[OpenLayers-Dev] RE: CacheRead and CacheWrite with Web SQL Storage?

[Vardeman, Andrew [CSSM]]

Dom,

Thanks for the response.  I've done something very similar to what you suggest.  The results I see suggest a race of some sort: sometimes I get the cached image, but sometimes I get a pink or transparent tile instead.  When I use the localStorage version (or any code where tile.url gets set synchronously within the fetch method), things are reliable and consistent.

>From what I see, lots of methods within OpenLayers.Tile.Image call setImgSrc, such that what I set from my database callback could be overwritten by one of them.  So here's a possible explanation from my admittedly poor understanding of the code:

My modified OpenLayers.Control.CacheRead.fetch method (responding to the tileloadstart event) kicks off an asynchronous database query.  Meanwhile, my fetch method returns without having modified the tile's URL.  Tile loading proceeds with the unmodified URL.  OpenLayers.Tile.Image.initImage calls setImgSrc to blank out the tile, and then there's a race between the blank image loading and my database query returning.  When my query returns, I call setImgSrc, briefly setting the image's src attribute to the loaded data URL.  If the blank image finishes loading after my query returns, the src attribute is set back to the unmodified tile.url, and if the user is offline, loading of that URL fails and the result is a pink tile.

Thanks for your help; I'll keep reading code to try to better understand the issue.

Andrew

-----Original Message-----
From: Dominik Mikiewicz [mailto:dominikmikiewicz at o2.pl] 
Sent: Wednesday, April 04, 2012 2:17 AM
To: 'Vardeman, Andrew [CSSM]'; openlayers-dev at lists.osgeo.org
Subject: RE: [OpenLayers-Dev] RE: CacheRead and CacheWrite with Web SQL Storage?

Andrew,

I haven't had a chance to play with these cool cotrols yet but from what I see fetching data asynchronously should not be a problem indeed.
In the fetch method you have the access to a tile so you can pass it along with your async calls and then modify the tile imgSrc when completed.

Try something like this (not tested):

getTileData : function(tile){
 		
	this.db.transaction(
		function(tx){
	 		tx.executeSql(
				'HereGoesYourSql;',
				[andParams],
				function (tx, results) { //success callback
					if(results.rows.length == 1){
						var tileCache =
results.rows.item(0);
						
	
tile.setImgSrc(tileCache.data);
					}
				},
				function(tx, error){ //error callback
				}
			);
		}
	);
}

Hope this help

dom 

-----Original Message-----
From: openlayers-dev-bounces at lists.osgeo.org
[mailto:openlayers-dev-bounces at lists.osgeo.org] On Behalf Of Vardeman, Andrew [CSSM]
Sent: Tuesday, April 03, 2012 10:47 PM
To: openlayers-dev at lists.osgeo.org
Subject: [OpenLayers-Dev] RE: CacheRead and CacheWrite with Web SQL Storage?

Okay, I've started on this and have the OpenLayers.Control.CacheWrite.cache
part working (tiles are getting cached to the Web SQL DB), but I think I've run into a problem with the OpenLayers.Control.CacheRead.fetch method.  I'm not quite sure what's happening, but it appears that the fetch method needs to complete synchronously for the dataURL it supplies to be used as the image src.  Does that sound right?  If so, I think I'm dead in the water.

Andrew

-----Original Message-----
From: openlayers-dev-bounces at lists.osgeo.org
[mailto:openlayers-dev-bounces at lists.osgeo.org] On Behalf Of Vardeman, Andrew [CSSM]
Sent: Monday, April 02, 2012 1:44 PM
To: openlayers-dev at lists.osgeo.org
Subject: [OpenLayers-Dev] CacheRead and CacheWrite with Web SQL Storage?

Hi, OpenLayers developers.  This is my first email; I hope I'm posting this to the proper list.

I'm interested in the work done here:

https://github.com/openlayers/openlayers/pull/301

The pull request says that "even asynchronous backends could be added."  By any chance, is anyone working on Web SQL Storage version or the mentioned pluggable version with multiple backends?  I need such a thing for my current project, so I'll probably try if no one else is already doing it.
If I get something working that's sufficiently generic, I'll submit it as a patch.

Thanks,

Andrew Vardeman

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