Retrieve distance from visible part of Google map

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I want to draw a static circle over a map with Google Maps. When the user pinches, the map will zoom in/out.

I need to know the map radius (related to the area contained in the circle) and change the seekbar at the bottom accordingly.

Does anybody know a solution how to retrieve the distance from the left to the right screen edge? I didn't find anything at the Google Maps API doc.

Something like this:

enter image description here

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