1. Pick the center point.
2. Trace the cursor with (Grread).
3. if the current location of the cursor is away from last point,
say, 0.01 units, insert the block pre-made.
4. Keep tracing the cursor, if no left-click, keep updating the block inerted.
5. If left-click happens, get the last location of cursor, then update the block or delete the block then draw up what you want.