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2. MeetingRoom NFT Contract Walkthrough

  • It is written in the Solidity Language compiled to WASM bytecode stored in an ethereum blockchain
  • Each deployment of this contract to the blockchain will create a seperate instance with an unique contract address
  • It will run in one of the ethereum blockchain's node

Business logic design

  • It is a ERC721 Non-Fungible Token (NFT)
    • Every Token has only ONE copy and is unique
    • Every Token has a unique URI (the UTC booking date)
    • If I own a Token on 13/07/2024, nobody can own a token on the same date
    • If I transfer my NFT to Mr. A, the ownership of 13/07/2024 booking will be changed to Mr. A as well
    • Each contract instance will have its own NFTs created under the contract instance's address
  • After booking a date, I can release it to make it vacant. I can also occupy it again.
    • Other users can search for vacant booking and request you to transfer the ownership
  • _tokenIds is a Counter from the Counters library
    • current() returns the current value
    • .increment() will add 1
  • Each NFT has a bookingDate and an occupied flag.
    • Booking Date is in Unix time, that is, number of seconds that have elapsed since 1970-01-01T00:00:00Z UTC
    • Booking Time is always 00:00:00.
    • online converter playground
    • In C#, it is a 'long' integer type.
  • _nfts[] = is a list of NFT created with the list index is the Token Id.
    • e.g. _nfts[17] is the NFT with token Id 17 having a bookingDate and an occupied flag.
  • _bookingDates[] = is a list of Token Id with the list index of Dates.
    • e.g. _bookingDates[1704326400] is 17
    • _nfts[17] is the NFT with token Id 17 having a bookingDate 1704326400 (2024年1月4日 00:00:00)
    • if _bookingDates[1704326400] is 0, meaning the list does not contain a booking for 4/1/2024.

Contract Functions

Function & property types

  • 'view' functions will not change anything. It can return a value directly to the caller
  • 'update' functions mine new blocks and write data with a transaction. It cannot directly return data to the caller and need to 'emit' events for the caller to receive.
  • pure: e.g. calculation, string concat. can return value directly to caller.

Access Scope

  • internal / private: can only be called within the contract
  • public: can be called within the contract or outside
  • external: can only be called from outside

Implementations

  • mintMEET will create a new NFT with the desired booking Date
    • Requirements are book Date not exist in _bookingDates[] and the Date must not in the past.
      • There is no DateTime.Now in Solidity.
      • We can only compare it to the Blockchain's block timestamp.
    • _tokenIds.current() gets the new Token Id
    • Create the entries in _nfts[] & _bookingDates[]
    • _safeMint is to create the NFT and transfer to the receipant
    • _setTokenURI is to set the NFT's unique URI
      • The URL that returns the metadata of the NFT, containing
        • NFT Id
        • Booking Date
        • Graphics of the booking date
    • emit is to emit an event for the calling program to receive the result.
      • Contract transactional functions are executed in an ethereum node
      • Return values are emitted to the blockchain's transaction log
      • The calling program needs to get the return values from the log
  • _exists(17) check whether there exist a NFT with Token Id 17
  • _beforeTokenTransfer is a hook override for additional check before transfer
    • if (to == address(0)), it is a burn operation with booking date in the past
    • otherwise, Requirement: cannot transfer NFT with booking dates in the past
    • This hook takes effect even when you do the transfer in your wallet
  • houseKeep() 'burn' NFTs with booking dates in the past
    • _burn(i) will 'burn' the NFT with token Id i
      • Need to delete the corresponding elements in _bookingDates[] & _nfts[]
      • That's why in most function, we need to check _exists(i) before accessing _nfts[i]
  • For the rest of the functions, please refer to the coding.