Count BApp for Android (Count) is an Android implementation (Java) demonstrating the simple use of caver-java. The primary purpose of Count is to highlight Klaytn Fee Delegation mechanism with an arbitrary fee delegator. Check out the article on this app from Klaytn Developers Forum.
You can fork this repository and run the project on Android Studio. This way, you can run emulators of your choice to test the app without having physical devices.
Count uses Klaytn Fee Delegation to enable users to send tranasctions without paying transaction fees.
The idea is that if there is a fee delegator, FD, with fee delegation key FDK, all transactions sent from accounts with zero balance can be included in blocks as long as they have additional signatures verfiable by the public key derived from FDK.
Users wishing to send transactions using fee delegation must find a fee delegator (just like FD) and send their transactions to him/her in order to get valid signatures. Detailed explanation regarding fee delegation is in here.
Count uses HTTP for sending fee delegated transactions, and assumes there exists a fee delegator, called payer, who has implemented a REST interface like this:
POST /payer
Here's an example payload that Count sends to a payer:
{
"senderRawTx": "0x1234ABCD5678...EF90"
}
Payers accept and process user submitted transactions as described in here, and send responses containing transaction hashes. In case of failures (e.g., server failure, network disconnection, failed transactions), the payer responds with proper error messages describing the nature of failure.
An example payer implementation is available at KlaytnDevelopers/payer-express. Simply clone and run the payer using npm start with required environment variables (FEE_PAYER_KEY and FEE_PAYER_ADDRESS; for more information, please follow the instruction available in the payer-express repository).
FYI, if you run payer-express on your local machine (i.e., localhost) and wish to use it from the emulator, make sure you point to 10.0.2.2:3000, not localhost:3000.