MarkerDatastore<T> class abstract

Abstract base class for managing collections of map markers with dynamic loading capabilities.

A MarkerDatastore provides an interface for storing, retrieving, and managing markers that can be dynamically loaded based on the current map view. This enables efficient handling of large marker datasets by only loading markers relevant to the visible area.

Key Concepts

• Dynamic Loading: Markers are loaded based on zoom level and visible bounding box
• Change Notification: Extends ChangeNotifier to trigger UI updates when markers change
• Zoom-Aware: Different markers can be shown at different zoom levels
• Boundary-Based: Markers are queried based on geographic boundaries
• Generic Type Support: Supports typed markers via generic parameter T

Lifecycle Methods

The overlay system calls these methods to manage marker loading:

1. askChangeZoomlevel: Called when zoom level changes - opportunity to load/filter markers
2. askChangeBoundingBox: Called when map moves - opportunity to load markers for new area
3. askRetrieveMarkersToPaint: Called during rendering - returns markers to display

Implementation Strategy

Implementing classes should consider:

• Caching: Cache markers to avoid repeated loading
• Filtering: Filter markers by zoom level and bounding box for performance
• Clustering: Group nearby markers at lower zoom levels
• Lazy Loading: Load markers on-demand from databases or network sources
• Memory Management: Remove off-screen markers to control memory usage

Usage Example

class MyMarkerDatastore extends MarkerDatastore<String> {
  final List<Marker<String>> _markers = [];
  BoundingBox? _currentBounds;
  int _currentZoom = -1;

  @override
  void askChangeZoomlevel(int zoomlevel, BoundingBox boundingBox, PixelProjection projection) {
    _currentZoom = zoomlevel;
    _currentBounds = boundingBox;
    _loadMarkersForView();
  }

  @override
  void askChangeBoundingBox(int zoomlevel, BoundingBox boundingBox) {
    _currentBounds = boundingBox;
    _loadMarkersForView();
  }

  @override
  Iterable<Marker<String>> askRetrieveMarkersToPaint() {
    return _markers.where((marker) => _isMarkerVisible(marker));
  }

  void _loadMarkersForView() {
    // Load markers from database/API based on _currentBounds and _currentZoom
    // Update _markers list
    requestRepaint(); // Notify UI of changes
  }
}

Performance Tips

• Implement efficient spatial indexing for marker queries
• Use marker clustering for high-density areas
• Cache marker data to minimize database/network requests
• Filter markers by zoom level to reduce rendering overhead
• Call requestRepaint only when markers actually change

See also:

• MarkerDatastoreOverlay for displaying markers from a datastore
• DefaultMarkerDatastore for a ready-to-use implementation
• SingleMarkerOverlay for displaying individual markers