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
}