4.3 合约编码方法和工具

合约的调用和查询，传入的参数是根据合约ABICode编码后函数和函数参数的data。详细可参考solidity - Application Binary Interface Specification
下面介绍几种方式取得合约ABICode方法调用时对应的编码data。

4.3.1 使用remix调用合约获取data

在使用remix执行合约方法时，每次交易都有对应的交易详情，可以在控制台看到交易信息里的函数调用的input，以及对应的decodeInput和decodeOutput。
input就是调用函数时向区块链环境发送的JSON里的data。对应开放平台，在使用合约调用时，传入的data可以直接使用remix的data。 remix input data

4.3.2 使用ethers.js实现编码和解码

ethers.js是一个以太坊的js依赖包。里面提供了根据ABICode编码函数名的方法和编码参数的静态方法。
例：

  1. //HelloWorld.sol
  2. pragma solidity^0.4.23;
  4. contract HelloWorld {
  5.     address public owner;
  6.     string public info;
  8.     constructor(address _owner) public {
  9.         owner = _owner;
  10.     }
  12.     function saySomething(string _str) public returns(string) {
  13.         info = _str;
  14.         return info;
  15.     }
  16. }
  1. //编码function saySomething函数的 abi
  2. var ethers = require('ethers')
  4. var  abi = [
  5.     {
  6.         "constant": false,
  7.         "inputs": [
  8.             {
  9.                 "name": "_str",
  10.                 "type": "string"
  11.             }
  12.         ],
  13.         "name": "saySomething",
  14.         "outputs": [
  15.             {
  16.                 "name": "",
  17.                 "type": "string"
  18.             }
  19.         ],
  20.         "payable": false,
  21.         "stateMutability": "nonpayable",
  22.         "type": "function"
  23.     },
  24.     {
  25.         "inputs": [
  26.             {
  27.                 "name": "_owner",
  28.                 "type": "address"
  29.             }
  30.         ],
  31.         "payable": false,
  32.         "stateMutability": "nonpayable",
  33.         "type": "constructor"
  34.     },
  35.     {
  36.         "constant": true,
  37.         "inputs": [],
  38.         "name": "info",
  39.         "outputs": [
  40.             {
  41.                 "name": "",
  42.                 "type": "string"
  43.             }
  44.         ],
  45.         "payable": false,
  46.         "stateMutability": "view",
  47.         "type": "function"
  48.     },
  49.     {
  50.         "constant": true,
  51.         "inputs": [],
  52.         "name": "owner",
  53.         "outputs": [
  54.             {
  55.                 "name": "",
  56.                 "type": "address"
  57.             }
  58.         ],
  59.         "payable": false,
  60.         "stateMutability": "view",
  61.         "type": "function"
  62.     }
  63. ];
  64. var interface = new ethers.Interface(abi)
  65. var abiInstance = interface.functions[abi[0].name] // abi[0]即saySomething function的abi
  66. return abiInstance.sighash // 0xfe6b3783

获取函数参数编码data的方法：
使用ethers.utils.ABICoder的encode方法，如

  1. // 编码 function saySomething 的输入参数 'Hello World'的data
  2. var abiCoder = new ethers.utils.AbiCoder()
  3. return abiCoder.encode(['string'], ['Hello World'])
  4. //0x0000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000b48656c6c6f20576f726c64000000000000000000000000000000000000000000

实际调用函数的input即为：函数saySomething encode的前八位 + 函数输入参数的encode。结果为

  1. 0xfe6b37830000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000b48656c6c6f20576f726c64000000000000000000000000000000000000000000

可对比上面使用remix调用方法时的input值，两者结果一致。

合约执行结束后，info值被设置为 “Hello World”。下面请求constant方法(同8.3 Demo合约查询)，根据结果解析info值。

  1. curl -k -X POST --data '{"jsonrpc":"2.0","method":"eth_call","params":[{"from":"0x7eff122b94897ea5b0e2a9abf47b86337fafebdc","to":"0xd5b0df861803a07f330868104eec92ebdcce4c79","data":"0x370158ea"}, "latest"],"id":1}' https://sandbox-blockchain.xunlei.com/call
  2. {"jsonrpc":"2.0","id":1,"result":"0x0000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000b48656c6c6f20576f726c64000000000000000000000000000000000000000000"}
  1. var ethers = require('ethers')
  3. var outputTypes = ['string']
  4. var response = "0x0000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000b48656c6c6f20576f726c64000000000000000000000000000000000000000000"
  6. var abiCoder = new ethers.utils.AbiCoder()
  7. abiCoder.decode(outputTypes, response)
  8. // [ 'Hello World' ]

decode结果根据传入的outputTypes数组长度，解析对应的参数。