use them, since we have nothing to compare to yet.
                if (idx == 0) {
                    northEastPoint = point;
                    southWestPoint = point;
                }
                else
                {
                    if (point.x > northEastPoint.x)
                        northEastPoint.x = point.x;
                    if(point.y > northEastPoint.y)
                        northEastPoint.y = point.y;
                    if (point.x < southWestPoint.x)
                        southWestPoint.x = point.x;
                    if (point.y < southWestPoint.y)
                        southWestPoint.y = point.y;
                }

                pointArr[idx] = point;
            }

            // create the polyline based on the array of points.
            self.routeLine = [MKPolyline polylineWithPoints:pointArr count:ar.count];

            _routeRect = MKMapRectMake(southWestPoint.x, southWestPoint.y, northEastPoint.x - southWestPoint.x, northEastPoint.y - southWestPoint.y);
            // clear the memory allocated earlier for the points
            free(pointArr);


            [self.mapView removeOverlays: self.mapView.overlays];
            // add the overlay to the map
            if (nil != self.routeLine) {
                [self.mapView addOverlay:self.routeLine];
            }

UPDATE

ViewDidLoad:

...
[self performSelectorInBackground:@selector(drawPathInBackground) withObject:nil];
...
-(void)drawPathInBackground{
for(int idx = 0; idx < ar.count; idx++)
    { ... }
[self.mapView performSelector:@selector(addOverlay:) onThread:[NSThread mainThread] withObject:self.routeLine waitUntilDone:YES];
}

I did like this and UI not freezes.
The only thing that left is how to draw MKPolyLine on every X points?

Answer 1

three approaches:

1. don't display every point but rather combine nearby points to just one. the saving depends on your data and the necessity to display all.
2. if possible load the data in a background thread and display in multiple batches on the main thread. the user will practically see how the data is loaded after time.
3. load and display data lazily. means: only display those points which are visible on screen

Answer 2

Do fetching from the database and processing in a background thread.

Then reduce the number of coordinates in the path using the Douglas–Peucker algorithm:

And cache the results.

Answer 3

If you have array of coordinates the use this code

here routes is array of coordinates.

NSLog(@"count %d",[routes count]);
MKMapPoint* pointArr = malloc(sizeof(CLLocationCoordinate2D) * [routes count]);

for(int idx = 0; idx < [routes count]; idx++)
{
    CLLocation* location = [routes objectAtIndex:idx];
    CLLocationCoordinate2D workingCoordinate;       
    workingCoordinate.latitude=location.coordinate.latitude;
    workingCoordinate.longitude=location.coordinate.longitude;  
    NSLog(@"loop = %f,%f",workingCoordinate.latitude, workingCoordinate.longitude);
    MKMapPoint point = MKMapPointForCoordinate(workingCoordinate);
    pointArr[idx] = point;      
}   
// create the polyline based on the array of points. 
self.routeLine = [MKPolyline polylineWithPoints:pointArr count:[routes count]];
[mapView addOverlay:self.routeLine];
free(pointArr);

Hope this helps.

Answer 4

google has a algorithm that can encode the locations to string. for your situation , 14000 coordinates will be encoded to a String nearly 14000 length. then put the String into sqlite. it will accelerate the speed to get data from DB