can a webcore piston send a variable to a he dashboard?
I believe there are some 'super tiles' available that might be able to take your variable from webcore. I have not tried any of them myself. What is in this variable?
it is a number
Yes. Theoretically, an RM Global Variable Connector device could be manipulated from webCoRE and displayed on Dashboard, but there are some naming conflicts between the commands you need to run on that device to change the value and a general webCoRE-provided command, which it does not handle well. This thread contains some discussion on that:
For a fix, scroll down to post 23 in that thread, and you'll see a custom driver you can use with command names that do not conflict with webCoRE's own. You can then put this device on a Dashboard with a "Custom Attribute" template, choosing the appropriate attribute (this device has two; you do not need to use both). Any "SuperTiles"-style app/device might be able to help with this, too, but I'm not sure any of them are designed with an outside app directly manipulating their contents in mind, a problem you'd have to address somehow either way.
I use this custom driver on my dashboard:
Here's the webcore piston that writes into the device I created using the custom driver:
This webcore issue for setVariable command in a device is fixed.