CCHMapClusterController

CCHMapClusterController solves the problem of displaying many annotations on an MKMapView and is available under the MIT license.

Note: With iOS 11, Apple introduced map clustering support in MapKit. You can continue using CCHMapClusterController on iOS 11, but for new projects, I suggest to check out if the built-in functionality is a match for your needs. I will still accept PRs for bug fixes and small enhancements but won't otherwise implement any new functionality.

See Changes for a high-level overview of recent updates.

Need to talk to a human? I'm @claushoefele on Twitter.

Getting started

If you have your project set up with an MKMapView, integrating clustering will take 4 lines of code:

#import "CCHMapClusterController.h"
  
@interface ViewController()

@property (strong, nonatomic) CCHMapClusterController *mapClusterController;

@end

- (void)viewDidLoad
{
  [super viewDidLoad]
    
  NSArray annotations = ...
  self.mapClusterController = [[CCHMapClusterController alloc] initWithMapView:self.mapView];
  [self.mapClusterController addAnnotations:annotations withCompletionHandler:NULL];
}

Don't worry about manually updating the clusters; CCHMapClusterController automatically knows when changes have occurred that require the clusters to regroup.

To try out the clustering, experiment with the example included in this project, or download the app “Stolpersteine in Berlin” .

Usage

• Installation
• Performance
• Cell size and margin factor
• Custom annotation views
• Custom titles and subtitles for callouts
• Positioning cluster annotations
• Cluster grouping
• Dynamically disabling clustering
• Animations
• Code recipes
• Finding a clustered annotation
• Centering the map without changing the zoom level
• Receiving taps on annotation views
• Zooming in to a cluster
• Showing callout accessory views for unclustered annotations only
• Additional reading
• License (MIT)

Installation

Use CocoaPods to easily integrate CCHMapClusterController into your project. Minimum deployment targets are 7.0 for iOS and 10.9 for OS X.

platform :ios, '7.0'
pod "CCHMapClusterController"

platform :osx, '10.9'
pod "CCHMapClusterController"

Performance

The clustering algorithm splits a rectangular area of the map into a grid of square cells. For each cell, a representation for the annotations in this cell is selected and displayed.

The quad tree implementation used to gather annotations for a cell is based on TBQuadTree and is very fast. For this reason, performance is less dependent on the number of clustered annotations, but rather on the number of visible clusters on the map. This number can be configured with the cell size and the margin factor (see below).

Other factors are the density of the clustered annotations (annotations spread over a large area cluster faster) and the way annotation views are implemented (if possible, use images instead of drawRect:).

The example in this project contains two data sets for testing: 5000+ annotations in a small area around Berlin and 80000+ annotations spread over the entire US. Both data sets perform fine on an iPhone 4S.

Cell size and margin factor

CCHMapClusterController has a property cellSize to configure the size of the cell in points (1 point = 2 pixels on Retina displays). This way, you can select a cell size that is large enough to display map icons with minimal overlap. More likely, however, you will choose the cell size to optimize clustering performance (the larger the size, the better the performance). The actual cell size used for clustering will be adjusted so that the map's width is a multiple of the cell size. This avoids realignment of cells when panning across the 180th meridian.

The marginFactor property configures the additional map area around the visible area that's included for clustering. This avoids sudden changes at the edges of the visible area when the user pans the map. Ideally, you would set this value to 1.0 (100% additional map area on each side), as this is the maximum scroll area a user can achieve with a panning gesture. However, this affects performance as this will cover 9x the map area for clustering. The default is 0.5 (50% additional area on each side).

To debug these settings, set the debugEnabled property to YES. This will display the clustering grid over the map.

Custom annotation views

Cluster annotations are of type CCHMapClusterAnnotation. Apart from implementing the MKAnnotation protocol, this class' property annotations exposes an array of annotations contained in the cluster. CCHMapClusterAnnotation also has two properties that can help you categorize the cluster annotation: isCluster returns YES if the cluster annotation has more than one annotation and isUniqueLocation if all annotations in this cluster have the same location.

Customizing the look of cluster annotations is possible via the standard mapView:viewForAnnotation: method that's part of MKMapViewDelegate.

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id<MKAnnotation>)annotation
{
    MKAnnotationView *annotationView;
    
    if ([annotation isKindOfClass:CCHMapClusterAnnotation.class]) {
        ...
        annotationView = clusterAnnotationView;
    }
    
    return annotationView;
}

In addition, the delegate method mapClusterController:willReuseMapClusterAnnotation: is called when a cluster annotation is reused for a cell. The reused cluster annotation will have the same location as before, but will contain different annotations. This avoids annotations moving around while adding more data (see the property reuseExistingClusterAnnotations below on how to disable this behavior).

Make sure that you implement both mapView:viewForAnnotation: and mapClusterController:willReuseMapClusterAnnotation: to always have the annotation view in a consistent state.

- (void)mapClusterController:(CCHMapClusterController *)mapClusterController willReuseMapClusterAnnotation:(CCHMapClusterAnnotation *)mapClusterAnnotation
{
    ClusterAnnotationView *clusterAnnotationView = (ClusterAnnotationView *)[self.mapView viewForAnnotation:mapClusterAnnotation];
    ...
}

The iOS example contains code that demonstrates how to display the current cluster size as part of the annotation view.

Custom titles and subtitles for callouts

You can customize titles and subtitles used for their callouts by registering as a CCHMapClusterControllerDelegate with CCHMapClusterController and implementing two delegate methods.

In these methods, CCHMapClusterAnnotation gives you access to the annotations contained in the cluster through the property annotations. An annotation in this array will always implement MKAnnotation, but is otherwise of same type as the instances you added to CCHMapClusterController when calling addAnnotations:withCompletionHandler:.

Here is an example:

- (NSString *)mapClusterController:(CCHMapClusterController *)mapClusterController titleForMapClusterAnnotation:(CCHMapClusterAnnotation *)mapClusterAnnotation
{
    NSUInteger numAnnotations = mapClusterAnnotation.annotations.count;
    NSString *unit = numAnnotations > 1 ? @"annotations" : @"annotation";
    return [NSString stringWithFormat:@"%tu %@", numAnnotations, unit];
}

- (NSString *)mapClusterController:(CCHMapClusterController *)mapClusterController subtitleForMapClusterAnnotation:(CCHMapClusterAnnotation *)mapClusterAnnotation
{
    NSUInteger numAnnotations = MIN(mapClusterAnnotation.annotations.count, 5);
    NSArray *annotations = [mapClusterAnnotation.annotations.allObjects subarrayWithRange:NSMakeRange(0, numAnnotations)];
    NSArray *titles = [annotations valueForKey:@"title"];
    return [titles componentsJoinedByString:@", "];
}

Positioning cluster annotations

For aesthetic reasons, cluster annotations are not lined up evenly as this would make the underlying grid obvious. This library comes with two implementations to configure the position of cluster annotations:

• CCHCenterOfMassMapClusterer (default): computes the average of the coordinates of all annotations in a cluster
• CCHNearCenterMapClusterer: uses the position of the annotation in a cluster that's closest to the center

Instances of these classes can be assigned to CCHMapClusterController's property clusterer. By implementing the protocol CCHMapClusterer, you can provide your own strategy for positioning cluster annotations.

In addition, CCHMapClusterController by default reuses cluster annotations for a cell. This is beneficial for incrementally adding more annotations to the clustering (e.g. when downloading batches of data) because you want to avoid the cluster annotation jumping around during updates. Set reuseExistingClusterAnnotations to NO if you don't want this behavior.

Cluster grouping

To have independent groups of clusters, more than one CCHMapClusterController can work on the same MKMapView instance. Each CCHMapClusterController can have its own settings.

// First cluster controller
self.mapClusterControllerRed = [[CCHMapClusterController alloc] initWithMapView:self.mapView];
self.mapClusterControllerRed.cellSize = ...;
self.mapClusterControllerRed.marginFactor = ...;
    
// Second cluster controller
self.mapClusterControllerBlue = [[CCHMapClusterController alloc] initWithMapView:self.mapView];
self.mapClusterControllerBlue.cellSize = ...;
self.mapClusterControllerBlue.marginFactor = ...;

Dynamically disabling clustering

Sometimes it's helpful to disable clustering based on the current map data. This allows you to show unclustered annotations if needed. There are two properties for this purpose:

• maxZoomLevelForClustering: controls clustering for the entire map based on how far the map has been zoomed in. To disable clustering, set the maxZoomLevelForClustering property to the zoom level where you want clustering to stop. A zoom level of 0 means that the entire map fits the screen width and the value increases when zooming in. You can retrieve the current zoom level from CCHMapClusterController's property zoomLevel. By default, maxZoomLevelForClustering is set to DBL_MAX, which means clustering is never disabled.
• minUniqueLocationsForClustering: controls clustering for a cell based on the number of unique locations in a cell. Clustering is disabled if the number of unique locations in a cell is below this value. By default, minUniqueLocationsForClustering is set to 0, which means clustering is never disabled.

The example in this project contains settings to experiment with these properties.

Animations

By default, annotation views for cluster annotations receive an animation that fades the view in when added and out when removed (CCHFadeInOutAnimator). You can provide your own animation code by implementing the protocol CCHMapAnimator and changing CCHMapClusterController's property animator.

Code recipes

This list contains solutions to a number of issues that people have encountered. Feel free to ask additional questions by creating a new issue.

Finding a clustered annotation

A common use case is to have a search field where the user can make a choice from a list of matching annotations. Selecting an annotation would then zoom to its position on the map.

For this to work, you have to figure out which cluster contains the selected annotation. In addition, the clustering changes while zooming thus requiring an incremental approach to finding the cluster that contains the annotation the user is looking for.

CCHMapClusterController contains an easy to use interface to help you with this. Note that you have to use an annotation that has previously been added to the clustering:

id<MKAnnotation> clusteredAnnotation = ...
[self.mapClusterController addAnnotations:@[clusteredAnnotation] withCompletionHandler:NULL];
    
...
    
[self.mapClusterController selectAnnotation:clusteredAnnotation andZoomToRegionWithLatitudinalMeters:1000 longitudinalMeters:1000];

Centering the map without changing the zoom level

MKMapView offers the method setCenterCoordinate:animated: to center the map on a new coordinate without changing the current zoom level. Unfortunately, this method doesn't work as advertised on iOS 7. Instead, calling it will zoom the map slightly thus causing the clusters to regroup with a different zoom level.

The following code avoids this problem:

- (void)mapView:(MKMapView *)mapView didSelectAnnotationView:(MKAnnotationView *)view
{
    MKMapPoint point = MKMapPointForCoordinate(view.annotation.coordinate);
    MKMapRect rect = [mapView visibleMapRect];
    rect.origin.x = point.x - rect.size.width * 0.5;
    rect.origin.y = point.y - rect.size.height * 0.5;
    [mapView setVisibleMapRect:rect animated:YES];
}

Receiving taps on annotation views

mapView:didSelectAnnotationView: behaves differently, depending on the state of the annotation view's canShowCallout property.

If canShowCallout is set to YES, a tap on the annotation view will open a callout. The map view will only call mapView:didSelectAnnotationView: if your annotation title is set to a non-zero-length string. For this reason, you will have to implement mapClusterController:titleForMapClusterAnnotation: to return a title for a cluster annotation.

If you don't want to show a callout on your annotation view, you have to set canShowCallout to NO (the default). Once you have done this, mapView:didSelectAnnotationView: will be called without setting a title.

One caveat is that the map view will remember the last selection. To be able to select the same annotation view again, you have to unselect its annotation first:

- (void)mapView:(MKMapView *)mapView didSelectAnnotationView:(MKAnnotationView *)view
{
    [mapView deselectAnnotation:view.annotation animated:NO];
}

Zooming in to a cluster

On iOS 7, you could use showAnnotations:animated:, but this will also add the given annotations to the MKMapView. Thus you will end up with all the clustered annotations on the screen in addition to the clusters.

Instead, CCHMapClusterAnnotation offers the method mapRect that manually calculates an MKMapRect that includes all clustered annotations:

- (void)mapView:(MKMapView *)mapView didSelectAnnotationView:(MKAnnotationView *)view
{
    if ([view.annotation isKindOfClass:CCHMapClusterAnnotation.class]) {
        CCHMapClusterAnnotation *clusterAnnotation = (CCHMapClusterAnnotation *)view.annotation;
        MKMapRect mapRect = [clusterAnnotation mapRect];
        UIEdgeInsets edgeInsets = UIEdgeInsetsMake(20, 20, 20, 20);
        [mapView setVisibleMapRect:mapRect edgePadding:edgeInsets animated:YES];
    }
}

Use CCHMapClusterController's maxZoomLevelForClustering property if you want to guarantee that each cluster annotation on the map will have one unique location when being zoomed in.

Showing callout accessory views for unclustered annotations only

This can be achieved by setting up the accessory views and then controlling their display status via the canShowCallout property.

- (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id<MKAnnotation>)annotation
{
    MKAnnotationView *annotationView;
    
    if ([annotation isKindOfClass:CCHMapClusterAnnotation.class]) {
        annotationView = ...
        annotationView.rightCalloutAccessoryView = ...

        CCHMapClusterAnnotation *clusterAnnotation = (CCHMapClusterAnnotation *)annotation;
        annotationView.canShowCallout = clusterAnnotation.isUniqueLocation;
    }
}

- (void)mapClusterController:(CCHMapClusterController *)mapClusterController willReuseMapClusterAnnotation:(CCHMapClusterAnnotation *)mapClusterAnnotation
{
    ClusterAnnotationView *clusterAnnotationView = (ClusterAnnotationView *)[self.mapClusterController.mapView viewForAnnotation:mapClusterAnnotation];
    clusterAnnotationView.canShowCallout = clusterAnnotation.isUniqueLocation;
}

Additional reading

• Theodore Calmes' in-depth explanation of how to implement a clustering algorithm
• Video of Claus' presentation at Macoun 2013 that explains some of the techniques used to implement CCHMapClusterController (German)
• Blog article covering Claus' Macoun presentation

License (MIT)

Copyright (C) 2013 Claus Höfele

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.