How is the action data encoded?

Question

Take for example this action belonging to a newaccount transaction:

{
      "account": "eosio",
      "name": "newaccount",
      "authorization": [{
          "actor": "eosio",
          "permission": "active"
        }
      ],
      "data": "0000000000ea305510c255191ba9b23f010000000100030a5d4c7f7d35c16028563c8ec58406bb084acf5ff95d386ada1de045b1d23dd501000000010000000100030a5d4c7f7d35c16028563c8ec58406bb084acf5ff95d386ada1de045b1d23dd501000000"
}

How is the data field encoded?

Answer 1

The action's ABI describes how to encode it. e.g. here's the newaccount ABI:

{
  "name": "newaccount",
  "base": "",
  "fields": [
    {"name":"creator", "type":"account_name"},
    {"name":"name",    "type":"account_name"},
    {"name":"owner",   "type":"authority"},
    {"name":"active",  "type":"authority"}
  ]
}

account_name is an alias for the built-in name type:

{
  "new_type_name": "account_name",
  "type": "name"
}

authority is another structure:

{
  "name": "authority",
  "base": "",
  "fields": [
    {"name":"threshold", "type":"uint32"},
    {"name":"keys",      "type":"key_weight[]"},
    {"name":"accounts",  "type":"permission_level_weight[]"},
    {"name":"waits",     "type":"wait_weight[]"}
  ]
}

The contract's ABI has defintions for all types it uses, except for the built-in types. e.g. name is a built-in type that contains up to 12 characters packed into a uint64. See this answer for name's encoding.

Answer 2

Here is how EOSDevHelper serialize data in new account. In fact, it serializing block num, block expiration, creator, permission and new name. You can see details of it from link.