Hi,1)Map has set of Layers to be viewed. layers property
layers:Object
This property lets you use either an Array, an ArrayCollection, or a single Layer...
Layer index in this collection is the layer location. Layer with greater index is on top of layer with smaller index.Example:Layer index=0, name='Topo'Layer index=1, name='Imagery'Layer index=2, name='Street' (the only visible to client)<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx"
xmlns:esri="http://www.esri.com/2008/ags">
<esri:Map>
<esri:ArcGISTiledMapServiceLayer url="http://server.arcgisonline.com/ArcGIS/rest/services/USA_Topo_Maps/MapServer"
name="Topo"/>
<esri:ArcGISTiledMapServiceLayer url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer"
name="Imagery"/>
<esri:ArcGISTiledMapServiceLayer url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer"
name="Street"/>
</esri:Map>
</s:Application>
2) Note that all layers extend UIComponent and thus include basic mouse events, such as: click, mouseOut, mouseOver, and mouseDown, as well as other events like show and hide, and general properties, such as alpha and visible.
Example:Layer index=0, name='Topo', visible=trueLayer index=1, name='Imagery', visible=true (the only visible to client)Layer index=2, name='Street', visible=false<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
xmlns:s="library://ns.adobe.com/flex/spark"
xmlns:mx="library://ns.adobe.com/flex/mx"
xmlns:esri="http://www.esri.com/2008/ags">
<esri:Map>
<esri:ArcGISTiledMapServiceLayer url="http://server.arcgisonline.com/ArcGIS/rest/services/USA_Topo_Maps/MapServer"
name="Topo"/>
<esri:ArcGISTiledMapServiceLayer url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer"
name="Imagery"/>
<esri:ArcGISTiledMapServiceLayer url="http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer"
name="Street"
visible="false"/>
</esri:Map>
</s:Application>
3)Also layer has such property as isInScaleRange. ...Returns true if the current scale of the map is between the maximum and minimum scale of the layer...
[/HR]U must collect layer information (alpha, visible, index in map layer collection ...), and compare it.The same logic when U need to find top most visible sublayer in a layer. Sample with sources.