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TUTORIAL: How to make wallet transactions on Android

Tutorial Overview

This tutorial is meant for android developers who want to begin to build interesting applications on the aeternity blockchain. In this tutorial we will cover the following:

  • Wallet Creation
  • Checking Wallet Balance
  • Token transfer

Prerequisites

  • A familiarity with the kotlin programming language
  • Basic understanding of some blockchain terminologies

Setup And Configuration

Create a new android studio project, change the language type to kotlin, set the minimum sdk version to 26(Android 8.0 oreo) Now, in your app level build.gradle file, add the code below just after the closing brace of the defaultConfig

 compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = "1.8"
    }
    packagingOptions {
        exclude 'META-INF/DISCLAIMER'
        exclude 'META-INF/io.netty.versions.properties'
        exclude 'META-INF/INDEX.LIST'
    }
    packagingOptions {
        exclude 'lib/x86_64/darwin/libscrypt.dylib'
        exclude 'lib/x86_64/freebsd/libscrypt.so'
        exclude 'lib/x86_64/linux/libscrypt.so'
    }

also make sure that Jcenter is present in the repository section of your Top level build.gradle file

repositories {
        jcenter()

}

and finally add this dependency to the dependency section of your project and click sync now

implementation 'com.kryptokrauts:aepp-sdk-java:2.2.1'

So simple, thats all you need to get set up

Wallet creation

To create a wallet, we need to make use of some classes which include the AeternityService,AeternityServiceFactory,AeternityServiceConfiguration,KeyPairService,Keypair.

  • AeternityService: It is the central access point to all Services
  • AeternityServiceFactory: Used to provide a Singleton instance of the AeternityService
  • AeternityServiceConfiguration: Used to configure the AeternityService object
  • KeyPairService: It is used to create A keypair
  • KeyPair: An existence of a wallet on the aeternity blockchain, contains access to publicKey, privateKey,...

Lets begin to Use some of all this classes in our Project,Open the MainActivity class of your project and write the following:

lateinit var keyPairService:KeyPairService
lateinit var userKeyPair:BaseKeyPair
lateinit  var mnemonicKeyPair:MnemonicKeyPair
lateinit var aeternityService:AeternityService
//You can change the variables below to also point to the mainnet, the testnet is enough for this example
private val aeternalBaseUrl="https://testnet.aeternal.io";
private val nodeBaseUrl="https://sdk-testnet.aepps.com"

Then in your onCreate function initialize your aeternityService and keypairService variables, we also need to disable Vert.x from creating a cache folder to avoid an exception,Since the aepp-sdk-java creates an instance of it behind the scenes

  System.setProperty("vertx.disableFileCPResolving", "true")
  
  aeternityService=AeternityServiceFactory().getService(
           AeternityServiceConfiguration.configure()
               .baseUrl(nodeBaseUrl)
               .aeternalBaseUrl(aeternalBaseUrl)
               .compile()
       )
       
  keyPairService=KeyPairServiceFactory().service

Creating a Keypair

We will cover two ways to create a KeyPair object, the first is creating a Keypair object using a mnemonic String and the other is creating a KeyPair object without a mnemonic string.

Creating A Keypair With A Mnemonic String

lets create a function called createAccountFromMnemonic that will create a keypair

  fun createAeAccountFromMnemonic(){
       mnemonicKeyPair=keyPairService.generateMasterMnemonicKeyPair(null)
       userKeyPair = EncodingUtils.createBaseKeyPair(keyPairService.generateDerivedKey(mnemonicKeyPair, true))
       Log.d("usersMnemonicSeedWord",mnemonicKeyPair.mnemonicSeedWords.joinToString{it+" "})
       Log.d("usersKeys",userKeyPair.privateKey+"\n ${userKeyPair.publicKey}")
   }

one thing to note presently is that there is a slight bug with creating a keypair with a mnemonic, you can follow the issue on github here kryptokrauts/aepp-sdk-java#126

Creating A KeyPair Without A Mnemonic String

fun createAccountWithoutMnemonic(){
 userKeyPair= keyPairService.generateBaseKeyPair()
  Log.d("usersKeys",userKeyPair.privateKey+"\n ${userKeyPair.publicKey}")
}

Then you can always recover the created Keypair by using the privateKey

 fun recoverWalletFromPrivateKey(){
 var recoverredKeyPair= keyPairService.generateBaseKeyPairFromSecret(userKeyPair.privateKey);
  Log.d("usersKeys",recoverredKeyPair.privateKey+"\n ${recoverredKeyPair.publicKey}")
 }

Transfer Tokens

Next we will create a function called sendAe that will receive as input two Strings, one for the publicAddress and the other for the amount

  fun sendAe(var pAddrToSend:String,var amount:String){
     var account = aeternityService.accounts.blockingGetAccount(Optional.of(pAddrToSend))
      var spendTx = SpendTransactionModel.builder().amount(UnitConversionUtil.toAettos(amount, UnitConversionUtil.Unit.AE).toBigInteger()).nonce(account.nonce.add(
          BigInteger.ONE)).recipient(
         pAddrToSend).sender(userKeyPair.publicKey).ttl(BigInteger.ZERO).build()
      var result = aeternityService.transactions.blockingPostTransaction(spendTx, userKeyPair.privateKey)
  
  }

Notice how we use the privateKey field on the userKeyPair object to access the private key of the already created user so as to send tokens from his wallet

Checking Wallet Balance

Lastly we will create a function to get the balance of a user using his public address

fun getAeBalance(var publicAddress:String){

        var account= aeternityService.accounts.blockingGetAccount(Optional.of(publicAddress))

        Thread({
            var account= aeternityService.accounts.blockingGetAccount(Optional.of(userKeyPair.publicKey));
            Log.d("balance","Balance is ${account.balance}")
        }).start()
    }

Things To Note

  • This tutorial was done using the gradle pugin 3.6.0 and Gradle build tools version 5.4.6
  • BigDecimal and BigInteger are used instead of double, float and int for a higher precision while making calculation
  • It is important to remember to put most of this operations in another thread using Coroutines, Thread or AsyncTasks
  • You can always check out the documentation here https://kryptokrauts.gitbook.io/aepp-sdk-java to understand any class better