add InsertedIntoTree event

solidity/contracts/hooks/MerkleTreeHook.sol

@@ -14,6 +14,8 @@ contract MerkleTreeHook is IPostDispatchHook, MailboxClient, Indexed {
     // An incremental merkle tree used to store outbound message IDs.
     MerkleLib.Tree internal _tree;
 
+    event InsertedIntoTree(bytes32 messageId, uint32 index);
+
     constructor(address _mailbox) MailboxClient(_mailbox) {}
 
     function count() public view returns (uint32) {
@@ -39,7 +41,9 @@ contract MerkleTreeHook is IPostDispatchHook, MailboxClient, Indexed {
         require(msg.value == 0, "MerkleTreeHook: no value expected");
         bytes32 id = message.id();
         require(isLatestDispatched(id), "message not dispatching");
+
         _tree.insert(id);
+        emit InsertedIntoTree(id, count() - 1);
     }
 
     function quoteDispatch(

solidity/test/MerkleTreeHook.t.sol

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: MIT or Apache-2.0
+pragma solidity ^0.8.13;
+
+import {Test, console} from "forge-std/Test.sol";
+
+import {Message} from "../contracts/libs/Message.sol";
+import {TypeCasts} from "../contracts/libs/TypeCasts.sol";
+import {TestMerkleTreeHook} from "../contracts/test/TestMerkleTreeHook.sol";
+import {TestMailbox} from "../contracts/test/TestMailbox.sol";
+
+contract MerkleTreeHookTest is Test {
+    using Message for bytes;
+    using TypeCasts for address;
+
+    TestMailbox internal mailbox;
+    TestMerkleTreeHook internal hook;
+
+    uint32 internal constant ORIGIN = 11;
+    uint32 internal constant DESTINATION = 22;
+    address internal constant RECIPIENT = address(0xdeadbeef);
+    uint256 internal MSG_COUNT = 3;
+    bytes[3] internal testMessage;
+    bytes32[3] internal expectedRoots = [
+        bytes32(
+            0x10df2f89cb24ed6078fc3949b4870e94a7e32e40e8d8c6b7bd74ccc2c933d760
+        ),
+        bytes32(
+            0x080ef1c2cd394de78363ecb0a466c934b57de4abb5604a0684e571990eb7b073
+        ),
+        bytes32(
+            0xbf78ad252da524f1e733aa6b83514dd83225676b5828f888f01487108f8f7cc7
+        )
+    ];
+
+    event InsertedIntoTree(bytes32 messageId, uint32 index);
+
+    function setUp() public {
+        mailbox = new TestMailbox(ORIGIN);
+        hook = new TestMerkleTreeHook(address(mailbox));
+
+        for (uint256 i = 0; i < MSG_COUNT; i++) {
+            testMessage[i] = mailbox.buildOutboundMessage(
+                DESTINATION,
+                RECIPIENT.addressToBytes32(),
+                abi.encodePacked(i)
+            );
+        }
+    }
+
+    function testPostDispatch_emit() public {
+        for (uint32 i = 0; i < MSG_COUNT; i++) {
+            bytes memory currMessage = testMessage[i];
+            mailbox.updateLatestDispatchedId(currMessage.id());
+
+            vm.expectEmit(false, false, false, true);
+            emit InsertedIntoTree(currMessage.id(), i);
+            hook.postDispatch("", currMessage);
+
+            assertEq(hook.count(), i + 1);
+            assertEq(hook.root(), expectedRoots[i]);
+        }
+    }
+
+    function testQuoteDispatch() public {
+        for (uint256 i = 0; i < MSG_COUNT; i++) {
+            bytes memory currMessage = testMessage[i];
+
+            assertEq(hook.quoteDispatch("", currMessage), 0);
+        }
+    }
+}