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Payable actions

Goal​

This tutorial illustrates how to write a smart contract that has payable actions. Payable actions are actions that require you to transfer some tokens to actions prior to use other functionality of the smart contract. Also, the Antelope asset type is covered in this tutorial.

As for the logic of this smart contract, we're going to write a contract that accepts a particular token but will not allow the tokens to be withdrawn for a specific amount of time.

The token to HODL​

First create a standard C++ class called "hodl" that extends eosio::contract.

#include <eosio/eosio.hpp>

using namespace eosio;

class [[eosio::contract("hodl")]] hodl : public eosio::contract{
  private:
  public:
};

This contract needs to set up a few constraints:

  • What symbol/token does this contract accept?
  • When is the hodl over?

Let us now define these constraints as constants:

  • hodl_symbol: the symbol of tokens this contract accepts. In this case, we use the "SYS" symbol.
  • the_party constant sets the hodl to end on Tuesday, February 22, 2022 10:22:22 PM.
#include <eosiolib/eosio.hpp>

using namespace eosio;

class [[eosio::contract("hodl")]] hodl : public eosio::contract {
  private:
    static const uint32_t the_party = 1645525342;
    const symbol hodl_symbol;
  public:

}

Next, let's define a table to track the number of tokens the hodl contract has received.

struct [[eosio::table]] balance
{
  eosio::asset funds;
  uint64_t primary_key() const { return funds.symbol.raw(); }
};

In this multiple index table declaration, a new type called asset is used. An asset is a type designed to represent a digital token asset. See more details in the asset reference documentation.

The symbol member of an asset instance will be used as the primary key. By calling the raw() function the symbol variable will be converted into an unsigned integer so it can be used as a primary key.

Constructor​

The constructor initializes the hodl_symbol as “SYS”, which is a token created in the Deploy, Issue and Transfer Tokens section.

public:
  using contract::contract;
  hodl(name receiver, name code, datastream<const char *> ds):contract(receiver, code, ds), hodl_symbol("SYS", 4){}

Get current UTC time​

In order to get time in UTC timezone throughout the code, create a function to easily access the current UTC time.

uint32_t now() {
  return current_time_point().sec_since_epoch();
}

Next, we'll write the actions of the contract.

Deposit​

To accept a transfer we need to have a deposit action.

[[eosio::on_notify("eosio.token::transfer")]]
void deposit(name hodler, name to, eosio::asset quantity, std::string memo)
{
  if (to != get_self() || hodler == get_self())
  {
    print("These are not the droids you are looking for.");
    return;
  }

  check(now() < the_party, "You're way late");
  check(quantity.amount > 0, "When pigs fly");
  check(quantity.symbol == hodl_symbol, "These are not the droids you are looking for.");

  balance_table balance(get_self(), hodler.value);
  auto hodl_it = balance.find(hodl_symbol.raw());

  if (hodl_it != balance.end())
    balance.modify(hodl_it, get_self(), [&](auto &row) {
      row.funds += quantity;
    });
  else
    balance.emplace(get_self(), [&](auto &row) {
      row.funds = quantity;
    });
}

This action should not introduce many new concepts if you have followed this tutorial from the beginning.

Firstly, the action checks that the contract is not transferring to itself:

if (to != get_self() || hodler == get_self()) {
  print("These are not the droids you are looking for.");
  return;
}

The contract needs to do so because transferring to the contract account itself would create an invalid booking situation in which an account could have more tokens than the account has in the eosio.token contract.

Then this action checks a few other conditions:

  • The time to withdraw has not already passed
  • The incoming transfer has a valid amount of tokens
  • The incoming transfer uses the token we specify in the constructor
check(now() < the_party, "You're way late");
check(quantity.amount > 0, "When pigs fly");
check(quantity.symbol == hodl_symbol, "These are not the droids you are looking for.");

If all constraints are passed, the action updates the balances accordingly:

balance_table balance(get_self(), hodler.value);
auto hodl_it = balance.find(hodl_symbol.raw());

if (hodl_it != balance.end())
  balance.modify(hodl_it, get_self(), [&](auto &row) {
    row.funds += quantity;
  });
else
  balance.emplace(get_self(), [&](auto &row) {
    row.funds = quantity;
  });

The important thing to note is the deposit function will actually be triggered by the eosio.token contract. To understand this behaviour we need to understand the on_notify attribute.

The on_notify attribute​

[[eosio::on_notify("eosio.token::transfer")]]

The on_notify attribute is one of the CDT attributes that annotates a smart contract action.

Annotating an action with an on_notify attribute ensures any incoming notification is forwarded to the annotated action if and only if the notification is dispatched from a specified contract and from a specified action.

In this case, the on_notify attribute ensures the incoming notification is forward to the deposit action only if the notification comes from the eosio.token contract and is from the eosio.token's transfer action.

This is also why we don't need to check if the hodler actually has the appropriate amount of tokens he or she claimed, as the eosio.token contract would have done this check prior to the transfer notification reaching the hodl deposit action.

Party!​

The party action will only allow withdrawals after the configured the_party time has elapsed. The party action has a similar construct as the deposit action with the following conditions:

  • check the withdrawing account is the account which made the deposit initially
  • find the locked balance
  • transfer the token from the hodl contract to the account itself
[[eosio::action]]
void party(name hodler)
{
  //Check the authority of hodler
  require_auth(hodler);

  //Check the current time has passed the the_party time
  check(now() > the_party, "Hold your horses");

  balance_table balance(get_self(), hodler.value);
  auto hodl_it = balance.find(hodl_symbol.raw());

  //Make sure the holder is in the table
  check(hodl_it != balance.end(), "You're not allowed to party");

  action{
    permission_level{get_self(), "active"_n},
    "eosio.token"_n,
    "transfer"_n,
    std::make_tuple(get_self(), hodler, hodl_it->funds, std::string("Party! Your hodl is free."))
  }.send();

  balance.erase(hodl_it);
}

The complete code listing is the following:

#include <eosio/eosio.hpp>
#include <eosio/print.hpp>
#include <eosio/asset.hpp>
#include <eosio/system.hpp>

using namespace eosio;

class [[eosio::contract("hodl")]] hodl : public eosio::contract {
  private:
    static const uint32_t the_party = 1645525342;
    const symbol hodl_symbol;

    struct [[eosio::table]] balance
    {
      eosio::asset funds;
      uint64_t primary_key() const { return funds.symbol.raw(); }
    };

    using balance_table = eosio::multi_index<"balance"_n, balance>;

    uint32_t now() {
      return current_time_point().sec_since_epoch();
    }

  public:
    using contract::contract;

    hodl(name receiver, name code, datastream<const char *> ds) : contract(receiver, code, ds),hodl_symbol("SYS", 4){}

    [[eosio::on_notify("eosio.token::transfer")]]
    void deposit(name hodler, name to, eosio::asset quantity, std::string memo) {
      if (hodler == get_self() || to != get_self())
      {
        return;
      }

      check(now() < the_party, "You're way late");
      check(quantity.amount > 0, "When pigs fly");
      check(quantity.symbol == hodl_symbol, "These are not the droids you are looking for.");

      balance_table balance(get_self(), hodler.value);
      auto hodl_it = balance.find(hodl_symbol.raw());

      if (hodl_it != balance.end())
        balance.modify(hodl_it, get_self(), [&](auto &row) {
          row.funds += quantity;
        });
      else
        balance.emplace(get_self(), [&](auto &row) {
          row.funds = quantity;
        });
    }

    [[eosio::action]]
    void party(name hodler)
    {
      //Check the authority of hodlder
      require_auth(hodler);

      // //Check the current time has pass the the party time
      check(now() > the_party, "Hold your horses");

      balance_table balance(get_self(), hodler.value);
      auto hodl_it = balance.find(hodl_symbol.raw());

      // //Make sure the holder is in the table
      check(hodl_it != balance.end(), "You're not allowed to party");

      action{
        permission_level{get_self(), "active"_n},
        "eosio.token"_n,
        "transfer"_n,
        std::make_tuple(get_self(), hodler, hodl_it->funds, std::string("Party! Your hodl is free."))
      }.send();

      balance.erase(hodl_it);
    }
};

Great, let's deploy it.

Test deposit​

First, create an account and deploy to it:

cleos create account eosio hodl YOUR_PUBLIC_KEY
cdt-cpp hodl.cpp -o hodl.wasm
cleos set contract hodl ./ -p hodl@active

As mentioned in a previous tutorial, this contract needs an eosio.code permission:

cleos set account permission hodl active --add-code

Next, create a testing account:

cleos create account eosio han YOUR_PUBLIC_KEY

Let's transfer some SYS tokens issued in the previous section to han:

cleos push action eosio.token transfer '[ "alice", "han", "100.0000 SYS", "Slecht geld verdrijft goed" ]' -p alice@active

Finally, transfer some SYS tokens to the hodl contract from han's account.

cleos transfer han hodl '0.0001 SYS' 'Hodl!' -p han@active

Test withdraw​

To test the withdrawal feature, the the_party variable needs to be updated. Update the the_party variable to a point in time in the past so the withdrawal functionality can be tested.


CONTRACT hodl : public eosio::contract {
  private:
    // 9 June 2018 01:00:00
    static const uint32_t the_party = 1528549200;

Withdrawing the funds:

cleos push action hodl party '["han"]' -p han@active

Should produce the following response:

executed transaction: 62b1e6848c8c5e6458b9a0f7600e65574eaf60445be114d224adccc5a962a09a  104 bytes  383 us
#          hodl <= hodl::party                  {"hodler":"han"}
#   eosio.token <= eosio.token::transfer        {"from":"hodl","to":"han","quantity":"0.0001 SYS","memo":"Party! Your hodl is free."}
#          hodl <= eosio.token::transfer        {"from":"hodl","to":"han","quantity":"0.0001 SYS","memo":"Party! Your hodl is free."}
#           han <= eosio.token::transfer        {"from":"hodl","to":"han","quantity":"0.0001 SYS","memo":"Party! Your hodl is free."}