Mastering Blockchain Programming with Solidity

The Approval event

The Approval event is triggered when the approval of a token is given or changed. 

The event takes three arguments:

• owner: The address of the approver who is the owner of the token
• approved: The address that will be approved for the given token
• tokenId: The tokenId of the NFT for which the approval is initiated

All of the preceding arguments are indexed in the event, which would allow the client applications to filter out the events:

event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

The Approval event is triggered only from the approve() function of the ERC721 implementation.