Rahul Vaidya’s Blog

This submission is in response to this gmaps-api-flex feature request. This is a hack that provides functionality like what was requested here.

So you’ve added a custom tile layer to your Google Maps app in Flex, which lets you show all your pretty spatial features. Fantastic! If you want to make this tile layer dynamic, however, you run into a few problems. The Google Maps Flex API caches tiles that it has loaded from a specific Tile Overlay, so if your data changes, the changes won’t be reflected on your map immediately. There have been various hacks that force a refresh of the map, but they don’t completely meet the requirements set in this feature request. The attached code here is a hack that tries to mimic the requested functionality, without refreshing the entire map.

After exploring a little, I saw that one way the Flex API differentiates overlays is by the request URL. If you create an overlay, add it to the map, remove it, create a new overlay with the same URL, and add the new overlay, it will load the tiles from the internal cache. Now, this functionality can be nice if you’re switching between two different overlays, and there haven’t been any changes in the data. For instance, if an app requests tiles from the following URLs in order:

mapserver.php?X=0&Y=0&Z=0&LAYERS=calicounties

mapserver.php?X=0&Y=0&Z=0&LAYERS=oregcounties

it will load both of these layers properly, and cache them both. If you then load the calicounties layer, using the same URL string, it’ll give you back the cached version.

However, if the data has changed, you’d want to reload the entire layer. To do this, I have appended a garbage GET variable to the URL. The requests will now look like:

mapserver.php?X=0&Y=0&Z=0&LAYERS=calicounties&GARBAGE=SDK34K

mapserver.php?X=0&Y=0&Z=0&LAYERS=oregcounties&GARBAGE=SDK34K

Simply change the GARBAGE variable when loading calicounties again and it’ll load from the source.

The included code implements four events that are dispatched to/listened from the map object:

• TileLayerRefresh
  • Tell the Tile Layer to refresh (do not change the GARBAGE variable)
• TileLayerReload
  • Tell the Tile Layer to reload (change the GARBAGE variable)
• TileLayerRefreshed
  • Refresh/Reload completed successfully (all tiles loaded)
  • Implemented by incrementing a counter whenever Loader instances are created, and decrementing the counter whenever they complete/fail.
• TileLayerRefreshError
  • If any loading errors occurred.

Note: You need to pass the TileLayer both the TileOverlay that holds it and the Map object, so it can handle refreshing and event dispatch by itself.

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var tileLayer:MapServerTileLayer = new MapServerTileLayer(map);
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var tileLayerOverlay:TileLayerOverlay = new TileLayerOverlay(tileLayer);
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tileLayer.setOverlay(tileLayerOverlay);
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map.addOverlay(tileLayerOverlay);
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For an example on using MapServer/PHP MapScript with Google Maps, go to this blog post.

A nice little function I found posted by “lazynitwit” on BlueHost forums:

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function getHeaders()
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{
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    $headers = array();
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    foreach ($_SERVER as $k => $v)
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    {
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        if (substr($k, 0, 5) == "HTTP_")
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        {
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            $k = str_replace('_', ' ', substr($k, 5));
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            $k = str_replace(' ', '-', ucwords(strtolower($k)));
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            $headers[$k] = $v;
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        }
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    }
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    return $headers;
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}  

The PHP function getallheaders() only works on Apache, and only if Apache has been installed as a module.

It turns out that this information is within the $_SERVER global variable, and can be parsed out.

This code is useful if, for instance, you want to differentiate between user browser requests and requests made through AJAX. Some of the JavaScript frameworks (jQuery, Prototype, YUI, MooTools) send a special header,

X-Requested-With: XMLHttpRequest

whenever a request is made. I have put this to use in my phpFreeChat chat box to allow channels to be specified in the URL as GET parameters.

via request headers php – bluehostforum.com.

Information on using SQLite for local storage on Cocoa isn’t all that easy to find. The best resource, as always, is Google, and you can find a few tutorials with step by step instructions and code samples on how to use it. Well, here’s another basic step by step set of instructions, but rather than including code samples, I have written a couple of wrapper classes for creating databases, making queries, and getting results.

Steps:

1: Include the SQLite framework in your project. It’s usually located somewhere like “/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS2.1.sdk/usr/lib/libsqlite3.0.dylib” (why is developer in there twice?)

2: Create a base sqlite database file, and modify SQLite.m so that the filename method refers to it. Place it in your main resource path. This will be copied out of your bundle into your application’s Documents directory. If you need help on creating one of these and managing it, there is a fantastic FireFox extension that does it for you!

3: Add the source files to your project.

Usage can be as simple as:

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SQLiteResult *result = [SQLite query:@"SELECT * from test;"];
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NSArray *row = [result.rows objectAtIndex:0];
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NSString *firstValue = [row objectAtIndex:0];
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NSArray columnNames contains NSStrings of all the names.
NSArray columnTypes contains NSStrings of all the types.
NSArray rows contains NSArrays, where each of these contains NSStrings of all the row values in that row.

Generating Google Maps tiles using MapServer is not completely intuitive. For static map applications, a new tile mode has recently been created for MapServer CGI mode ( http://mapserver.org/output/tile_mode.html ).

For more dynamic mapping applications, PHP MapScript is an ideal way execute this, as it only requires a very small static component (an initial map file, which won’t even be needed when MapServer 5.4 comes). Included is a short PHP example on how do go about doing this. The PROJ.4 projection parameters included are for the Google Maps mercator projection.

The sample code includes WGS84 to Google Maps mercator projection as well as the calculation of mercator extents from Google Maps tile indices.

This example has been extended from the previous iteration to include dynamic layer generation and POINT geometry feature symbols via image files. The $phplayers variable holds layer definitions that have been queried from a database table, obtained using the getLayersForSessionID($sessionID) function. The table includes layer table name, layer geometrytype layer geometry name, and a symbol definition for the layer. For point layers, this is an image file (ie. “image.gif”), and for polygon and line layers, this is a line color (255 0 0 for red).

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