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

The other answers here seem to be about using Apple Maps, for using Google Maps I found I had to make some modifications to @SedateAlien's great category.

MODIFIED CATEGORY

+ (GMSPolyline *)polylineWithEncodedString:(NSString *)encodedString {
    const char *bytes = [encodedString UTF8String];
    NSUInteger length = [encodedString lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
    NSUInteger idx = 0;

    NSUInteger count = length / 4;
    CLLocationCoordinate2D *coords = calloc(count, sizeof(CLLocationCoordinate2D));
    NSUInteger coordIdx = 0;

    float latitude = 0;
    float longitude = 0;
    while (idx < length) {
        char byte = 0;
        int res = 0;
        char shift = 0;

        do {
            byte = bytes[idx++] - 63;
            res |= (byte & 0x1F) << shift;
            shift += 5;
        } while (byte >= 0x20);

        float deltaLat = ((res & 1) ? ~(res >> 1) : (res >> 1));
        latitude += deltaLat;

        shift = 0;
        res = 0;

        do {
            byte = bytes[idx++] - 0x3F;
            res |= (byte & 0x1F) << shift;
            shift += 5;
        } while (byte >= 0x20);

        float deltaLon = ((res & 1) ? ~(res >> 1) : (res >> 1));
        longitude += deltaLon;

        float finalLat = latitude * 1E-5;
        float finalLon = longitude * 1E-5;

        CLLocationCoordinate2D coord = CLLocationCoordinate2DMake(finalLat, finalLon);
        coords[coordIdx++] = coord;

        if (coordIdx == count) {
            NSUInteger newCount = count + 10;
            coords = realloc(coords, newCount * sizeof(CLLocationCoordinate2D));
            count = newCount;
        }
    }

    GMSMutablePath *path = [[GMSMutablePath alloc] init];

    int i;
    for (i = 0; i < coordIdx; i++)
    {
        [path addCoordinate:coords[i]];
    }

    GMSPolyline *polyline = [GMSPolyline polylineWithPath:path];
    free(coords);

    return polyline;
}

USAGE

// Here I make the call to the Google Maps API to get the routes between two points...

....

// Get the encoded array of points.
NSString *points = routes[@"routes"][0][@"overview_polyline"][@"points"];

// Use the modified category to get a polyline from the points.
GMSPolyline *polyline = [GMSPolyline polylineWithEncodedString:points];

// Add the polyline to the map.
polyline.strokeColor = [UIColor redColor];
polyline.strokeWidth = 10.f;
polyline.map = theMapView;
}