produceValueWithKey
Exposes a value this tile currently holds, addressed by key — the mechanism behind the Tile data source (tile(tileId, dataKey) in the server DSL), read by GetData/EvaluateData. The default implementation exposes nothing (null); override it on a TileHolder whose tile has a value worth reading this way (see CheckboxTileHolder, which exposes mapOf(key to tile.checked) — key is ignored there since a checkbox only ever has one value to expose, but a tile with several distinct values would branch on it).
Return
a single-entry map pairing key with the requested value, or null if this tile has nothing to expose (or doesn't recognize key).
Parameters
key
the data key requested — meaning is entirely up to this tile's own implementation.