如何使用EOS多索引表

時間 2019-11-15

標籤如何使用 eos 索引简体版

原文原文鏈接

下面是多索引表的使用指南。編程

爲了深刻和清晰的瞭解多索引表，最終的.cpp文件部分將被進一步詳細討論和討論。注意，完整的.cpp文件能夠在頁面的底部找到。app

詞彙表

code :是指已公佈智能合約的account_name。
scope:account_name所涉及數據範圍。
table_name: 存儲在內存中的表的名稱。

代碼分解

要存儲的結構

要在多索引表中存儲的數據是limit_order結構。primary_key()，get_expiration()，get_price()函數用於返回表。返回的表將根據調用的函數排序。ide

struct limit_order {
  uint64_t     id;
  uint128_t    price;
  uint64_t     expiration;
  account_name owner;

  auto primary_key() const { return id; }
  uint64_t get_expiration() const { return expiration; }
  uint128_t get_price() const { return price; }

  EOSLIB_SERIALIZE( limit_order, ( id )( price )( expiration )( owner ) )
};

建一個多索引表

auto payer = ilm.get_account();
...

payer是保存賬戶的變量，它將帳單元素添加到多索引表中，並修改已經在多索引表中的元素。函數

...
eosio::multi_index< N( orders ), limit_order, 
...

N(orders)是多索引表的名稱，limit_order是要存儲在表中的數據。工具

...  
indexed_by< N( byexp ), const_mem_fun< limit_order, uint64_t, 
&limit_order::get_expiration> >,
...

indexed_by< N( byexp ), const_mem_fun< limit_order, uint64_t, &limit_order::get_expiration> >定義了多索引表的索引方式。N(byexp)是這個索引的名稱。const_mem_fun表示正在查詢的數據類型、limit_order的變量的類型是uint64_t，將使用get_expiration函數獲取變量。區塊鏈

...
  indexed_by< N( byprice ), const_mem_fun< limit_order, uint128_t, &limit_order::get_price> >
...

indexed_by< N( byprice ), const_mem_fun< limit_order, uint128_t, &limit_order::get_price> >定義了多索引表的索引方式。N(byprice)是這個索引的名稱。const_mem_fun表示正在查詢的數據類型、limit_order的變量的類型是uint128_t，將使用get_price函數獲取變量。ui

orders( N( limitorders ), N( limitorders )

orders便是多索引表。this

auto payer = ilm.get_account();

print("Creating multi index table 'orders'.\n");
eosio::multi_index< N( orders ), limit_order, 
  indexed_by< N( byexp ),   const_mem_fun< limit_order, uint64_t, &limit_order::get_expiration> >,
  indexed_by< N( byprice ), const_mem_fun< limit_order, uint128_t, &limit_order::get_price> >
    > orders( N( limitorders ), N( limitorders ) );

添加多索引表

下面，將兩個limit_order添加到orders表中。請注意，payer是正在修改orders表的「帳單」賬戶。spa

orders.emplace( payer, [&]( auto& o ) {
  o.id = 1;
  o.expiration = 300;
  o.owner = N(dan);
});

auto order2 = orders.emplace( payer, [&]( auto& o ) {
  o.id = 2;
  o.expiration = 200;
  o.owner = N(thomas);
});

按照主鍵排序

默認的orders表按照主鍵排序。code

print("Items sorted by primary key:\n");
for( const auto& item : orders ) {
  print(" ID=", item.id, ", expiration=", item.expiration, ", owner=", name{item.owner}, "\n");
}

按第二索引expiration排序

orders表經過expiration進行排序並分配給expidx。

auto expidx = orders.get_index<N(byexp)>();

print("Items sorted by expiration:\n");
for( const auto& item : expidx ) {
  print(" ID=", item.id, ", expiration=", item.expiration, ", owner=", name{item.owner}, "\n");
}

按第二索引price排序

orders表經過price進行排序並分配給oridx。

auto pridx = orders.get_index<N(byprice)>();

print("Items sorted by price:\n");
for( const auto& item : pridx ) {
  print(" ID=", item.id, ", expiration=", item.expiration, ", owner=", name{item.owner}, "\n");
}

修改一個輸入值

下面，「ID＝2」的條目被修改。請注意，payer是正在修改orders表的「帳單」賬戶。

print("Modifying expiration of order with ID=2 to 400.\n");
orders.modify( order2, payer, [&]( auto& o ) {
  o.expiration = 400;
});

獲得一個最小值

auto lower = expidx.lower_bound(100);

print("First order with an expiration of at least 100 has ID=", lower->id, " and expiration=", lower->get_expiration(), "\n");

完整的.cpp文件

#include <eosiolib/eosio.hpp>
#include <eosiolib/dispatcher.hpp>
#include <eosiolib/multi_index.hpp>

using namespace eosio;

namespace limit_order_table {

    struct limit_order {
        uint64_t     id;
        uint128_t    price;
        uint64_t     expiration;
        account_name owner;

        auto primary_key() const { return id; }
        uint64_t get_expiration() const { return expiration; }
        uint128_t get_price() const { return price; }

        EOSLIB_SERIALIZE( limit_order, ( id )( price )( expiration )( owner ) )
    };

    class limit_order_table {
        public:

        ACTION( N( limitorders ), issue_limit_order ) {
            EOSLIB_SERIALIZE( issue_limit_order )
        };

        static void on( const issue_limit_order& ilm ) {
            auto payer = ilm.get_account();

            print("Creating multi index table 'orders'.\n");
            eosio::multi_index< N( orders ), limit_order, 
                indexed_by< N( byexp ),   const_mem_fun< limit_order, uint64_t, &limit_order::get_expiration> >,
                indexed_by< N( byprice ), const_mem_fun< limit_order, uint128_t, &limit_order::get_price> >
                > orders( N( limitorders ), N( limitorders ) );

            orders.emplace( payer, [&]( auto& o ) {
                o.id = 1;
                o.expiration = 300;
                o.owner = N(dan);
            });

            auto order2 = orders.emplace( payer, [&]( auto& o ) {
                o.id = 2;
                o.expiration = 200;
                o.owner = N(thomas);
            });

            print("Items sorted by primary key:\n");
            for( const auto& item : orders ) {
                print(" ID=", item.id, ", expiration=", item.expiration, ", owner=", name{item.owner}, "\n");
            }

            auto expidx = orders.get_index<N(byexp)>();

            print("Items sorted by expiration:\n");
            for( const auto& item : expidx ) {
                print(" ID=", item.id, ", expiration=", item.expiration, ", owner=", name{item.owner}, "\n");
            }

            auto pridx = orders.get_index<N(byprice)>();

            print("Items sorted by price:\n");
            for( const auto& item : pridx ) {
                print(" ID=", item.id, ", expiration=", item.expiration, ", owner=", name{item.owner}, "\n");
            }

            print("Modifying expiration of order with ID=2 to 400.\n");
            orders.modify( order2, payer, [&]( auto& o ) {
                o.expiration = 400;
            });

            auto lower = expidx.lower_bound(100);

            print("First order with an expiration of at least 100 has ID=", lower->id, " and expiration=", lower->get_expiration(), "\n");
   };

} /// limit_order_table

namespace limit_order_table {
   extern "C" {
      /// The apply method implements the dispatch of events to this contract
      void apply( uint64_t code, uint64_t action ) {
         require_auth( code );
         eosio_assert( eosio::dispatch< limit_order_table, limit_order_table::issue_limit_order >( code, action ), "Could not dispatch" );
      }
   }
}