feat: add baseTokenURI to ERC721Metadata (#1970)

* feat: add baseTokenURI

* fix: tests

* chore: dev notation

* chore: changelog

* chore: typo

* Remove extra getters, return empty URI by default

* Update docs

* Rename baseTokenURI to baseURI

* Roll back visibility change of tokenURI

* Update changelog entry

* Version setBaseURI docs

* Improve internal names and comments

* Fix compilation errors

* Add an external getter for baseURI

contracts/mocks/ERC721FullMock.sol

@@ -26,4 +26,8 @@ contract ERC721FullMock is ERC721Full, ERC721Mintable, ERC721MetadataMintable, E
     function setTokenURI(uint256 tokenId, string memory uri) public {
         _setTokenURI(tokenId, uri);
     }
+
+    function setBaseURI(string memory baseURI) public {
+        _setBaseURI(baseURI);
+    }
 }

contracts/token/ERC721/ERC721Metadata.sol

@@ -12,6 +12,9 @@ contract ERC721Metadata is Context, ERC165, ERC721, IERC721Metadata {
     // Token symbol
     string private _symbol;
 
+    // Base URI
+    string private _baseURI;
+
     // Optional mapping for token URIs
     mapping(uint256 => string) private _tokenURIs;
 
@@ -52,24 +55,58 @@ contract ERC721Metadata is Context, ERC165, ERC721, IERC721Metadata {
     }
 
     /**
-     * @dev Returns an URI for a given token ID.
-     * Throws if the token ID does not exist. May return an empty string.
-     * @param tokenId uint256 ID of the token to query
+     * @dev Returns the URI for a given token ID. May return an empty string.
+     *
+     * If the token's URI is non-empty and a base URI was set (via
+     * {_setBaseURI}), it will be added to the token ID's URI as a prefix.
+     *
+     * Reverts if the token ID does not exist.
      */
     function tokenURI(uint256 tokenId) external view returns (string memory) {
         require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");
-        return _tokenURIs[tokenId];
+
+        string memory _tokenURI = _tokenURIs[tokenId];
+
+        // Even if there is a base URI, it is only appended to non-empty token-specific URIs
+        if (bytes(_tokenURI).length == 0) {
+            return "";
+        } else {
+            // abi.encodePacked is being used to concatenate strings
+            return string(abi.encodePacked(_baseURI, _tokenURI));
+        }
+    }
+
+    /**
+    * @dev Returns the base URI set via {_setBaseURI}. This will be
+    * automatically added as a preffix in {tokenURI} to each token's URI, when
+    * they are non-empty.
+    */
+    function baseURI() external view returns (string memory) {
+        return _baseURI;
     }
 
     /**
      * @dev Internal function to set the token URI for a given token.
+     *
      * Reverts if the token ID does not exist.
-     * @param tokenId uint256 ID of the token to set its URI
-     * @param uri string URI to assign
+     *
+     * TIP: if all token IDs share a prefix (e.g. if your URIs look like
+     * `http://api.myproject.com/token/<id>`), use {_setBaseURI} to store
+     * it and save gas.
      */
-    function _setTokenURI(uint256 tokenId, string memory uri) internal {
+    function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal {
         require(_exists(tokenId), "ERC721Metadata: URI set of nonexistent token");
-        _tokenURIs[tokenId] = uri;
+        _tokenURIs[tokenId] = _tokenURI;
+    }
+
+    /**
+     * @dev Internal function to set the base URI for all token IDs. It is
+     * automatically added as a prefix to the value returned in {tokenURI}.
+     *
+     * _Available since v2.5.0._
+     */
+    function _setBaseURI(string memory baseURI) internal {
+        _baseURI = baseURI;
     }
 
     /**