Question

I add a NSTimer to record the location from location manager ,and put ever location into a NSMutableArray.

-(void)OnTimer:(NSTimer *)param{
     [self.locationRecoder addObject:self.manager.location];
}

and I add a button to UI, when I click the button, it invoke this method

-(IBAction)Click:(id)sender(){
   NSArray *coordinateArray = [self.locationRecorder valueForKeyPath:@"coordinate"];
   MKPolyline *lines = [MKPolyline ploylineWithCoordinates:(CLLocationCoordinate2D *)coordinateArray count:coordinateArray.count];
   [self.map addOverlay:lines];
}

then there is nothing drawn. did i do something wrong in type cast?

Answer 1

The polylineWithCoordinates method requires a plain C array of structs of type CLLocationCoordinate2D.

After the call to valueForKeyPath, coordinateArray is an NSArray of NSValue objects.
That is not the same thing as a C array of structs.

Casting that NSArray to (CLLocationCoordinate2D *) doesn’t convert it to a C array of structs.

Instead, you have to create the C array manually using malloc and looping through the locationRecoder array:

CLLocationCoordinate2D *coordinateArray 
    = malloc(sizeof(CLLocationCoordinate2D) * locationRecorder.count);

int caIndex = 0;
for (CLLocation *loc in locationRecorder) {
    coordinateArray[caIndex] = loc.coordinate;
    caIndex++;
}

MKPolyline *lines = [MKPolyline polylineWithCoordinates:coordinateArray 
                        count:locationRecorder.count];

free(coordinateArray);

[self.map addOverlay:lines];