How to decode the Google Directions API polylines field into lat long points in Objective-C for iPhone?

Question

I want to draw routes on a map corresponding to directions JSON which I am getting through the Google Directions API: https://developers.google.com/maps/documentation/direct

Answer 1

Here's how I do it in my directions app. keyPlace is your destination object

- (void)getDirections {

  CLLocation *newLocation;// = currentUserLocation;
  MKPointAnnotation *annotation = [[[MKPointAnnotation alloc] init] autorelease];
  annotation.coordinate = CLLocationCoordinate2DMake(newLocation.coordinate.latitude, newLocation.coordinate.longitude);
  annotation.title = @"You";
  [mapView addAnnotation:annotation];

  CLLocationCoordinate2D endCoordinate;

  NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"https://maps.googleapis.com/maps/api/directions/json?origin=%f,%f&destination=%f,%f&sensor=false&mode=walking", newLocation.coordinate.latitude, newLocation.coordinate.longitude, keyPlace.lat, keyPlace.lon]];
  ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
  [request startSynchronous];

  if ([[request.responseString.JSONValue valueForKey:@"status"] isEqualToString:@"ZERO_RESULTS"]) {
    [[[[UIAlertView alloc] initWithTitle:@"Error"
                                 message:@"Could not route path from your current location"
                                delegate:nil
                       cancelButtonTitle:@"Close"
                       otherButtonTitles:nil, nil] autorelease] show];
    self.navigationController.navigationBar.userInteractionEnabled = YES;
    return; 
  }

  int points_count = 0;
  if ([[request.responseString.JSONValue objectForKey:@"routes"] count])
    points_count = [[[[[[request.responseString.JSONValue objectForKey:@"routes"] objectAtIndex:0] objectForKey:@"legs"] objectAtIndex:0] objectForKey:@"steps"] count];

  if (!points_count) {
    [[[[UIAlertView alloc] initWithTitle:@"Error"
                                 message:@"Could not route path from your current location"
                                delegate:nil
                       cancelButtonTitle:@"Close"
                       otherButtonTitles:nil, nil] autorelease] show];
    self.navigationController.navigationBar.userInteractionEnabled = YES;
    return;     
  }
  CLLocationCoordinate2D points[points_count * 2];

  int j = 0;
  NSArray *steps = nil;
  if (points_count && [[[[request.responseString.JSONValue objectForKey:@"routes"] objectAtIndex:0] objectForKey:@"legs"] count])
    steps = [[[[[request.responseString.JSONValue objectForKey:@"routes"] objectAtIndex:0] objectForKey:@"legs"] objectAtIndex:0] objectForKey:@"steps"];
  for (int i = 0; i < points_count; i++) {

    double st_lat = [[[[steps objectAtIndex:i] objectForKey:@"start_location"] valueForKey:@"lat"] doubleValue];
    double st_lon = [[[[steps objectAtIndex:i] objectForKey:@"start_location"] valueForKey:@"lng"] doubleValue];
    //NSLog(@"lat lon: %f %f", st_lat, st_lon);
    if (st_lat > 0.0f && st_lon > 0.0f) {
      points[j] = CLLocationCoordinate2DMake(st_lat, st_lon);
      j++;
    }
    double end_lat = [[[[steps objectAtIndex:i] objectForKey:@"end_location"] valueForKey:@"lat"] doubleValue];
    double end_lon = [[[[steps objectAtIndex:i] objectForKey:@"end_location"] valueForKey:@"lng"] doubleValue];

    if (end_lat > 0.0f && end_lon > 0.0f) {
      points[j] = CLLocationCoordinate2DMake(end_lat, end_lon);
      endCoordinate = CLLocationCoordinate2DMake(end_lat, end_lon);
      j++;
    }
  }

  MKPolyline *polyline = [MKPolyline polylineWithCoordinates:points count:points_count * 2];
  [mapView addOverlay:polyline];


}

#pragma mark - MapKit
- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id )annotation {
  MKPinAnnotationView *annView = [[[MKPinAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"currentloc"] autorelease];
  annView.canShowCallout = YES;
  annView.animatesDrop = YES;
  return annView;
}

- (MKOverlayView *)mapView:(MKMapView *)mapView
            viewForOverlay:(id)overlay {
  MKPolylineView *overlayView = [[[MKPolylineView alloc] initWithOverlay:overlay] autorelease];
  overlayView.lineWidth = 5;
  overlayView.strokeColor = [UIColor purpleColor];
  overlayView.fillColor = [[UIColor purpleColor] colorWithAlphaComponent:0.5f];
  return overlayView;
}