DynamicLayer Auto-Refresh for ESRI Silverlight API

Despite recent news regarding Silverlight, I expect some of my projects to continue using it for the near term. Others may be taking the same tack, so I thought I’d go ahead and offer this up.

Several of the projects I support have the requirement to periodically refresh specific layers in order to track change or movement. These layers can range from weather to vehicle locations and such. I have typically accomplished this with a timer that refreshes the layer(s) on a specified interval. This can get rather cumbersome if you have different layers that require different refresh intervals.

Working within Silverlight, I have the option of using an existing layer class as a base class and extending it to include an automatic refresh capability. However, some classes, such as the ArcGISDynamicMapServiceLayer, are sealed and cannot be extended. Luckily, the Expression Blend SDK enables me to get around this by attaching a custom behavior.

The ArcGISDynamicMapServiceLayer descends from the DynamicLayer inheritance hierarchy, which provides access to a Refresh method. So I developed the following behavior to attach to a DynamicLayer (or anything that derives from it). It makes use of a DispatcherTimer to refresh the layer to which it is attached. In this particular implementation, each layer gets its own timer. That met my requirement of having different refresh rates for different layers. It may be preferable in some instances to pass in a reference to a single timer to control all layers. In either case, you can simply attach the behavior only to those layers that you need to refresh. This can get you around iterating the map’s Layers collection at each timer tick (or other such approaches).

This is the code for the behavior itself. Scroll down for a snippet showing how to implement it.

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using System;
using System.Windows.Input;
using System.Windows.Interactivity;
using System.Windows.Threading;
using ESRI.ArcGIS.Client;
using ESRI.ArcGIS.Client.Geometry;
using System.Windows;

namespace Zekiah.ArcGIS.Behaviors
{
    /// <summary>
    /// Adds automatic refresh to a DynamicLayer object (or any
    /// class that inherits from DynamicLayer. 
    /// </summary>
    public class AutoRefresh : Behavior<DynamicLayer>
    {
         private DispatcherTimer _tmr = null;

        /// <summary>
        /// Called after the behavior is attached to an AssociatedObject.
        /// </summary>
        /// <remarks>Override this to hook up functionality to the AssociatedObject.</remarks>
        protected override void OnAttached()
        {
            base.OnAttached();
            
            initTimer();
        }

        public TimeSpan Interval
        {
            get
            {
                if (_tmr == null)
                {
                    initTimer();
                }
                return _tmr.Interval;
            }
            set
            {
                if (_tmr == null)
                {
                    initTimer();
                }
                _tmr.Interval = value;
            }
        }

        public void Start()
        {
            if (_tmr == null)
                initTimer();
            if (!_tmr.IsEnabled)
                _tmr.Start();
        }

        public void Stop()
        {
            if ((_tmr != null) && (_tmr.IsEnabled))
                _tmr.Stop();
        }

        /// <summary>
        /// Called when the behavior is being detached from its AssociatedObject, 
        /// but before it has actually occurred.
        /// </summary>
        /// <remarks>Override this to unhook functionality from the AssociatedObject.</remarks>
        protected override void OnDetaching()
        {
            base.OnDetaching();
            _tmr = null;
        }

        private void initTimer()
        {
            _tmr = new DispatcherTimer();
            _tmr.Interval = new TimeSpan(0, 0, 30); //default thirty second interval
            _tmr.Tick += new EventHandler(_tmr_Tick);
        }

        void _tmr_Tick(object sender, EventArgs e)
        {
            this.AssociatedObject.Refresh();
        }

     }
}

The following code shows how to attach the behavior. In this case, I just kept the default 30 second interval (more than enough for this weather layer). For layers of type ArcGISDynamicMapServiceLayer, it is necessary to set DisableClientCaching to true or you’ll never see a refresh.

            var lyr = new ESRI.ArcGIS.Client.ArcGISDynamicMapServiceLayer();
            lyr.DisableClientCaching = true;
            lyr.Url = "http://services.nationalmap.gov/ArcGIS/rest/services/NEXRAD_Weather/MapServer";

            var autoRefresh = new Zekiah.ArcGIS.Behaviors.AutoRefresh();
            var behaviors = System.Windows.Interactivity.Interaction.GetBehaviors(lyr);
            behaviors.Add(autoRefresh);
            autoRefresh.Start();
            this.Map.Layers.Add(lyr);

I’ve used this for both ArcGIS Server dynamic layers as well as WMS layers so far. I also have a version that attached to GraphicsLayer objects so that I can refresh GeoRSS layers. So far this technique has worked pretty well for me.

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6 Responses to DynamicLayer Auto-Refresh for ESRI Silverlight API

  1. Pingback: Tweets that mention Blogged - DynamicLayer Auto-Refresh for ESRI Silverlight API: -- Topsy.com

  2. Morten says:

    Perhaps it was an idea to set DisableClientCaching to true in the onattached method, so you don’t forget?

    • Bill Dollins says:

      Great idea. I’ll do that. Thanks!

    • Bill Dollins says:

      I remember now why I didn’t do it that way. This behavior attaches to anything that descends from DynamicLayer. The DynamicLayer and DynamicMapServiceLayer classes don’t have a DisableClientCaching property. That only appears on the ArcGISDynamicMapServiceLayer class. I didn’t want this behavior to assume that it was always dealing with an ArcGIS Server layer.

  3. Geo_Rube says:

    So, how will this help me get a GISP?

    • Bill Dollins says:

      I suppose it won’t help you. If I bothered to track such things, I guess blogging it could help *me* get a GISP. Then again, if I were interested in that, I would have grandfathered when I could have.

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