[OpenLayers-Users] how to caculate the required tiles in the current map div

[maven apache]

Hi:

For some reasons,we can not use the google map or openlayers in our project.

So we have to make our own map library. We have the map tiles in the server
side, the orignal point is (-180,90),tilesize is 256*256.

The core point is requesting related tiles according to the center of the
map div .

I can make it ,that's to say,when the map is first inited,the tile will be
loaded correctly, but when user drag  the div,new tiles have to be
loaded,now ,I have no idea how to relayout the tiles,since some tiles have
been layouted inner the div, so only new tiles should be loaded and
layouted.

How to decide which tiles is loaded yet and which should to be loaded? and
after the new tiles are loaded,how to layout them with the existed tiles?

I try to read the openlayer source codes,but I can find the right way,I can
not get the key point.


So I wonder any one can tell me the right manner?

Maven.
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