Add provider-engine provider supporting signing

provider-engine.js

@@ -0,0 +1,24 @@
+'use strict'
+
+const inherits = require('util').inherits
+const HookedWalletEthTxSubprovider = require('web3-provider-engine/subproviders/hooked-wallet-ethtx')
+
+module.exports = WalletSubprovider
+
+inherits(WalletSubprovider, HookedWalletEthTxSubprovider)
+
+function WalletSubprovider (wallet, opts) {
+  opts.getAccounts = function (cb) {
+    cb(null, [ wallet.getAddressesString() ])
+  }
+
+  opts.getPrivateKey = function (address, cb) {
+    if (address !== wallet.getAddressString()) {
+      return cb('Account not found')
+    }
+
+    cb(null, wallet.getPrivateKey())
+  }
+
+  WalletSubprovider.super_.call(this, opts)
+}