Interacting with a Universal App

Calling a Universal App

To call a universal app and transfer native gas tokens from a connected chain, a user makes a token transfer transaction to a special address called TSS.

To call a universal app and transfer supported ERC-20 tokens from a connected chain, a user calls the deposit method of the ERC-20 custody contract.

Calling a universal app executes the onCrossChainCall method.

The message is passed as the bytes calldata message argument and can contain arbitrary data.

Native gas or supported ERC-20 tokens get locked in the TSS address or ERC-20 custody contract on connected chain. onCrossChainCall receives representations of these tokens called ZRC-20. For example, if you make a call with 1 ETH on Ethereum, a universal app will receive 1 ZRC-20 ETH.

Advantages:

• Bitcoin support
• Sending messages to a TSS address is cheaper than general message passing
• Does not require deploying contracts on connected chains. For example, a user on Ethereum can call a universal app by sending ETH and a message to a TSS address.

Limitations:

• ERC-20 custody contract accepts only supported tokens.
• Messages can only be sent to universal apps, not from them.

Withdrawing ZRC-20 Tokens

Native gas and tokens ERC-20 tokens transferred to a universal app are represented as ZRC-20 tokens. ZRC-20 extends ERC-20 with a withdraw method.

Call the withdraw method to send ZRC-20 tokens back to the chain from which they originate. The withdrawer must have sufficient tokens to cover the withdraw fee.

Sending Messages to/from a Universal App

To send a message to or from a universal app you can also use a connector API. Connector is an API that offers two features:

• General message passing to and from universal apps
• Transfer of ZETA tokens to and from universal apps

Advantages:

• Messages can be sent to and from universal apps
• Messages can be sent between messaging contracts on connected chains. For example, a messaging contract on Ethereum can send a message to a contract on BNB chain.

Limitations:

• Contracts need to be deployed on connected chains to receive and send messages

Summary

• Universal apps are contracts deployed on ZetaChain's EVM that can accept contract calls, messages and token transfers from connected chains.
• Universal apps can contract calls, and make token transfers on connected chains.
• A universal app can be implemented as a single contract on ZetaChain. You don't need to have contracts on all connected chains.
• You can use the connector API for general message passing to and from universal apps.

Hands-on Practice

Eager to start building universal apps? 🙌

Jump directly into the tutorials section, and create your first universal app.