Registers a callback that's fired when the map bounds change. This is fired during pan, zoom, and when the map's bounds are changed programmatically.
Returns an ID which can be passed to unlisten() to unregister the callback.
The ui.Map instance.
The callback to fire when the map bounds change. The callback is passed two parameters: an object containing the coordinates of the new map center (with keys lon, lat, and zoom) and the map widget itself.
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