Gear 合约大揭秘

#[no_mangle]
pub unsafe extern "C" fn init() {}
#[no_mangle]
pub unsafe extern "C" fn handle() {}

# Gear 合约揭秘
Gearfans 举办了 3 次 Gear workshop，从第一次的 [ping-pong](https://wiki.gear-tech.io/zh-cn/developing-contracts/examples/ping/) 合约，到 [去中心化 twitter](https://wiki.gear-tech.io/zh-cn/developing-contracts/examples/feeds)，再到 [Erc20](https://wiki.gear-tech.io/zh-cn/developing-contracts/examples/erc20) 。
那我们来介绍下 Gear 合约的基本结构。
## Gear 合约最小必要结构
```rust
#[no_mangle]
pub unsafe extern "C" fn init() {}
#[no_mangle]
pub unsafe extern "C" fn handle() {}

#![no_std]
use gstd::{msg, prelude::*};
#[no_mangle]
pub unsafe extern "C" fn handle() {
    let new_msg = String::from_utf8(msg::load_bytes()).expect("Invalid message");
    if new_msg == "PING" {
        msg::reply_bytes("PONG", 12_000_000, 0);
    }
}
#[no_mangle]
pub unsafe extern "C" fn init() {}

msg::load() // 获取当前正在处理的消息
msg::source() // 获取消息的发送者
msg::reply() // 发送一条新消息作为对当前正在的消息的回应
msg::reply_bytes() // 发送一条新消息作为对当前正在的消息的回应
msg::reply_to() // 获取当前消息的id
msg::send() // 发送一个新的消息给合约或用户

#![no_std]
use gstd::msg;
gstd::metadata! {
  title: "ERC20",
  init:
    input: InitConfig,
  handle:
    input: Action,
    output: Event,
  state:
    input: State,
    output: StateReply,
}
#[no_mangle]
pub unsafe extern "C" fn handle() {}
#[no_mangle]
pub unsafe extern "C" fn init() {}
#[no_mangle]
pub unsafe extern "C" fn meta_state() -> *mut [i32; 2] {}

gstd::metadata! {
  title: "ERC20",
  init:
    input: InitConfig, // 定义初始化参数
  handle:
    input: Action, // 定义合约方法，供 handle使用
    output: Event, // 定义事件，在调用合约方法后，会调用合约事件，供 handle 使用
  state:
    input: State, // 定义 meta_state 的输入格式，供 meta_state使用
    output: StateReply, // 定义 meta_state 的输出格式，供 meta_state使用
}

#[no_mangle]
pub unsafe extern "C" fn meta_state() -> *mut [i32; 2] {
    let query: State = msg::load().expect("failed to decode input argument");
    let encoded = match query {
        State::Name => StateReply::Name(FUNGIBLE_TOKEN.name.clone()).encode(),
        State::Symbol => StateReply::Name(FUNGIBLE_TOKEN.symbol.clone()).encode(),
        State::Decimals => StateReply::Decimals(18).encode(),
        State::TotalSupply => StateReply::TotalSupply(FUNGIBLE_TOKEN.total_supply).encode(),
        State::BalanceOf(account) => {
            StateReply::Balance(FUNGIBLE_TOKEN.balance_of(&account)).encode()
        }
    };
    let result = gstd::macros::util::to_wasm_ptr(&(encoded[..]));
    core::mem::forget(encoded);
    result
}

#[no_mangle]
pub unsafe extern "C" fn handle() {
    let action: ChannelAction = msg::load().expect(&format!(
        "CHANNEL {:?}: Unable to decode Channel Action",
        STATE.name()
    ));
    match action {
        ChannelAction::Meta => {
            msg::reply(meta, 100_000_000, 0);
        }
        ChannelAction::Subscribe => {}
        ChannelAction::Unsubscribe => {
            msg::reply((), 0, 0);
        }
        ChannelAction::Post(text) => {
            msg::reply((), 0, 0);
        }
    }
}