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The callback that is invoked when the export button is clicked. You may want to call one of the following functions in this callback: ly.img.engine.BlockApi.export, ly.img.engine.BlockApi.exportWithColorMask, ly.img.engine.BlockApi.exportVideo. Note that the "export" coroutine job will survive configuration changes and will be cancelled only if the editor is closed or the process is killed when in the background.