Merge pull request #2 from BaldyAsh/erc20fix

Fixed erc20 precompiled contract

Author: TonioMacaronio

web3swift/PrecompiledContracts/ERC20/Web3+ERC20.swift

@@ -9,10 +9,19 @@ import BigInt
 import EthereumAddress
 import PromiseKit
 
+protocol IERC20 {
+    func getBalance(account: EthereumAddress) throws -> BigUInt
+    func getAllowance(originalOwner: EthereumAddress, delegate: EthereumAddress) throws -> BigUInt
+    func transfer(from: EthereumAddress, to: EthereumAddress, amount: String) throws -> WriteTransaction
+    func transferFrom(from: EthereumAddress, to: EthereumAddress, originalOwner: EthereumAddress, amount: String) throws -> WriteTransaction
+    func setAllowance(from: EthereumAddress, to: EthereumAddress, newAmount: String) throws -> WriteTransaction
+    func approve(from: EthereumAddress, spender: EthereumAddress, amount: String) throws -> WriteTransaction
+}
+
 // This namespace contains functions to work with ERC20 tokens.
 // variables are lazyly evaluated or global token information (name, ticker, total supply)
 // can be imperatively read and saved
-public class ERC20 {
+public class ERC20: IERC20 {
 
     @available(*, deprecated, renamed: "transactionOptions")
     public var options: Web3Options = .init()
@@ -99,7 +108,7 @@ public class ERC20 {
         }.wait()
     }
 
-    func getBalance(account: EthereumAddress) throws -> BigUInt{
+    public func getBalance(account: EthereumAddress) throws -> BigUInt {
         let contract = self.contract
         var transactionOptions = TransactionOptions()
         transactionOptions.callOnBlock = .latest
@@ -185,5 +194,26 @@ public class ERC20 {
         return tx
     }
 
+    public func approve(from: EthereumAddress, spender: EthereumAddress, amount: String) throws -> WriteTransaction {
+        let contract = self.contract
+        var basicOptions = TransactionOptions()
+        basicOptions.from = from
+        basicOptions.callOnBlock = .latest
+        
+        // get the decimals manually
+        let callResult = try contract.read("decimals", transactionOptions: basicOptions)!.call()
+        var decimals = BigUInt(0)
+        guard let dec = callResult["0"], let decTyped = dec as? BigUInt else {
+            throw Web3Error.inputError(desc: "Contract may be not ERC20 compatible, can not get decimals")}
+        decimals = decTyped
+        
+        let intDecimals = Int(decimals)
+        guard let value = Web3.Utils.parseToBigUInt(amount, decimals: intDecimals) else {
+            throw Web3Error.inputError(desc: "Can not parse inputted amount")
+        }
+        
+        let tx = contract.write("approve", parameters: [spender, value] as [AnyObject], transactionOptions: basicOptions)!
+        return tx
+    }
 
 }