为了防止重复打开控制器，导致内存暴增 在dealloc方法释放
-(void)dealloc {
[self.mapView removeFromSuperview];
self.mapView.delegate = nil;
self.mapView = nil;
}
地图缩放地图产生的内存这样释放
-(void)mapView:(MKMapView *)mapView regionDidChangeAnimated:(BOOL)animated{
[_mapView removeFromSuperview];
[self.view addSubview:mapView];
}
