How to automatically pass the value to custom SignedExtension

Question

For now I have custom SignedExtension in my substrate node:

#[derive(Encode, Decode, Clone, Eq, PartialEq, TypeInfo)]
#[scale_info(skip_type_params(T))]
pub struct CheckAccount<T> {
    timestamp: u64,
    _marker: PhantomData<T>,
}

In my polkadot-js-api I define this like so:

let signedExtensions = {
    CheckAccount: {
      extrinsic: {
        timestamp: "u64",
      },
      payload: {},
    },
  };

const api = await ApiPromise.create({
    ...
    SignedExtensions,
  });

And to perform transaction I have specify extension value manually

api.tx.balances
    .transfer(bob, 1000)
    .signAndSend(alice, { timestamp: Date.now() })

The questing is how can I get behaviour for my custom SignedExtension, for example like CheckNonce have, when it automatically initialised with api and don't have to specify it manually?

Update

I want to get something like this:

api.setGetterForSignedExtension("CheckAccount", Date.now);

And then I just simply can call

api.tx.balances
    .transfer(bob, 1000)
    .signAndSend(Alice);

P.S.

And if this possible can custom SignedExtension be integrated with polkadot/apps to allow submit extrinsic throw polkadot ui. Now when I try do it got this error

1002: Verification Error: Runtime error: Execution failed: Runtime panicked: Bad input data provided to validate_transaction: Could not decode `RuntimeCall`, variant doesn't exist: RuntimeApi, Execution failed: Runtime panicked: Bad input data provided to validate_transaction: Could not decode `RuntimeCall`, variant doesn't exist

Answer 1

Something like following can be tried out:

const { ApiPromise, Keyring } = require('@polkadot/api');

// Create a custom type registry  

const types = {
  CheckAccount: {
    timestamp: 'u64',
  },
};

// Create the api instance with the custom type registry  

const api = await ApiPromise.create({
  provider: wsProvider,
  types,
});

// Create a wrapper function for the transfer transaction  

const transferWithTimestamp = async (from, to, amount) => {
  const transaction = api.tx.balances.transfer(to, amount);

  // Get the current timestamp  
  const timestamp = Date.now();

  // Set the timestamp in the transaction payload  
  const payload = {
    CheckAccount: {
      timestamp,
    },
  };

  // Sign and send the transaction with the payload  
  await transaction.signAndSend(from, { payload });
};

// Use the wrapper function to perform the transfer with the timestamp
transferWithTimestamp(alice, bob, 1000);

here timestamp = Date.now() is evaluated every time and passed in without need to enter manually