Add onChangeAccount callback and selectedAccount context variable

src/react-web3/Web3Provider.js

@@ -1,24 +1,88 @@
-import React, { Component } from 'react';
+import React, { Component } from 'react'
 import PropTypes from 'prop-types'
+import Web3 from 'web3'
+
+const ONE_SECOND = 1000;
 
 class Web3Provider extends Component {
+  constructor(props, context) {
+    super(props, context)
+
+    this.state = {
+      selectedAccount: null
+    }
+
+    this.web3 = null
+    this.interval = null
+
+  }
   render() {
+    const { web3UnavailableScreen: Web3UnavailableScreen } = this.props
+
     if (window.web3) {
+      if (!this.web3) {
+        this.web3 = new Web3(window.web3.currentProvider);
+        this.fetchAccounts()
+      }
+
       return this.props.children
     }
 
-    return <this.props.web3UnavailableScreen />
+    return <Web3UnavailableScreen />
+  }
+
+  componentDidMount() {
+    this.initPoll();
+  }
+
+  initPoll = () => {
+    if (!this.interval) {
+      this.interval = setInterval(this.fetchAccounts, ONE_SECOND)
+    }
+  }
+
+  fetchAccounts = () => {
+    const { web3  } = this
+    const { onChangeAccount } = this.props
+
+    if (!web3 || !web3.eth) {
+      return
+    }
+
+    return web3.eth.getAccounts()
+      .then(accounts => {
+        if (!accounts || !accounts.length) {
+          return
+        }
+
+        let curr = this.state.selectedAccount
+        let next = accounts[0]
+        curr = curr && curr.toLowerCase()
+        next = next && next.toLowerCase()
+
+        const didChange = curr && next && (curr !== next)
+
+        if (didChange && typeof onChangeAccount === 'function') {
+          onChangeAccount(next)
+        }
+
+        this.setState({
+          selectedAccount: next || null
+        })
+      })
   }
 
   getChildContext() {
     return {
-      web3: window.web3
+      web3: this.web3,
+      selectedAccount: this.state.selectedAccount
     }
   }
 }
 
 Web3Provider.childContextTypes = {
-  web3: PropTypes.object
+  web3: PropTypes.object,
+  selectedAccount: PropTypes.string
 }
 
 export default Web3Provider;