LayerBase.ViewportChanged Event

Occurs every time the current viewport is changed.

Assembly : DevExpress.XtraMap.v25.2.dll

Declaration

C#
VB.NET

csharp

public event ViewportChangedEventHandler ViewportChanged

Public Event ViewportChanged As ViewportChangedEventHandler

Event Data

The ViewportChanged event's data class is ViewportChangedEventArgs. The following properties provide information specific to this event:

Property	Description
Angle	Returns the Map Control’s rotation angle in degrees.
BottomRight	Gets the bottom right coordinate visible on a map.
IsAnimated	Gets whether or not the animation effect is used while the position of the current viewport is changed.
TopLeft	Gets the top left coordinate visible on the map.
ZoomLevel	Returns the zoom level of the current viewport.

Remarks

How to Detect Visible Items

The following example shows how to determine the number of visible vector items (in this example, map dots) on the map surface. A list box contains the coordinates of the visible items and a label displays their number. The list of visible items is re-calculated when the map viewport is changed. For example, when a user zooms the map.

To do so, handle the ViewportChanged event of the layer that contains vector items. In the event handler, determine whether item coordinates are in the viewport. The viewport boundaries are defined by the e.TopLeft and e.BottomRight properties:

C#
VB.NET

csharp

using DevExpress.Map;
using DevExpress.XtraMap;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Windows.Forms;

namespace MapVisibleItems {
    public partial class Form1 : Form {
        VectorItemsLayer Layer { get { return (VectorItemsLayer)mapControl1.Layers[0]; } }
        MapItemStorage Data { get { return (MapItemStorage)Layer.Data; } }

        public Form1() {
            InitializeComponent();
            Layer.ViewportChanged += Layer_ViewportChanged;
            Data.Items.AddRange(CreateItems());
            UpdateVisibleItemsList(new GeoPoint(90, -180), new GeoPoint(-90, 180));
        }
        static bool IsPointInside(CoordPoint point, CoordPoint topLeft, CoordPoint bottomRight) {
            return (point.GetY() >= bottomRight.GetY()) && (point.GetY() <= topLeft.GetY()) &&
                   (point.GetX() >= topLeft.GetX()) && (point.GetX() <= bottomRight.GetX());
        }
        static List<MapDot> CreateItems() {
            List<MapDot> result = new List<MapDot>();
            for (double lat = -80; lat <= 80; lat += 20)
                for (double lon = -180; lon <= 180; lon += 20)
                    result.Add(new MapDot() { Location = new GeoPoint(lat, lon), Size = 15 });
            return result;
        }
        void UpdateVisibleItemsList(CoordPoint topLeft, CoordPoint bottomRight) {
            IEnumerable<CoordPoint> visibleItems = Data.Items.Select(item => ((MapDot)item).Location).
                                                              Where(location => IsPointInside(location, topLeft, bottomRight));
            listBoxControl1.Items.Clear();
            listBoxControl1.Items.AddRange(visibleItems.ToArray());
            labelControl1.Text = $"Item Count: {listBoxControl1.ItemCount.ToString()}";
        }
        void Layer_ViewportChanged(object sender, ViewportChangedEventArgs e) {
            if (!e.IsAnimated)
                UpdateVisibleItemsList(e.TopLeft, e.BottomRight);
        }
    }
}

Imports DevExpress.Map
Imports DevExpress.XtraMap
Imports System.Collections.Generic
Imports System.Linq
Imports System.Windows.Forms

Namespace MapVisibleItems
    Public Partial Class Form1
        Inherits Form

        Private ReadOnly Property Layer As VectorItemsLayer
            Get
                Return CType(mapControl1.Layers(0), VectorItemsLayer)
            End Get
        End Property

        Private ReadOnly Property Data As MapItemStorage
            Get
                Return CType(Layer.Data, MapItemStorage)
            End Get
        End Property

        Public Sub New()
            InitializeComponent()
            Layer.SelectedItemStyle.Fill = Drawing.Color.Red
            Layer.SelectedItemStyle.StrokeWidth = 10
            Layer.SelectedItemStyle.Stroke = Drawing.Color.Red
            Me.Layer.ViewportChanged += AddressOf Layer_ViewportChanged
            Data.Items.AddRange(CreateItems())
            UpdateVisibleItemsList(New GeoPoint(90, -180), New GeoPoint(-90, 180))
        End Sub

        Private Shared Function IsPointInside(ByVal point As CoordPoint, ByVal topLeft As CoordPoint, ByVal bottomRight As CoordPoint) As Boolean
            Return (point.GetY() >= bottomRight.GetY()) AndAlso (point.GetY() <= topLeft.GetY()) AndAlso (point.GetX() >= topLeft.GetX()) AndAlso (point.GetX() <= bottomRight.GetX())
        End Function

        Private Shared Function CreateItems() As List(Of MapDot)
            Dim result As List(Of MapDot) = New List(Of MapDot)()

            For lat As Double = -80 To 80 Step 20

                For lon As Double = -180 To 180 Step 20
                    result.Add(New MapDot() With {
                        .Location = New GeoPoint(lat, lon),
                        .Size = 15
                    })
                Next
            Next

            Return result
        End Function

        Private Sub UpdateVisibleItemsList(ByVal topLeft As CoordPoint, ByVal bottomRight As CoordPoint)
            Dim visibleItems As IEnumerable(Of CoordPoint) = Data.Items.[Select](Function(item) CType(item, MapDot).Location).Where(Function(location) IsPointInside(location, topLeft, bottomRight))
            listBoxControl1.Items.Clear()
            listBoxControl1.Items.AddRange(visibleItems.ToArray())
            labelControl1.Text = $"Item Count: {listBoxControl1.ItemCount.ToString()}"
        End Sub

        Private Sub Layer_ViewportChanged(ByVal sender As Object, ByVal e As ViewportChangedEventArgs)
            If Not e.IsAnimated Then UpdateVisibleItemsList(e.TopLeft, e.BottomRight)
        End Sub
    End Class
End Namespace