[区块链]以太坊合约获取Event事件(web3j)

智能合约Event事件

智能合约尽管具备很多高级语言的特性，但是本身还是存在很多限制。对于业务的精准处理，需要采用Event事件进行跟踪，对于不同的合约和方法，可以编写不同的Event事件，如下：

pragma solidity >=0.7.0 <0.9.0;

contract Owner {

    address private owner;
    
    // event for EVM logging
    event OwnerSet(address indexed oldOwner, address indexed newOwner);
    
    // modifier to check if caller is owner
    modifier isOwner() {
        require(msg.sender == owner, "Caller is not owner");
        _;
    }  

    constructor() {
        // 'msg.sender' is sender of current call, contract deployer for a constructor
        owner = msg.sender; 
        emit OwnerSet(address(0), owner);
    }

    function changeOwner(address newOwner) public isOwner {
        emit OwnerSet(owner, newOwner);
        owner = newOwner;
    }

    function getOwner() external view returns (address) {
        return owner;
    }
}

获取Event事件

对应的java智能合约如何获取Event事件呢？见下

@GetMapping("/setOwner")
@ResponseBody
public String getUserCopyrights(String oldePK,String newPK) {
    try {
        Credentials oldOwner = Credentials.create(oldePK);
        Credentials newOwner = Credentials.create(newPK);
        // 加载合约
        Owner contract = Owner.load(contractAddress, web3j, oldOwner, new DefaultGasProvider());
        // 调用合约方法
        TransactionReceipt transactionReceipt = contract.changeOwner(newOwner.getAddress()).send();
        // 获取Event事件
        List<Owner.OwnerSetEventResponse> ownerSetEvents = contract.getOwnerSetEvents(transactionReceipt);
        Owner.OwnerSetEventResponse ownerSetEventResponse = ownerSetEvents.get(0);
        // 简单对比链上返回的事件信息与传入的是否一致
        if (Objects.equals(ownerSetEventResponse.newOwner, newOwner.getAddress())){
            System.out.println("oldOwner:" + ownerSetEventResponse.oldOwner);
            return "change owner successful";
        }else{
            return "change owner failed";
        }
    } catch (Exception e) {
        e.printStackTrace();
        return "error happend";
    }
}

运行结果

扩展

FISCO BCOS开源社区：如何优雅地编写智能合约

• 统一接口

合约中可以采用require的方式进行处理，不过require方式不支持动态变量，每个require处理后需要填入特定的报错内容，耦合性太重，且不便于扩展。

基于智能合约的SDK开发，对于每一个交易（方法）由于Event事件不同，需要编写大量的不可复用的代码，解析Event事件。这种写法，对于代码的理解和维护性都是非常差的。要解决这个问题，只需要编写一个基合约CommonLib，如下所示：

contract CommonLib {
    // tx code
    bytes32 constant public ADD_STUDENT = "1";
    bytes32 constant public MODIFY_STUDENT_NAME = "2";
    
    // return code
    bytes32 constant public STUDENT_EXIST = "1001";
    bytes32 constant public STUDENT_NOT_EXIST = "1002";
    bytes32 constant public TX_SUCCESS = "0000";
    
  event commonEvent(bytes id, bytes32 txCode, bytes32 rtnCode);
}

contract StudentController is CommonLib {
     function addStudent(bytes32 studentId, bytes32 studenntName) public returns(bool) {
      	// 处理过程
      	// .......
      
      	// 根据处理结果来调用不同事件
        if(add success){
          	commonEvent(studentId, ADD_STUDENT, TX_SUCCESS);
            return true;
        }else {
          	commonEvent(studentId, ADD_STUDENT, STUDENT_EXIST);
            return false;
        }
    }
}