[OpenLayers-Dev] bug report and fix proposal for TypeError: this.layer is null

Balázs Bámer Balazs.Bamer at grintec.com
Wed Mar 26 01:14:53 PDT 2014

Hi Developers,

We use OpenLayers V2.13.1 in several projects. It works well in most cases,
but in one case it throws
TypeError: this.layer is null
during loading layers. It occurs in FF and IE.

The error rises in OpenLayers.Tile.shouldDraw in line 228:
maxExtent = this.layer.maxExtent;

I have found a similar phenomenon here:
where one corrects the bug by not drawing the already destroyed tiles.

However, this solution does not corrects the root of the problem. My solution
below may be not optimal, too, since we do not have time to dig deep in the
event-based tile management architecture, but is probably closer. This
solution also prohibits a memory leak.

The problem occurs this way:

OpenLayers performs Map layout calculations. First it requests a tile (one WMS
layer with singleTile = true), it draws it, then it realises the viewport
layout is changed, and calls OpenLayers.Tile.destroy. This function makes the
Tile instance practically unusable by setting all important properties to
null. Further operations on this instance ar not feasible any more.

After it (probably after several other calculations) it calls
OpenLayers.TileManager.clearTileQueue to delete all tiles of this layer.
Since the queue contains tiles for all layers in a map, it checks each tile
if it belongs to the layer in question. This check takes place in line 441.
if (tileQueue[i].layer === layer) {

In our case, the layer property of the tile is null, so the invalidated tile
instance escapes from deleting. Later, in a new viewport layout step OpenLayers
wants to draw it, which is not possible any more.

My idea is to delete tile instances with layer===null, too:
if (tileQueue[i].layer === null || tileQueue[i].layer === layer) {

If we let them gather, and somehow OpenLayers won't draw them (as usually
there is no problem), they result in a memory leak.

Best regards:
 Balázs Bámer
Anzengrubergasse 6, 8010 Graz, Austria
Tel: +43(316)383706-0
balazs.bamer at grintec.com<mailto:balazs.bamer at grintec.com>

FN 47845k Handelsgericht Graz

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