## Gear 智能合约 -- Vara Network
### 使用 Gas Vouchers API 实现 Payless Transactions
本 PPT 链接: https://hackmd.io/@btwiuse/vouchers
参考文档: https://wiki.gear-tech.io/docs/api/vouchers
---
# Gas vouchers
Vouchers, issued by any actor empower users with gas-free interactions, enabling them to send messages to specific programs seamlessly.
An example of using vouchers is shown in the [Battleship](/examples/Gaming/battleship.md) game. Users without tokens on their balance can make moves by sending messages to a program using a voucher.
---
### Issue a voucher
Use `api.voucher.issue` method to issue a new voucher for a user to be used to pay for sending messages to `program_id` program.
```javascript
import { VoucherIssued } from '@gear-js/api';
const programId = '0x..';
const account = '0x...';
const tx = api.voucher.issue(account, programId, 10000);
tx.signAndSend(account, (events) => {
const voucherIssuedEvent = events.events.filter(({event: {method}}) => method === 'VoucherIssued') as VoucherIssued;
console.log(voucherIssuedEvent.toJSON());
})
```
---
### Check voucher
Use `api.voucher.exists` method to check that the voucher exists for a particular user and program:
```javascript
const voucherExists = await api.voucher.exists(programId, accountId);
```
---
### Send a message using voucher
To send message with voucher you can use `api.voucher.call` method:
```javascript
const messageTx = api.message.send({
destination: destination,
payload: somePayload,
gasLimit: 10000000,
value: 1000
}, meta);
const voucherTx = api.voucher.call({ SendMessage: messageTx });
await voucherTx.signAndSend(account, (events) => {
console.log(events.toHuman());
});
```
---
### Send a reply using voucher
Sending a reply with issued voucher works similar to sending message with voucher:
```javascript
const messageTx = api.message.sendReply(...);
const voucherTx = api.voucher.call({ SendReply: messageTx });
await voucherTx.signAndSend(account, (events) => {
console.log(events.toHuman());
});
```
---
总结
- check voucher
- send a message using voucher
- send a reply using voucher
课后作业：使用 Gas Voucher API 向自定义用户地址发放免 Gas 券，并进行一次交易
https://github.com/gear-tech/gear/issues/3399
https://github.com/gear-tech/gear/pull/3606
---
# Questions
<img src="https://hackmd.io/_uploads/SyK8P72d2.jpg" width="240" alt="GEAR">
<img src="https://hackmd.io/_uploads/By_kgE3_2.png" width="240" alt="VARA">
课后答疑频道: https://t.me/Gear_CN
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