All Players come with the ability to add Items to their backpacks. They can even add other Players to their backpack too, provided those Players are also declared to be Items, something completely possible in the Raindrops framework. Something to think about here would be multiple fighters being stored in a Battlecruiser. Here's how to add an Item to a Player. First, you'll need a JSON file that meets the following basic format:

  "playerMint": "edo3DzeqXsFzoaEhGHVn1GRCcFf4CWTaUSU4DDDVpJY",
  "playerClassMint": "GwB8ZtjDXYNR4iheuSS2uLa7EYC9J7b8yaRY9YmXXqUh",
  "addItemPermissivenessToUse": { "tokenHolder": true },
  "index": 3,
  "metadataUpdateAuthority": null,

and metadataUpdateAuthority need only be present if you plan to use updateAuthority as updatePermissivenessToUse instead of something else. The playerMint key is the mint of the Player you wish to add the item to, and the playerClassMint is the mint of the class that spawned that Player. The index is the offset on the playerMint. You can read more about index offsets here.

Then you can issue call from the player-cli using this json file and a few additional arguments, which we'll break down:

player-cli add_item -cp myconfig.json     
        -k ~/new-metaplex/mykey.json --log-level debug 
        -e mainnet-beta 
        -m 7LmihZJPobESB6nrWwk8xPneNFdeVACwg7PSjdAMrkPV 
        -i 1  
        -a 1 
        -cm 7t5G3x4Zq5p2ZbAX8owcSi1tiLwLwYTDMJc1FmAYYpb7

In this command, -m is saying the Item mint I wish to add is 7lmih, it's index offset is 1, the amount of that Item is 1(this could be an SFT Item like a Potion that has many, hence the explicitness), and then finally the -cm stands for "Class Mint", which in this case is 7t5G3x, and points at the ItemClass mint for this particular Item. Between the config file passed and the arguments provided, the player-cli is able to construct the command for the Player contract to have it move the Item from your wallet to your Player's backpack!

A similar command called remove_item exists that operates in reverse and takes the same commands.

If you wish to operate this command from your website, you can use the Raindrops SDK, which will look more like this:

  await (
      await anchorProgram.addItem(
          index: playerIndex,
            config.addItemPermissivenessToUse ||
          amount: new BN(amount),
          metadataUpdateAuthority: config.metadataUpdateAuthority
            ? new PublicKey(config.metadataUpdateAuthority)
            : keypair.publicKey,
          playerClassMint: new PublicKey(config.playerClassMint),
          itemClassMint: classMint ? new PublicKey(classMint) : null,

When added, the Item token account is stored as a PDA relative to the Player PDA with seeds [ PREFIX.as_bytes(), item.key().as_ref(), player.key().as_ref() ] It is explicitly not an associated token account, as to make it one would require an additional nested CPI which is a waste of CPU.

The benefit of this design is that your Player can carry many items while appearing as a single NFT in your wallet. Only Items carried by your Player can be equipped.

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