MKMapView缩放显示全部annotation

在地图上加载许多annotation，是不是想一屏幕显示完所有的annotation?下面代码就可以实现。

 #define MINIMUM_ZOOM_ARC 0.014 //approximately 1 miles (1 degree of arc ~= 69 miles)
 #define ANNOTATION_REGION_PAD_FACTOR 1.15
 #define MAX_DEGREES_ARC 360
 //size the mapView region to fit its annotations
 - (void)zoomMapViewToFitAnnotations:(MKMapView *)mapView animated:(BOOL)animated
 {
     NSArray *annotations = mapView.annotations;
     int count = [mapView.annotations count];
     if ( count == 0) { return; } //bail if no annotations

     //convert NSArray of id <MKAnnotation> into an MKCoordinateRegion that can be used to set the map size
     //can't use NSArray with MKMapPoint because MKMapPoint is not an id
     MKMapPoint points[count]; //C array of MKMapPoint struct
     for( int i=0; i<count; i++ ) //load points C array by converting coordinates to points
     {
         CLLocationCoordinate2D coordinate = [(id <MKAnnotation>)[annotations objectAtIndex:i] coordinate];
         points[i] = MKMapPointForCoordinate(coordinate);
     }
     //create MKMapRect from array of MKMapPoint
     MKMapRect mapRect = [[MKPolygon polygonWithPoints:points count:count] boundingMapRect];
     //convert MKCoordinateRegion from MKMapRect
     MKCoordinateRegion region = MKCoordinateRegionForMapRect(mapRect);

     //add padding so pins aren't scrunched on the edges
     region.span.latitudeDelta  *= ANNOTATION_REGION_PAD_FACTOR;
     region.span.longitudeDelta *= ANNOTATION_REGION_PAD_FACTOR;
     //but padding can't be bigger than the world
     if( region.span.latitudeDelta > MAX_DEGREES_ARC ) { region.span.latitudeDelta  = MAX_DEGREES_ARC; }
     if( region.span.longitudeDelta > MAX_DEGREES_ARC ){ region.span.longitudeDelta = MAX_DEGREES_ARC; }

     //and don't zoom in stupid-close on small samples
     if( region.span.latitudeDelta  < MINIMUM_ZOOM_ARC ) { region.span.latitudeDelta  = MINIMUM_ZOOM_ARC; }
     if( region.span.longitudeDelta < MINIMUM_ZOOM_ARC ) { region.span.longitudeDelta = MINIMUM_ZOOM_ARC; }
     //and if there is a sample of 1 we want the max zoom-in instead of max zoom-out
     if( count == 1 )
     {
         region.span.latitudeDelta = MINIMUM_ZOOM_ARC;
         region.span.longitudeDelta = MINIMUM_ZOOM_ARC;
     }
     [mapView setRegion:region animated:animated];
 }

 - (void)viewWillAppear:(BOOL)animated
 {
     [self zoomMapViewToFitAnnotations:self.mapView animated:animated];
     //or maybe you would do the call above in the code path that sets the annotations array
 }