The `MyPaymaster` contract extends `BasePaymaster` to facilitate gas fee payments using ERC20 tokens. It validates user operations by ensuring the sender has enough tokens to cover the maximum possible cost. It tracks nonces for senders and calculates token value equivalent to ETH. Post-operation, it attempts to charge the sender's token balance for the gas used, reverting to use deposits if the transfer fails.

The `SimpleAccount` contract functions as a wallet that can execute transactions and manage deposits. It includes signature validation for secure transaction execution, supports batch transactions, and integrates with an `EntryPoint` for account abstraction. The contract is upgradeable via UUPS and can be initialized with an owner. It also handles deposits and checks balances within the `EntryPoint`.

The `SimpleAccountFactory` contract is a factory for creating `SimpleAccount` instances. It uses the `Create2` opcode to deterministically deploy accounts, ensuring the same address is returned even if the account already exists. The `createAccount` function deploys a new `SimpleAccount` using an `ERC1967Proxy` if it doesn't already exist, and the `getAddress` function computes the potential address of an account based on the owner and a salt value.

The `TokenCallbackHandler` contract implements interfaces to handle callbacks for receiving ERC777, ERC721, and ERC1155 tokens. It provides empty implementations for `tokensReceived` (ERC777), `onERC721Received` (ERC721), and `onERC1155Received`/`onERC1155BatchReceived` (ERC1155). It also includes a `supportsInterface` function to indicate support for these interfaces.

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