Add custom events

arbitrum-docs/launch-orbit-chain/how-tos/customize-precompile.mdx

@@ -19,10 +19,11 @@ To support these additional use-cases, follow the instructions described in [How
 
 :::
 
-There are two ways to customize your chain's precompiles:
+There are three primary ways to customize your chain's precompiles:
 
 1.  Add new methods to an existing [precompile][precompile_impl_dir_link].
 2.  Create a new precompile.
+3.  Define a new event.
 
 ### Prerequisites
 
@@ -162,6 +163,111 @@ You should see something like this:
 hi
 ```
 
+## Option 3: Define a new event
+
+We'll reuse the `Arbsys` precompile from Option 1 above to demonstrate how to emit a simple `Hi` event from the `SayHi` method in `ArbSys.sol`.
+
+First, go to the [precompiles implementation][precompile_impl_dir_link] directory, find `ArbSys.go`, and edit the `ArbSys` struct:
+
+```go
+// ArbSys provides system-level functionality for interacting with L1 and understanding the call stack.
+type ArbSys struct {
+	Address                 addr // 0x64
+	L2ToL1Tx                func(ctx, mech, addr, addr, huge, huge, huge, huge, huge, huge, []byte) error
+	L2ToL1TxGasCost         func(addr, addr, huge, huge, huge, huge, huge, huge, []byte) (uint64, error)
+	SendMerkleUpdate        func(ctx, mech, huge, bytes32, huge) error
+	SendMerkleUpdateGasCost func(huge, bytes32, huge) (uint64, error)
+	InvalidBlockNumberError func(huge, huge) error
+
+	// deprecated event
+	L2ToL1Transaction        func(ctx, mech, addr, addr, huge, huge, huge, huge, huge, huge, huge, []byte) error
+	L2ToL1TransactionGasCost func(addr, addr, huge, huge, huge, huge, huge, huge, huge, []byte) (uint64, error)
+
+	// Add your customize event here:
+	Hi						func(ctx, mech, addr) error
+	// This is needed and will tell you how much gas it will cost, the param is the same as your event but without the first two (ctx, mech), the return param is always (uint64, error)
+	HiGasCost				func(addr) (uint64, error)
+}
+```
+
+Then add the event to the `SayHi` method:
+
+```go
+func (con *ArbSys) SayHi(c ctx, evm mech) (string, error) {
+	err := con.Hi(c, evm, c.caller)
+	return "hi", err
+}
+```
+
+Now navigate to the [precompiles interface][precompile_interface_dir_link] directory, open `Arbsys.sol`, and add the required interface. Ensure that the event name on the interface matches the name of the function you introduced in `ArbSys` struct in the previous step:
+
+```solidity
+event Hi(address caller);
+```
+
+If you want to [index the parameter](https://docs.soliditylang.org/en/latest/contracts.html#events) of the event (if you want to filter by that parameter in the future, for example), just add `indexed` to the Solidity interface:
+
+```solidity
+event Hi(address indexed caller);
+```
+
+Our function now emits an event, which means that when calling it, the state will change and a gas cost will be incurred. So we have to remove the `view` function behavior:
+
+```solidity
+function sayHi() external returns(string memory);
+```
+
+Next, build Nitro by following the instructions in [How to build Nitro locally](/node-running/how-tos/build-nitro-locally). Note that if you've already built the Docker image, you still need run the last step to rebuild.
+
+Run Nitro with the following command:
+
+```shell
+docker run --rm -it  -v /some/local/dir/arbitrum:/home/user/.arbitrum -p 0.0.0.0:8547:8547 -p 0.0.0.0:8548:8548 @latestNitroNodeImage@ --parent-chain.connection.url=<YourParentChainUrl> --chain.id=<YourOrbitChainId> --http.api=net,web3,eth,debug --http.corsdomain=* --http.addr=0.0.0.0 --http.vhosts=*
+```
+
+:::info
+
+Note that the instructions provided in [How to run a full node](/node-running/how-tos/running-a-full-node) **will not** work with your Orbit node. See [Command-line options (Orbit)](/launch-orbit-chain/reference/command-line-options) for Orbit-specific CLI flags.
+
+:::
+
+### Send the transaction and get the transaction receipt
+
+To send a transaction to `ArbSys`, we need to include a gas cost, because the function is no longer a view/pure function:
+
+```
+cast send 0x0000000000000000000000000000000000000064 "sayHi()(string)"
+```
+
+Call `eth_getTransactionReceipt` with the returned transaction hash result. You should see something like this:
+
+```
+{"jsonrpc":"2.0","id":1,"result":{"blockHash":"Your_blockHash","blockNumber":"Your_blockNumber","contractAddress":null,"cumulativeGasUsed":"0x680b","effectiveGasPrice":"0x5f5e100","from":"Your_address","gasUsed":"0x680b","gasUsedForL1":"0xe35","l1BlockNumber":"l1_blockNumber","logs":[{"address":"0x0000000000000000000000000000000000000064","topics":["0xa9378d5bd800fae4d5b8d4c6712b2b64e8ecc86fdc831cb51944000fc7c8ecfa","0x000000000000000000000000{Your_address}"],"data":"0x","blockNumber":"Your_blockNumber","transactionHash":"Your_txHash","transactionIndex":"0x1","blockHash":"Your_blockHash","logIndex":"0x0","removed":false}],"logsBloom":"0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000008000000000000000000000000000000000000000000000100000000000000040000000000000080004000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000040000000000000000000000000000000004000000000000200000000000000000000000000000000000000000000000000000000000000000000000000000000000","status":"0x1","to":"0x0000000000000000000000000000000000000064","transactionHash":"Your_txHash","transactionIndex":"0x1","type":"0x2"}}
+```
+
+Note the `logs` field within the transaction receipt:
+
+```
+"logs":[
+         {
+            "address":"0x0000000000000000000000000000000000000064",
+            "topics":[
+               "0xa9378d5bd800fae4d5b8d4c6712b2b64e8ecc86fdc831cb51944000fc7c8ecfa",
+               "0x000000000000000000000000{Your_address}"
+            ],
+            "data":"0x",
+            "blockNumber":"0x40",
+            "transactionHash":"{Your_txHash}",
+            "transactionIndex":"0x1",
+            "blockHash":"0x0b367d705002b3575db99354a0964c033f929f26f4442ed347e47ae43a8f28e4",
+            "logIndex":"0x0",
+            "removed":false
+         }
+      ]
+```
+
+From the `logs` field, we can see the `topics[0]` is `0xa937..cfa`, which is the event signature of `Hi(address)`. You can verify this using [4 byte directory](https://www.4byte.directory/event-signatures/?bytes_signature=0xa9378d5bd800fae4d5b8d4c6712b2b64e8ecc86fdc831cb51944000fc7c8ecfa) - `topics[1]` is `Your_address`, which is exactly what we defined above!
+
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