使用Oraclize让智能合约调用外部数据

简介

Oraclize对于以太坊来说，是一份智能合约，继承它之后，自定义的合约可以通过api访问外部的数据。但需要给一定的费用。

• Orcalize的数据源有：

  • URL （合约外部的API接口）
  • WolframAlpha （新一代的搜索引擎，能根据问题直接给出答案，如London的天气）
  • IPFS （星际文件系统，一个分布式的存储系统）
  • random （随机数引擎，能产生一个安全的随机数）

• Orcalize的数据源调用的费用： 
  

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使用步骤

• （1）继承Oraclize合约
• （2）定义外部数据源的访问方式
• （3）定义数据返回的回调函数__callback()

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官方例子

• 查看Youtube某个视频的观看人数

/*
   Youtube video views

   This contract keeps in storage a views counter
   for a given Youtube video.
*/


pragma solidity ^0.4.0;
import "github.com/oraclize/ethereum-api/oraclizeAPI.sol";

contract YoutubeViews is usingOraclize {

    string public viewsCount;

    event newOraclizeQuery(string description);
    event newYoutubeViewsCount(string views);

    function YoutubeViews() {
        update();
    }

    function __callback(bytes32 myid, string result) {
        if (msg.sender != oraclize_cbAddress()) throw;
        viewsCount = result;
        newYoutubeViewsCount(viewsCount);
        // do something with viewsCount. like tipping the author if viewsCount > X?
    }

    function update() payable {
        newOraclizeQuery("Oraclize query was sent, standing by for the answer..");
        oraclize_query('URL', 'html(https://www.youtube.com/watch?v=9bZkp7q19f0).xpath(//*[contains(@class, "watch-view-count")]/text())');
    }

}                                          

• 调用WolframAlpha查看伦敦的天气

/*
   WolframAlpha example

   This contract sends a temperature measure request to WolframAlpha
*/


pragma solidity ^0.4.0;
import "github.com/oraclize/ethereum-api/oraclizeAPI.sol";

contract WolframAlpha is usingOraclize {

    string public temperature;

    event newOraclizeQuery(string description);
    event newTemperatureMeasure(string temperature);

    function WolframAlpha() {
        update();
    }

    function __callback(bytes32 myid, string result) {
        if (msg.sender != oraclize_cbAddress()) throw;
        temperature = result;
        newTemperatureMeasure(temperature);
        // do something with the temperature measure..
    }

    function update() payable {
        newOraclizeQuery("Oraclize query was sent, standing by for the answer..");
        oraclize_query("WolframAlpha", "temperature in London");
    }
} 

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备注

• 官方文档：http://docs.oraclize.it/#home

• 官方例子在线IDE：http://dapps.oraclize.it/browser-solidity/#gist=9817193e5b05206847ed1fcd1d16bd1d&version=soljson-v0.4.24+commit.e67f0147.js
• 查询api: http://www.realitykeys.com/

转载自：https://blog.csdn.net/ns2250225/article/details/80498838