##ClusterAnnotation

self.coordinateQuadTree = [[CoordinateQuadTree alloc] init];

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    /* 建立四叉树. */
    [self.coordinateQuadTree buildTreeWithPOIs:respons.pois];
        
    dispatch_async(dispatch_get_main_queue(), ^{
            /* 计算当前mapView区域内需要显示的annotation. */
            NSLog(@"First time calculate annotations.");
            [self addAnnotationsToMapView:self.mapView];
    });
});

- (void)addAnnotationsToMapView:(MAMapView *)mapView
{
    /* 判断是否已建树. */
    if (self.coordinateQuadTree.root == nil)
    {
        return;
    }
    /* 根据当前zoomLevel和zoomScale 进行annotation聚合. */
    double zoomScale = self.mapView.bounds.size.width / self.mapView.visibleMapRect.size.width;
    /* 基于先前建立的四叉树索引，计算当前需要显示的annotations. */
    NSArray *annotations = [self.coordinateQuadTree clusteredAnnotationsWithinMapRect:mapView.visibleMapRect
                                withZoomScale:zoomScale
                                 andZoomLevel:mapView.zoomLevel];
   
    /* 更新annotations. */
    [self updateMapViewAnnotationsWithAnnotations:annotations];
}

/* 更新annotation. */
- (void)updateMapViewAnnotationsWithAnnotations:(NSArray *)annotations
{
    /* 用户滑动时，保留仍然可用的标注，去除屏幕外标注，添加新增区域的标注 */
    NSMutableSet *before = [NSMutableSet setWithArray:self.mapView.annotations];
    [before removeObject:[self.mapView userLocation]];
    NSSet *after = [NSSet setWithArray:annotations];
    
    /* 保留仍然位于屏幕内的annotation. */
    NSMutableSet *toKeep = [NSMutableSet setWithSet:before];
    [toKeep intersectSet:after];
    
    /* 需要添加的annotation. */
    NSMutableSet *toAdd = [NSMutableSet setWithSet:after];
    [toAdd minusSet:toKeep];
    
    /* 删除位于屏幕外的annotation. */
    NSMutableSet *toRemove = [NSMutableSet setWithSet:before];
    [toRemove minusSet:after];
    
    /* 更新. */
    dispatch_async(dispatch_get_main_queue(), ^{
        [self.mapView addAnnotations:[toAdd allObjects]];
        [self.mapView removeAnnotations:[toRemove allObjects]];
    });
}

-(MAAnnotationView *)mapView:(MAMapView *)mapView viewForAnnotation:(id<MAAnnotation>)annotation
{
    if ([annotation isKindOfClass:[ClusterAnnotation class]])
    {
        /* dequeue重用annotationView代码. */
        /* ... */
        
        /* 设置annotationView的属性. */
        annotationView.annotation = annotation;
        annotationView.count = [(ClusterAnnotation *)annotation count];
        
        /* 设置annotationView的callout属性和calloutView代码. */
        /* ... */
        return annotationView;
    }
    return nil;
}

- (void)mapView:(MAMapView *)mapView regionDidChangeAnimated:(BOOL)animated
{
    /* mapView区域变化时重算annotation. */
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [self addAnnotationsToMapView:self.mapView];
    });
}