Compile Your Contract

Run the following command in your flipper directory to compile your smart contract:

cargo +nightly contract build

This command will build the following for your contract: a Wasm binary, a metadata file (which contains the contract's ABI) and a .contract file which bundles both. This .contract file can be used to deploy your contract to a chain. If all goes well, you should see a target folder which contains these files:

target
  └─ ink
    └─ flipper.contract
    └─ flipper.wasm
    └─ metadata.json

Let's take a look at the structure of the metadata.json:

{
  "metadataVersion": "0.1.0",
  "source": {...},
  "contracts": {...},
  "spec": {
    "constructors": [...],
    "docs": [],
    "events": [],
    "messages": [...],
  },
  "storage": {...},
  "types": [...]
}

This file describes all the interfaces that can be used to interact with your contract:

• types provides the custom data types used throughout the rest of the JSON.
• storage defines all the storage items managed by your contract and how to ultimately access them.
• spec stores information about the callable functions like constructors and messages a user can call to interact with the contract. It also has helpful information like the events that are emitted by the contract or any docs.

If you look closely at the constructors and messages, you will also notice a selector which contains a 4-byte hash of the function name and is used to route your contract calls to the correct functions.

In the next section we will start a Substrate Smart Contracts node and configure the Contracts UI to interact with it.