Add MixinAssetProxy to reuse redundant code

packages/contracts/src/contracts/current/protocol/AssetProxy/MixinAssetProxy.sol

@@ -0,0 +1,75 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.4.21;
+pragma experimental ABIEncoderV2;
+
+import "./mixins/MAssetProxy.sol";
+import "./IAssetProxy.sol";
+import "../../utils/Authorizable/Authorizable.sol";
+
+contract MixinAssetProxy is 
+    IAssetProxy,
+    MAssetProxy,
+    Authorizable
+{
+
+    /// @dev Transfers assets. Either succeeds or throws.
+    /// @param assetMetadata Encoded byte array.
+    /// @param from Address to transfer asset from.
+    /// @param to Address to transfer asset to.
+    /// @param amount Amount of asset to transfer.
+    function transferFrom(
+        bytes assetMetadata,
+        address from,
+        address to,
+        uint256 amount)
+        external
+        onlyAuthorized
+    {
+        transferFromInternal(
+            assetMetadata,
+            from,
+            to,
+            amount
+        );
+    }
+
+    /// @dev Makes multiple transfers of assets. Either succeeds or throws.
+    /// @param assetMetadata Array of byte arrays encoded for the respective asset proxy.
+    /// @param from Array of addresses to transfer assets from.
+    /// @param to Array of addresses to transfer assets to.
+    /// @param amounts Array of amounts of assets to transfer.
+    function batchTransferFrom(
+        bytes[] memory assetMetadata,
+        address[] memory from,
+        address[] memory to,
+        uint256[] memory amounts)
+        public
+        onlyAuthorized
+    {
+        for (uint256 i = 0; i < assetMetadata.length; i++) {
+            transferFromInternal(
+                assetMetadata[i],
+                from[i],
+                to[i],
+                amounts[i]
+            );
+        }
+    }
+}

packages/contracts/src/contracts/current/protocol/AssetProxy/mixins/MAssetProxy.sol

@@ -0,0 +1,35 @@
+/*
+
+  Copyright 2018 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.4.21;
+pragma experimental ABIEncoderV2;
+
+contract MAssetProxy {
+
+    /// @dev Internal version of `transferFrom`.
+    /// @param assetMetadata Encoded byte array.
+    /// @param from Address to transfer asset from.
+    /// @param to Address to transfer asset to.
+    /// @param amount Amount of asset to transfer.
+    function transferFromInternal(
+        bytes memory assetMetadata,
+        address from,
+        address to,
+        uint256 amount)
+        internal;
+}

packages/contracts/src/contracts/current/protocol/AssetProxy/proxies/ERC20Proxy.sol

@@ -19,65 +19,20 @@
 pragma solidity ^0.4.21;
 pragma experimental ABIEncoderV2;
 
-import "../IAssetProxy.sol";
+
 import "../../../utils/LibBytes/LibBytes.sol";
-import "../../../utils/Authorizable/Authorizable.sol";
 import "../../../tokens/ERC20Token/IERC20Token.sol";
+import "../MixinAssetProxy.sol";
 
 contract ERC20Proxy is
     LibBytes,
-    Authorizable,
-    IAssetProxy
+    MixinAssetProxy
 {
 
     uint8 constant PROXY_ID = 1;
 
-    /// @dev Transfers ERC20 tokens. Either succeeds or throws.
-    /// @param assetMetadata ERC20-encoded byte array.
-    /// @param from Address to transfer asset from.
-    /// @param to Address to transfer asset to.
-    /// @param amount Amount of asset to transfer.
-    function transferFrom(
-        bytes assetMetadata,
-        address from,
-        address to,
-        uint256 amount)
-        external
-        onlyAuthorized
-    {
-        transferFromInternal(
-            assetMetadata,
-            from,
-            to,
-            amount
-        );
-    }
-
-    /// @dev Makes multiple transfers of assets. Either succeeds or throws.
-    /// @param assetMetadata Array of byte arrays encoded for the respective asset proxy.
-    /// @param from Array of addresses to transfer assets from.
-    /// @param to Array of addresses to transfer assets to.
-    /// @param amounts Array of amounts of assets to transfer.
-    function batchTransferFrom(
-        bytes[] memory assetMetadata,
-        address[] memory from,
-        address[] memory to,
-        uint256[] memory amounts)
-        public
-        onlyAuthorized
-    {
-        for (uint256 i = 0; i < assetMetadata.length; i++) {
-            transferFromInternal(
-                assetMetadata[i],
-                from[i],
-                to[i],
-                amounts[i]
-            );
-        }
-    }
-
     /// @dev Internal version of `transferFrom`.
-    /// @param assetMetadata ERC20-encoded byte array.
+    /// @param assetMetadata Encoded byte array.
     /// @param from Address to transfer asset from.
     /// @param to Address to transfer asset to.
     /// @param amount Amount of asset to transfer.

packages/contracts/src/contracts/current/protocol/AssetProxy/proxies/ERC721Proxy.sol

@@ -19,65 +19,19 @@
 pragma solidity ^0.4.21;
 pragma experimental ABIEncoderV2;
 
-import "../IAssetProxy.sol";
 import "../../../utils/LibBytes/LibBytes.sol";
-import "../../../utils/Authorizable/Authorizable.sol";
 import "../../../tokens/ERC721Token/ERC721Token.sol";
+import "../MixinAssetProxy.sol";
 
 contract ERC721Proxy is
     LibBytes,
-    Authorizable,
-    IAssetProxy
+    MixinAssetProxy
 {
 
     uint8 constant PROXY_ID = 2;
 
-    /// @dev Transfers ERC721 tokens. Either succeeds or throws.
-    /// @param assetMetadata ERC721-encoded byte array
-    /// @param from Address to transfer asset from.
-    /// @param to Address to transfer asset to.
-    /// @param amount Amount of asset to transfer.
-    function transferFrom(
-        bytes assetMetadata,
-        address from,
-        address to,
-        uint256 amount)
-        external
-        onlyAuthorized
-    {
-        transferFromInternal(
-            assetMetadata,
-            from,
-            to,
-            amount
-        );
-    }
-
-    /// @dev Makes multiple transfers of assets. Either succeeds or throws.
-    /// @param assetMetadata Array of byte arrays encoded for the respective asset proxy.
-    /// @param from Array of addresses to transfer assets from.
-    /// @param to Array of addresses to transfer assets to.
-    /// @param amounts Array of amounts of assets to transfer.
-    function batchTransferFrom(
-        bytes[] memory assetMetadata,
-        address[] memory from,
-        address[] memory to,
-        uint256[] memory amounts)
-        public
-        onlyAuthorized
-    {
-        for (uint256 i = 0; i < assetMetadata.length; i++) {
-            transferFromInternal(
-                assetMetadata[i],
-                from[i],
-                to[i],
-                amounts[i]
-            );
-        }
-    }
-
     /// @dev Internal version of `transferFrom`.
-    /// @param assetMetadata ERC20-encoded byte array.
+    /// @param assetMetadata Encoded byte array.
     /// @param from Address to transfer asset from.
     /// @param to Address to transfer asset to.
     /// @param amount Amount of asset to transfer.