Add ERC721APausable as equivalent to ERC721Pausable (chiru-labs#205)

* Add ERC721APausable equivalent to ERC721Pausable

* CR: Docs and ContractPaused() error throw

* Unit tests

* Include unit test for burnable composability

* run linter

contracts/ERC721A.sol

@@ -326,8 +326,7 @@ contract ERC721A is Context, ERC165, IERC721, IERC721Metadata {
      * Tokens start existing when they are minted (`_mint`),
      */
     function _exists(uint256 tokenId) internal view returns (bool) {
-        return _startTokenId() <= tokenId && tokenId < _currentIndex &&
-            !_ownerships[tokenId].burned;
+        return _startTokenId() <= tokenId && tokenId < _currentIndex && !_ownerships[tokenId].burned;
     }
 
     function _safeMint(address to, uint256 quantity) internal {

contracts/extensions/ERC721ABurnable.sol

@@ -10,7 +10,6 @@ import '../ERC721A.sol';
  * @dev ERC721A Token that can be irreversibly burned (destroyed).
  */
 abstract contract ERC721ABurnable is ERC721A {
-
     /**
      * @dev Burns `tokenId`. See {ERC721A-_burn}.
      *
@@ -21,4 +20,4 @@ abstract contract ERC721ABurnable is ERC721A {
     function burn(uint256 tokenId) public virtual {
         _burn(tokenId, true);
     }
-}
+}

contracts/extensions/ERC721APausable.sol

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: MIT
+// Creator: Chiru Labs
+
+pragma solidity ^0.8.4;
+
+import '../ERC721A.sol';
+import '@openzeppelin/contracts/security/Pausable.sol';
+
+error ContractPaused();
+
+/**
+ * @dev ERC721A token with pausable token transfers, minting and burning.
+ *
+ * Based off of OpenZeppelin's ERC721Pausable extension.
+ *
+ * Useful for scenarios such as preventing trades until the end of an evaluation
+ * period, or having an emergency switch for freezing all token transfers in the
+ * event of a large bug.
+ */
+abstract contract ERC721APausable is ERC721A, Pausable {
+    /**
+     * @dev See {ERC721A-_beforeTokenTransfers}.
+     *
+     * Requirements:
+     *
+     * - the contract must not be paused.
+     */
+    function _beforeTokenTransfers(
+        address from,
+        address to,
+        uint256 startTokenId,
+        uint256 quantity
+    ) internal virtual override {
+        super._beforeTokenTransfers(from, to, startTokenId, quantity);
+        if (paused()) revert ContractPaused();
+    }
+}

contracts/mocks/ERC721ABurnableMock.sol

@@ -15,12 +15,12 @@ contract ERC721ABurnableMock is ERC721A, ERC721ABurnable {
     function safeMint(address to, uint256 quantity) public {
         _safeMint(to, quantity);
     }
-    
+
     function getOwnershipAt(uint256 index) public view returns (TokenOwnership memory) {
         return _ownerships[index];
     }
 
     function totalMinted() public view returns (uint256) {
         return _totalMinted();
     }
-}
+}

contracts/mocks/ERC721ABurnableOwnersExplicitMock.sol

@@ -24,4 +24,4 @@ contract ERC721ABurnableOwnersExplicitMock is ERC721A, ERC721ABurnable, ERC721AO
     function getOwnershipAt(uint256 index) public view returns (TokenOwnership memory) {
         return _ownerships[index];
     }
-}
+}

contracts/mocks/ERC721APausableMock.sol

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MIT
+// Creators: Chiru Labs
+
+pragma solidity ^0.8.4;
+
+import '../extensions/ERC721APausable.sol';
+import '../extensions/ERC721ABurnable.sol';
+
+contract ERC721APausableMock is ERC721A, ERC721APausable, ERC721ABurnable {
+    constructor(string memory name_, string memory symbol_) ERC721A(name_, symbol_) {}
+
+    function safeMint(address to, uint256 quantity) public {
+        _safeMint(to, quantity);
+    }
+
+    function _beforeTokenTransfers(
+        address from,
+        address to,
+        uint256 startTokenId,
+        uint256 quantity
+    ) internal virtual override(ERC721A, ERC721APausable) {
+        super._beforeTokenTransfers(from, to, startTokenId, quantity);
+    }
+
+    function pause() external {
+        _pause();
+    }
+
+    function unpause() external {
+        _unpause();
+    }
+}

contracts/mocks/StartTokenIdHelper.sol

@@ -14,4 +14,4 @@ contract StartTokenIdHelper {
     constructor(uint256 startTokenId_) {
         startTokenId = startTokenId_;
     }
-}
+}

test/extensions/ERC721APausable.test.js

@@ -0,0 +1,73 @@
+const { deployContract } = require('../helpers.js');
+const { expect } = require('chai');
+
+const createTestSuite = ({ contract, constructorArgs }) =>
+  function () {
+    context(`${contract}`, function () {
+      beforeEach(async function () {
+        this.erc721aPausable = await deployContract(contract, constructorArgs);
+
+        this.startTokenId = this.erc721aPausable.startTokenId
+          ? (await this.erc721aPausable.startTokenId()).toNumber()
+          : 0;
+      });
+
+      beforeEach(async function () {
+        const [owner, addr1, addr2] = await ethers.getSigners();
+        this.owner = owner;
+        this.addr1 = addr1;
+        this.addr2 = addr2;
+        this.existingTokenID = 0;
+        this.numTestTokens = 1;
+        await this.erc721aPausable['safeMint(address,uint256)'](this.addr1.address, this.numTestTokens);
+      });
+
+      it('cannot burn a valid token id while paused', async function () {
+        await this.erc721aPausable.connect(this.owner).pause()
+        const query = this.erc721aPausable
+          .connect(this.addr1)
+          .burn(this.existingTokenID);
+
+        await expect(query).to.be.revertedWith('ContractPaused');
+        expect(await this.erc721aPausable.ownerOf(this.existingTokenID)).to.be.equal(this.addr1.address);
+      });
+
+      it('cannot transfer a valid token id while paused', async function () {
+        await this.erc721aPausable.connect(this.owner).pause()
+        const query = this.erc721aPausable
+          .connect(this.addr1)
+          .transferFrom(this.addr1.address, this.addr2.address, this.existingTokenID);
+
+        await expect(query).to.be.revertedWith('ContractPaused');
+        expect(await this.erc721aPausable.ownerOf(this.existingTokenID)).to.be.equal(this.addr1.address);
+      });
+
+      it('can transfer a valid token id while unpaused', async function () {
+        await this.erc721aPausable.connect(this.owner).pause()
+        await this.erc721aPausable.connect(this.owner).unpause()
+        const query = this.erc721aPausable
+          .connect(this.addr1)
+          .transferFrom(this.addr1.address, this.addr2.address, this.existingTokenID);
+
+        await expect(query).to.not.be.revertedWith('ContractPaused');
+        expect(await this.erc721aPausable.ownerOf(this.existingTokenID)).to.be.equal(this.addr2.address);
+      });
+
+      it('can burn a valid token id while unpaused', async function () {
+        await this.erc721aPausable.connect(this.owner).pause()
+        await this.erc721aPausable.connect(this.owner).unpause()
+        const query = this.erc721aPausable
+          .connect(this.addr1)
+          .burn(this.existingTokenID);
+
+        await expect(query).to.not.be.revertedWith('ContractPaused');
+        await expect(this.erc721aPausable.ownerOf(this.existingTokenID))
+          .to.be.revertedWith('OwnerQueryForNonexistentToken');
+      });
+    });
+  };
+
+describe(
+  'ERC721APausable',
+  createTestSuite({ contract: 'ERC721APausableMock', constructorArgs: ['Azuki', 'AZUKI'] })
+);