SamplePing.sol

pragma ton-solidity ^0.57.1;
pragma AbiHeader expire;
pragma AbiHeader pubkey;


interface ISample {
    function ping(address recipient) external;
    function pong() external;

    event Ping(address recipient);
    event Pong(address sender);
}


contract Sample is ISample {
    uint16 static _nonce;

    uint state;

    event StateChange(uint _state);

    constructor(uint _state) public {
        tvm.accept();

        setState(_state);
    }

    function setState(uint _state) public {
        tvm.accept();
        state = _state;

        emit StateChange(_state);
    }

    function getDetails()
        external
        view
    returns (
        uint _state
    ) {
        return state;
    }

    function ping(address recipient) external override {
        tvm.accept();

        emit Ping(recipient);

        ISample(recipient).pong();
    }

    function pong() external override {
        // FAILS
        require(msg.sender == address(this), 10001);

        emit Pong(msg.sender);
    }
}