Struct

Defining Structs

The Struct type in the Antelope library represents one entry in the structs array found within an ABI.

{
  structs: [
    {
      base: "",
      name: "transfer",
      fields: [
        { name: "from", type: "name" },
        { name: "to", type: "name" },
        { name: "quantity", type: "asset" },
        { name: "memo", type: "string" },
      ],
    },
  ]
}

This on-chain representation of a Struct allows the Antelope blockchain to encode and decode data of this type for use in smart contracts. In a similar way, the Struct found in the Antelope library also allows the Serializer to encode and decode this data within Javascript applications. Many of these Struct objects exist as core types offered by the library, such as Asset, Name, and PermissionLevel.

These Struct types are automatically generated and used while using the Serializer, but that requires retrieving and passing the entire ABI. To help optimize applications and their need to retrieve data from remote endpoints, the Struct class can also be used to define ABI-like definitions as code.

This can be done using TypeScript and the experimental decorators compiler option:

@Struct.type("transfer")
export class Transfer extends Struct {
  @Struct.field(Name) from!: Name
  @Struct.field(Name) to!: Name
  @Struct.field(Asset) quantity!: Asset
  @Struct.field("string") memo!: string
}

They can also be created in plain Javascript by manually defining all the properties:

class Transfer extends Struct {
  static abiName = "transfer"
  static abiFields = [
    {
      name: "from",
      type: Name,
    },
    {
      name: "to",
      type: Name,
    },
    {
      name: "quantity",
      type: Asset,
    },
    {
      name: "memo",
      type: "string",
    },
  ]
}

Using Structs

Once a Struct class is defined, they can be used to easily create encodable data within an Action without needing the ABI. Utilizing the definitions from the section above, a token transfer action can be written as:

const data = Transfer.from({
  from: "teamgreymass",
  to: "funds.gm",
  quantity: "0.0001 EOS",
  memo: "",
})

These can also be passed directly into an Action for automatic serialization.

const action = Action.from({
  account: "eosio.token",
  name: "transfer",
  authorization: [{ actor: "foo", permission: "active" }],
  data: Transfer.from({
    from: "teamgreymass",
    to: "funds.gm",
    quantity: "0.0001 EOS",
    memo: "",
  }),
})

Gotchas

When using the decorated versions of a Struct in a TypeScript codebase, there are a few things that may be required depending on your version of TypeScript and the bundler in use.

Within the tsconfig.json in your project, one thing that is required is the use of "experimentalDecorators": true. An example of this can be found in Unicove’s tsconfig.json file.

Another flag that may be required depending on configuration is the "useDefineForClassFields": true. You’ll know you need this one if you define a Struct, attempt to use it, and all the values within it come back as undefined.