Solidity之Event

Event是以太坊只能合约里面的一个成员就像下图里一样:

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那么Event到底是什么呢？是用来干什么的呢？下面是官网上给出的介绍：

Events are convenience interfaces with the EVM logging facilities

Event可以方便地使用EVM日志记录工具，而这些工具又可以在一个Dapp的用户界面中“调用”JavaScript callbacks，这些JavaScript callbacks是用来listen for these events的。

Events are inheritable members of contracts. When they are called, they cause the arguments to be stored in the transaction’s log - a special data structure in the blockchain. These logs are associated with the address of the contract and will be incorporated into the blockchain and stay there as long as a block is accessible

Event是可继承的合同成员。当它们被调用时，它们使得参数被存储在 交易的日志 中 - 区块链中的一块特殊数据结构。这些日志与合同所属的地址相关联，将被并入区块链中并保存在上面，和所属的该区块共存不离不弃。日志和事件数据不能从合同内访问（连从创建它们的合同内都不行）。

  pragma solidity ^0.4.0;
  
  contract ClientReceipt {
      event Deposit(
          address indexed _from,
          bytes32 indexed _id,
          uint _value
      );
  
      function deposit(bytes32 _id) public payable {
          // Any call to this function (even deeply nested) can
          // be detected from the JavaScript API by filtering
          // for `Deposit` to be called.
          Deposit(msg.sender, _id, msg.value);
      }
  }