Approval events on transferFrom and burnFrom (#1524)

* transferFrom now emits an Approval event, indicating the updated allowance.

* Updated burnFrom to also emit Approval.

* Added notices about the extra Approval events.

contracts/token/ERC20/ERC20.sol

@@ -9,6 +9,10 @@ import "../../math/SafeMath.sol";
  * @dev Implementation of the basic standard token.
  * https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
  * Originally based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
+ *
+ * This implementation emits additional Approval events, allowing applications to reconstruct the allowance status for
+ * all accounts just by listening to said events. Note that this isn't required by the specification, and other
+ * compliant implementations may not do it.
  */
 contract ERC20 is IERC20 {
     using SafeMath for uint256;
@@ -73,14 +77,17 @@ contract ERC20 is IERC20 {
     }
 
     /**
-     * @dev Transfer tokens from one address to another
+     * @dev Transfer tokens from one address to another.
+     * Note that while this function emits an Approval event, this is not required as per the specification,
+     * and other compliant implementations may not emit the event.
      * @param from address The address which you want to send tokens from
      * @param to address The address which you want to transfer to
      * @param value uint256 the amount of tokens to be transferred
      */
     function transferFrom(address from, address to, uint256 value) public returns (bool) {
         _allowed[from][msg.sender] = _allowed[from][msg.sender].sub(value);
         _transfer(from, to, value);
+        emit Approval(from, msg.sender, _allowed[from][msg.sender]);
         return true;
     }
 
@@ -90,6 +97,7 @@ contract ERC20 is IERC20 {
      * allowed value is better to use this function to avoid 2 calls (and wait until
      * the first transaction is mined)
      * From MonolithDAO Token.sol
+     * Emits an Approval event.
      * @param spender The address which will spend the funds.
      * @param addedValue The amount of tokens to increase the allowance by.
      */
@@ -107,6 +115,7 @@ contract ERC20 is IERC20 {
      * allowed value is better to use this function to avoid 2 calls (and wait until
      * the first transaction is mined)
      * From MonolithDAO Token.sol
+     * Emits an Approval event.
      * @param spender The address which will spend the funds.
      * @param subtractedValue The amount of tokens to decrease the allowance by.
      */
@@ -165,13 +174,13 @@ contract ERC20 is IERC20 {
      * @dev Internal function that burns an amount of the token of a given
      * account, deducting from the sender's allowance for said account. Uses the
      * internal burn function.
+     * Emits an Approval event (reflecting the reduced allowance).
      * @param account The account whose tokens will be burnt.
      * @param value The amount that will be burnt.
      */
     function _burnFrom(address account, uint256 value) internal {
-        // Should https://github.com/OpenZeppelin/zeppelin-solidity/issues/707 be accepted,
-        // this function needs to emit an event with the updated approval.
         _allowed[account][msg.sender] = _allowed[account][msg.sender].sub(value);
         _burn(account, value);
+        emit Approval(account, msg.sender, _allowed[account][msg.sender]);
     }
 }

test/token/ERC20/ERC20.test.js

@@ -199,6 +199,16 @@ contract('ERC20', function ([_, owner, recipient, anotherAccount]) {
               value: amount,
             });
           });
+
+          it('emits an approval event', async function () {
+            const { logs } = await this.token.transferFrom(owner, to, amount, { from: spender });
+
+            expectEvent.inLogs(logs, 'Approval', {
+              owner: owner,
+              spender: spender,
+              value: await this.token.allowance(owner, spender),
+            });
+          });
         });
 
         describe('when the owner does not have enough balance', function () {
@@ -521,14 +531,22 @@ contract('ERC20', function ([_, owner, recipient, anotherAccount]) {
             (await this.token.allowance(owner, spender)).should.be.bignumber.equal(expectedAllowance);
           });
 
-          it('emits Transfer event', async function () {
+          it('emits a Transfer event', async function () {
             const event = expectEvent.inLogs(this.logs, 'Transfer', {
               from: owner,
               to: ZERO_ADDRESS,
             });
 
             event.args.value.should.be.bignumber.equal(amount);
           });
+
+          it('emits an Approval event', async function () {
+            expectEvent.inLogs(this.logs, 'Approval', {
+              owner: owner,
+              spender: spender,
+              value: await this.token.allowance(owner, spender),
+            });
+          });
         });
       };