How to create and use action wrappers

Overview

This guide provides instructions on how to create and use an action wrapper in a smart contract.

Code Reference

See the following code reference guide for action wrapper:

Before you begin

Make sure you have the following prerequisites in place:

  • An EOSIO development environment, for details consult the Get Started Guide.
  • A smart contract named multi_index_example, defined in file multi_index_example.hpp.
  • An action mod which modifies the integer value n stored for row with key user.

Refer to the following reference implementation for your starting point:

class [[eosio::contract]] multi_index_example : public contract {
  // ...
  [[eosio::action]] void mod( name user, uint32_t n );
  // ...
}

Procedure

Complete the following steps to create and use mod_action action wrapper for the existing mod action in the smart contract:

  1. Define the action wrapper
  2. Use the action wrapper

1. Define The Action Wrapper

To define an action wrapper for the mod action, make use of the eosio::action_wrapper template, with the first parameter the action name as a eosio::name and second parameter as the reference to the action method:

class [[eosio::contract]] multi_index_example : public contract {
  // ...
  [[eosio::action]] void mod(name user);
  // ...
+  using mod_action = action_wrapper<"mod"_n, &multi_index_example::mod>;
  // ...
}

2. Use The Action Wrapper

2.1. Include Header File

To use the action wrapper, you have to include the header file where the action wrapper is defined:

#include <multi_index_example.hpp>

2.2. Instantiate The Action Wrapper

Instantiate the mod_action defined above, specifying the contract to send the action to as the first argument. In this case, it is assumed the contract is deployed to multiindexex account, and a structure which is defined by two parameters: the self account, obtained by get_self() call, and the active permission (you can modify these two parameters based on your requirements).

#include <multi_index_example.hpp>

+multi_index_example::mod_action modaction("multiindexex"_n, {get_self(), "active"_n});

2.3. Send The Action Using The Action Wrapper

Call the send method of the action wrapper and pass in the mod action's parameters as positional arguments:

#include <multi_index_example.hpp>

multi_index_example::mod_action modaction("multiindexex"_n, {get_self(), 1});

+modaction.send("eostutorial"_n, 1);

For a full example see the multi_index contract implementation.

Summary

In conclusion, the above instructions show how to create and use action wrapper in a smart contract.