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--- tecnica:gps_cartografia_gis:openlayers_900913 [2010/11/05 14:21] – niccolo
+++ tecnica:gps_cartografia_gis:openlayers_900913 [2010/11/05 14:23] – niccolo
@@ Line 19: / Line 19: @@
 Questo trucco funziona sul sito di OpenStreetMap, purtroppo non funziona su altri simili che usano sempre OpenLayers, ad esempio http://www.informationfreeway.org/.
-===== Dynamic layer update =====
-We want to refresh the contents of a layer using a timer, every time we want to fetch new data from the server.
-==== OpenLayers.Layer.GeoRSS ====
-This is an example of a **GeoRSS** layer, updated every 120 seconds. Every point is drawn with the same icon, a pop-up will appear when the icon is clicked.
-<code javascript>
-var friends;
-function init() {
-    ...
-    // Add tangoGPS friends layer.
-    friends = new OpenLayers.Layer.GeoRSS("Friends", "tangogps-friend.php", {
-        projection: map.displayProjection
-    });
-    friends.icon = new OpenLayers.Icon(
-        "icons/tangogps-friend.png");
-    map.addLayer(friends);
-    ...
-    var tim = setInterval ("updateFriends()", 120000);
-}
-function updateFriends() {
-    friends.clearMarkers();
-    friends.clearFeatures();
-    friends.loaded = false;
-    friends.loadRSS();
-}
-</code>
-The code **''tangogps-friend.php''** should return GeoRSS formatted text (like **{{.openlayers:georss.xml|this example}}**), with proper expire and no cache headers:
-<code php>
-header("Cache-Control: no-cache, must-revalidate");
-header("Expires: Sat, 26 Jul 1997 05:00:00 GMT");
-header("Content-Type: text/xml");
-...
-</code>
-==== OpenLayers.Layer.GML ====
-This is a more complex example, using a **GML** layer in GML format. The GML geometry is drawn with the default style. The layer is reloaded by a timer every 120 seconds. We use a BBOX strategy, so that the layer is reloaded also when the bounding box changes. Layer is loaded via HTTP protocol.
-<code javascript>
-var friends;
-function init() {
-    ...
-    // Add tangoGPS friends layer.
-    friends = new OpenLayers.Layer.GML("GML", "friends-gml.php", {
-        strategies: [new OpenLayers.Strategy.BBOX()],
-        projection: map.displayProjection,
-        protocol: new OpenLayers.Protocol.HTTP({
-            url: "friends-gml.php",
-            format: new OpenLayers.Format.GML()
-        }),
-    });
-    map.addLayer(friends);
-    ...
-    var tim = setInterval ("updateFriends()", 120000);
-}
-function updateFriends() {
-    var now = new Date().valueOf();
-    var bbox = map.getExtent().toBBOX();
-    friends.setUrl("friends-gml.php?bbox=" + bbox + "&time=" + now);
-}
-</code>
-The **''friends-gml.php''** should return a valid GML file (like **{{.openlayers:gml.xml|this example}}**) and it should be non cacheable (using the same headers of GeoRSS layer above). The script will be called with the following arguments:
-<file>
-friends-gml.php
-</file>
-The first call (without query parameters) is triggered by the second argument of the layer constructor. To optimize memory usage in the browser, the script **''friends-gml.php''** should honor the **''bbox''** request and return only the relevant features. Called without the parameter, it can return no features.
-<file>
-friends-gml.php?bbox=-10.77,41.32,11.72,52.57
-</file>
-Immediately after the layer creation, a new call is triggered by the BBOX strategy. This time the **bbox** parameter is automatically appended to the **''url''** declared with **''OpenLayers.Protocol.HTTP''**. A similar call is performed whenever a bounding box change event is detected.
-<file>
-friends-gml.php?bbox=-15.15,44.13,16.12,49.75&time=1232668479793
-</file>
-Every timer interval, the layer is refreshed. Reading new data is forced by setting a new and unique **''time''** parameter. In this case ''bbox'' is not automatically added (it was the timer which trigger the call, not the BBOX strategy), so we have to add it programmatically.
-==== OpenLayers.Layer.Text ====
-This is a very versatile example: every point can be represented by a different icon, a pop-up appears when you click an icon. The disadvantage is that all the layer features are loaded at every timer interval, no bounding box limit is enforced.
-<code javascript>
-var friends;
-function init() {
-    ...
-    // Add tangoGPS friends layer.
-    friends = new OpenLayers.Layer.Text("Friends", {
-        location:"friends-text.php",
-        projection: map.displayProjection});
-    map.addLayer(friends);
-    ...
-    var tim = setInterval ("updateFriends()", 120000);
-}
-function updateFriends() {
-    // Remove markers and features.
-    friends.clearMarkers();
-    friends.clearFeatures();
-    // Pretend the layers is not yet loaded.
-    friends.loaded = false;
-    friends.location = "friends-text.php";
-    friends.loadText();
-}
-</code>
-The text returned by **''friends-text.php''** should be non cacheable as always and contains a tab separated text, like **{{.openlayers:layertext.txt|this example}}**.
-This recipe can be improved by adding a ''bbox'' parameter (calculated at runtime) to the location of the layer, and forcing a call to ''updateFriends()'' whenever the viewport of the map changes.