# Callback

Callback is used to make a function call to an external contract. The external contract can perform an arbitrary logic using the bytes `data` and payable `value` provided by the call arguments.

One key usage for the callback is to make a token swap from a DEX contract outside of INIT.

{% hint style="info" %}
The external contract must implement `coreCallback payable` function.
{% endhint %}

{% hint style="info" %}
It is recommended to check that `msg.sender` of `coreCallback` is `INIT_CORE` to avoid unintended external calls from other users.
{% endhint %}

```solidity
// Example external contract that uses InitCore's callback
contract ExternalContract {
    function coreCallback(address sender, bytes calldata data) external payable returns (bytes memory result) {
        // check msg.sender is InitCore
        require(msg.sender == INIT_CORE, 'sender not allowed');

        // perform a swap on a DEX
        
        // returns bytes result
    }
    
    function executeCallback(bytes calldata data) payable {
        IInitCore(INIT_CORE).callback{value: msg.value}(msg.sender, data);
    }
}
```