pin drop animation

The default pin drop animation doesn't work in my app, here is some code I wrote:

- (void)viewDidLoad {
    /*
        some code...
    */
    [theMapView addAnnotation:addAnnotation];
    [addAnnotation release];
}

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation{
    MKPinAnnotationView *annoView = [[MKPinAnnotationView alloc] initWithAnnotation:annotation 
                                                            reuseIdentifier:@"current"];
    annoView.animatesDrop = YES;
    annoView.pinColor = MKPinAnnotationColorGreen;
    return annoView;
}

Right now the pin just appear in the screen as default red one without any animation, the MKM开发者_运维百科apViewDelegate protocol has been adopted, anyone can see what's wrong here?

First use:

[yourMap_view setDelegate:self];

in ViewDidLoad

Then call this for the drop animation :

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id <MKAnnotation>)annotation
{
MKPinAnnotationView *pinView = nil;
if(annotation!= map_view.userLocation)
{
    static NSString *defaultPin = @"pinIdentifier";
    pinView = (MKPinAnnotationView*)[map_view dequeueReusableAnnotationViewWithIdentifier:defaultPin];
    if(pinView == nil)
        pinView = [[[MKPinAnnotationView alloc]initWithAnnotation:annotation reuseIdentifier:defaultPin]autorelease];
    pinView.pinColor = MKPinAnnotationColorPurple; //Optional
    pinView.canShowCallout = YES; // Optional
    pinView.animatesDrop = YES;
}
else
{
    [map_view.userLocation setTitle:@"You are Here!"];
}
return pinView;
}