改变MKAnnotationView的大小，使之随MKMapView的缩放而缩放

如果要完成如题功能，有一个技术点需要实现。就是如何detect KMapView的pinch事件, iOS中有UIPinchGestureRecognizer,所以我们可以用这个来detect.

首先向MKMapView加一个gesture recognizer, 如下代码：

UIPinchGestureRecognizer *pinchGesture = [[UIPinchGestureRecognizer alloc] initWithTarget:self action:@selector(handlePinchGesture:)];
[pinchGesture setDelegate:self];
[mapView addGestureRecognizer:pinchGesture];
[pinchGesture release];

一定要在你的controller中实现下面delegate方法，不然会被系统过滤掉，因为MKMapView自己有系统的pinch Gesture recognizer。

- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer
{
    return YES;
}

然后就在回调方法中实现：

- (void)handlePinchGesture:(UIGestureRecognizer *)gesture
{
    if (gesture.state != UIGestureRecognizerStateChanged) {
        return;
    }


    [self refreshAnnotationView];
}

其中refreshAnnotationView代码如下，找mapView中所有的annotationView,并更新大小

- (void)refreshAnnotationView
{
    for (id <MKAnnotation>annotation in _mapView.annotations) {
        if ([annotation isKindOfClass:[MKAnnotation class]])
        {
            MKAnnotationView *pinView = [_mapView viewForAnnotation:annotation];
            [self formatAnnotationView:pinView];
            double zoomLevel = [_mapView getZoomLevel];
            double scale = -1 * sqrt((double)(1 - pow((zoomLevel/16.0), 2.0))) + 2.0; // This is a circular scale function where at zoom level 0 scale is 0.1 and at zoom level 20 scale is 1.1

            pinView.transform = CGAffineTransformMakeScale(scale, scale);

        }
    }
}

其中getZoomLevel是MKMapView的一个category方法，它可以获取当前地图的缩放等级，在以这儿有介绍。

Reference:http://stackoverflow.com/questions/8286711/resize-mkannotationview-image-when-map-zooms-in-and-out?answertab=votes#tab-top