EOS智能合约授权限制和数据存储

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在EOS合约中，调用合约需要来自账户的授权，同时还要指定需要调用的动作。当然，有的合约并不是所有账户都可以调用的，这就需要用到授权限制。接下来我们就来看看如何限制合约的授权账户。

合约案例

为了更好的演示，写了一个下课和吃饭的智能合约小例子。这个合约有两个动作，下课和吃饭。教师账户可以调用下课动作，学生账户可以调用吃饭动作。教师调用下课动作后，学生才能调用吃饭动作。接下来我们来看代码：

teacher.hpp

头文件teacher.hpp定义了两个动作，over是class over 下课动作，eat是吃饭动作。

#include <eosiolib/eosio.hpp>
#include <eosiolib/print.hpp>

using namespace eosio;

class teacher_stu : public eosio::contract{

    using contract::contract;
  public:
    teacher_stu( account_name self ) :
        contract( self ) {}

    void over(account_name teacher);

    void eat(account_name student);

  private:
    static uint64_t id;

    // @abi table
    struct mshare {
      uint64_t id;
      uint64_t start = 0;

      uint64_t primary_key() const { return id; }
    };

    typedef multi_index<N(mshare), mshare> mshares;

};

EOSIO_ABI( teacher_stu, (over)(eat))

teacher.cpp

teacher.cpp中主要是over和eat动作的实现。

#include "teacher.hpp"

uint64_t teacher_stu::id = 1;
uint64_t start = 0;

void teacher_stu::over(account_name teacher) {
  require_auth(teacher);
  print("teache:Class over!");

  mshares mshare_table(_self, _self);

  start = 1;//存储动作调用状态
  mshare_table.emplace( teacher, [&]( auto& mshare ) {
    mshare.id = id;
    mshare.start = start;
  });
}

void teacher_stu::eat(account_name student) {
  require_auth(student);
  mshares mshare_table(_self, _self);
  auto it = mshare_table.find( id );
  if (it->start == 1) {
    print("student:Class over, go to eat!");
    mshare_table.erase( it );
  }
  else
    print("teacher:Class time, you can't eat");
}

仔细观察这段代码就会发现，over和eat动作中都有个"require_auth()"语句。在over动作中,"requir_auth(teacher)"的作用是限制只有"teacher"账户才可以调用over动作。在eat动作中则是限制只有"student"账户才可调用eat动作。

合约数据存储

此合约设计为下课后才可以吃饭，所以当教师账户调用合约的over动作后，需要存储一个合约调用状态值。EOS合约的数据存储需要用数据库，把数据存到一张表中，这是令人很难受的。

teacher.hpp

在teacher.hpp中创建一个结构体。下段代码中的"//@abi table"注释非常重要，必须要写的，如果不写数据将无法存储。

    static uint64_t id;

    // @abi table
    struct mshare {
      uint64_t id;
      uint64_t start = 0;

      uint64_t primary_key() const { return id; }
    };

    typedef multi_index<N(mshare), mshare> mshares;

DAWN 3.0 使用eosio::multi_index作为容器，我们可以使用emplace来插入数据，使用modify来修改数据，使用erase删除数据。

teacher.cpp

  mshares mshare_table(_self, _self);

  start = 1;
  mshare_table.emplace( teacher, [&]( auto& mshare ) {
    mshare.id = id;
    mshare.start = start;
  });

在over动作中创建数据，学生调用eat动作后再修改或删除数据即可

  mshares mshare_table(_self, _self);
  auto it = mshare_table.find( id );
  if (it->start == 1) {
    print("student:Class over, go to eat!");
    mshare_table.erase( it );
  }

合约调用效果展示

• 授权限制

$ cleos push action class over '{"teacher":"teacher","student":"student"}' -p student
Error 3030001: missing required authority
Ensure that you have the related authority inside your transaction!;
If you are currently using 'cleos push action' command, try to add the relevant authority using -p option.
Error Details:
missing authority of teacher

• 学生没下课就吃饭

$ cleos push action class eat '{"teacher":"teacher","student":"student"}' -p student
executed transaction: 02918b223230cb9ea1fd383e0499ea22d22ced8f2108db3233bdfd51c06f3b37  232 bytes  102400 cycles
#         class <= class::eat                   {"student":"student"}
>> teacher:Class time, you can't eat

• 正常下课吃饭

$ cleos push action class over '{"teacher":"teacher","student":"student"}' -p teacher
executed transaction: a96520fa28c8412e0998080126734337507811638ecf6b939e904818a4892e35  232 bytes  103424 cycles
#         class <= class::over                  {"teacher":"teacher"}
>> teache:Class over!
$ cleos push action class eat '{"teacher":"teacher","student":"student"}' -p student
executed transaction: 2955a693b626c539d20da9d4f5d41a6b53bb8ca2b2651b63cf4a67012fb3dd7e  232 bytes  103424 cycles
#         class <= class::eat                   {"student":"student"}
>> student:Class over, go to eat!

• 查看表中数据

$ cleos get table class class mshare
{
  "rows": [{
      "id": 1,
      "start": 1
    }
  ],
  "more": false
}

整个合约写下来调通也是费了我很多脑细胞，数据存储也比较坑爹啊。现在把我的一点经验分享出来，希望大家在学习EOS的路上少踩一些坑。

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