Implement ERC721 token with metadata

contracts/mocks/ERC721TokenMetadataMock.sol

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+pragma solidity ^0.4.18;
+
+import "./ERC721TokenMock.sol";
+import "../token/ERC721/ERC721TokenMetadata.sol";
+
+/**
+ * @title ERC721TokenMetadataMock
+ * This mock just provides a public mint and burn functions for testing purposes.
+ */
+contract ERC721TokenMetadataMock is ERC721TokenMetadata, ERC721TokenMock {
+  function ERC721TokenMetadataMock(string name, string symbol)
+  ERC721TokenMock(name, symbol)
+  ERC721TokenMetadata(name, symbol)
+  public
+  { }
+}

contracts/token/ERC721/ERC721Metadata.sol

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+pragma solidity ^0.4.18;
+
+import "./ERC721.sol";
+
+
+/**
+ * @title Full ERC721 interface with metadata
+ * @dev see https://github.com/ethereum/eips/issues/721 and https://github.com/ethereum/EIPs/pull/841
+ */
+contract ERC721Metadata is ERC721 {
+  event MetadataUpdated(address _owner, uint256 _tokenId, string _newMetadata);
+
+  function setTokenMetadata(uint256 _tokenId, string _metadata) public;
+  function tokenMetadata(uint256 _tokenId) public view returns (string infoUrl);
+}

contracts/token/ERC721/ERC721TokenMetadata.sol

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+pragma solidity ^0.4.18;
+
+import "./ERC721Token.sol";
+import "./ERC721Metadata.sol";
+
+
+/**
+ * @title Full ERC721 Token with metadata
+ * This implementation includes all the functionality of the ERC721 standard besides metadata functionality
+ * @dev see https://github.com/ethereum/eips/issues/721 and https://github.com/ethereum/EIPs/pull/841
+ */
+contract ERC721TokenMetadata is ERC721Metadata, ERC721Token {
+  // Tokens metadata
+  mapping (uint256 => string) private _metadata;
+
+  /**
+  * @dev Constructor function
+  */
+  function ERC721TokenMetadata(string name, string symbol) ERC721Token(name, symbol) public {}
+
+  /**
+  * @dev Gets the metadata of the given token ID
+  * @param _tokenId uint256 ID of the token to query the metadata of
+  * @return string representing the metadata of the given token ID
+  */
+  function tokenMetadata(uint256 _tokenId) public view returns (string) {
+    return _metadata[_tokenId];
+  }
+
+  /**
+  * @dev Sets the metadata of the given token ID
+  * @param _tokenId uint256 ID of the token to set the metadata of
+  * @param _newMetadata string representing the new metadata to be set
+  */
+  function setTokenMetadata(uint256 _tokenId, string _newMetadata) public onlyOwnerOf(_tokenId) {
+    _metadata[_tokenId] = _newMetadata;
+    MetadataUpdated(msg.sender, _tokenId, _newMetadata);
+  }
+}

test/token/ERC721/ERC721TokenMetadata.test.js

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+import assertRevert from '../../helpers/assertRevert';
+const BigNumber = web3.BigNumber;
+const ERC721TokenMetadata = artifacts.require('ERC721TokenMetadataMock.sol');
+
+require('chai')
+  .use(require('chai-as-promised'))
+  .use(require('chai-bignumber')(BigNumber))
+  .should();
+
+contract('ERC721TokenMetadata', accounts => {
+  let token = null;
+  const _name = 'Non Fungible Token';
+  const _symbol = 'NFT';
+  const _firstTokenId = 1;
+  const _secondTokenId = 2;
+  const _unknownTokenId = 3;
+  const _creator = accounts[0];
+
+  beforeEach(async function () {
+    token = await ERC721TokenMetadata.new(_name, _symbol, { from: _creator });
+    await token.mint(_creator, _firstTokenId, { from: _creator });
+    await token.mint(_creator, _secondTokenId, { from: _creator });
+  });
+
+  describe('tokenMetadata', function () {
+    describe('when no metadata was set', function () {
+      it('the given token has no metadata', async function () {
+        const metadata = await token.tokenMetadata(_firstTokenId);
+
+        metadata.should.be.equal('');
+      });
+    });
+
+    describe('when some metadata was set', function () {
+      it('returns the metadata of the given token', async function () {
+        await token.setTokenMetadata(_firstTokenId, 'dummy metadata', { from: _creator });
+
+        const metadata = await token.tokenMetadata(_firstTokenId);
+
+        metadata.should.be.equal('dummy metadata');
+      });
+    });
+  });
+
+  describe('setTokenMetadata', function () {
+    describe('when the sender is not the token owner', function () {
+      const sender = accounts[1];
+
+      it('reverts', async function () {
+        await assertRevert(token.setTokenMetadata(_firstTokenId, 'new metadata', { from: sender }));
+      });
+    });
+
+    describe('when the sender is the owner of the token', function () {
+      const sender = _creator;
+
+      describe('when the given token ID was tracked by this contract before', function () {
+        const tokenId = _firstTokenId;
+
+        it('updates the metadata of the given token ID', async function () {
+          await token.setTokenMetadata(_firstTokenId, 'new metadata', { from: sender });
+
+          const metadata = await token.tokenMetadata(tokenId);
+
+          metadata.should.be.equal('new metadata');
+        });
+
+        it('emits a metadata updated event', async function () {
+          const { logs } = await token.setTokenMetadata(tokenId, 'new metadata', { from: sender });
+
+          logs.length.should.be.equal(1);
+          logs[0].event.should.be.eq('MetadataUpdated');
+          logs[0].args._owner.should.be.equal(sender);
+          logs[0].args._tokenId.should.be.bignumber.equal(tokenId);
+          logs[0].args._newMetadata.should.be.equal('new metadata');
+        });
+      });
+
+      describe('when the given token ID was not tracked by this contract before', function () {
+        const tokenId = _unknownTokenId;
+
+        it('reverts', async function () {
+          await assertRevert(token.setTokenMetadata(tokenId, 'new metadata'), { from: sender });
+        });
+      });
+    });
+  });
+});