per comments from nventuro, add an event to flag the existence of this token ID

Author: KaiRo-at

contracts/token/ERC1155/ERC1155.sol

@@ -201,10 +201,17 @@ contract ERC1155 is ERC165, IERC1155
     /**
      * @dev Register a token ID so other contract functionality knows this token
      * actually exists and this ID is valid. Minting will automatically call this.
+     * Also emits an event to flag the existence, see the spec:
+     * "To broadcast the existence of a token ID with no initial balance, the contract
+     * SHOULD emit the TransferSingle event from 0x0 to 0x0, with the token creator as
+     * _operator, and a _value of 0."
      * @param id uint256 ID of the token to register
      */
     function _registerToken(uint256 id) internal virtual {
-        _tokenExists[id] = true;
+        if (!_tokenExists[id]) {
+            _tokenExists[id] = true;
+            emit TransferSingle(msg.sender, address(0), address(0), id, 0);
+        }
     }
 
     /**