Cross-Chain Swap

Here is an example app that allows swapping one token on chain1 to another token on chain2 through cBridge and DEXes on both chain1 and chain2.

Source code on GitHub.

For the simplicity of explanation, let's say we deploy this contract on chain1 and chain2, and we want to input tokenA on chain1 and gain tokenC on chain2.

Public functions transferWithSwap and transferWithSwapNative are called by a user to initiate the entire process. These functions takes in a SwapInfo struct that specifies the behavior or "route" of the execution, and execute the process in the following fashion:

1. Swap tokenA on the source chain to gain tokenB

2. Packages a SwapRequest as a "message", which indicates the swap behavior on chain2

3. sendMessageWithTransfer is then called internally to send the message along with the tokenB through the bridge to chain2

4. On chain2, executeMessageWithTransfer is automatically called when the bridge determines that the execution conditions are met.

5. This contract parses the message received to a SwapRequest struct, then executes the swap using the tokenB received to gain tokenC. (Note: when executeMessageWithTransfer is called, it is guaranteed that tokenB already arrives at the TransferSwap contract address on chain2. You can check out this part of verification logic in MessageBusReceiver.sol's executeMessageWithTransfer).

6. If the execution of executeMessageWithTransfer of TransferSwap contract on chain2 reverts, or if the executeMessageWithTransfer call returns false, then MessageBus would call executeMessageWithTransferFallback. This is the place where you implement logic to decide what to do with the received tokenB.

The following is a more graphical explanation of all the supported flows of this demo app:

1. swap bridge swap

|--------chain1--------|-----SGN-----|---------chain2--------|
tokenA -> swap -> tokenB -> bridge -> tokenB -> swap -> tokenC -> out

2. swap bridge

|--------chain1--------|-----SGN-----|---------chain2--------|
tokenA -> swap -> tokenB -> bridge -> tokenB -> out

3. bridge swap

|--------chain1--------|-----SGN-----|---------chain2--------|
                  tokenA -> bridge -> tokenA -> swap -> tokenB -> out

4. just swap

|--------chain1--------|
tokenA -> swap -> tokenB -> out