Add proxy example (#225)

* Add proxy example

* Updated after review comments

proxy-blocker/background/proxy-handler.js

@@ -0,0 +1,55 @@
+// Location of the proxy script, relative to manifest.json
+const proxyScriptURL = "proxy/proxy-script.js";
+
+// Default settings. If there is nothing in storage, use these values.
+const defaultSettings = {
+   blockedHosts: ["example.com", "example.org"]
+ }
+
+// Register the proxy script
+browser.proxy.registerProxyScript(proxyScriptURL);
+
+// Log any errors from the proxy script
+browser.proxy.onProxyError.addListener(error => {
+  console.error(`Proxy error: ${error.message}`);
+});
+
+// Initialize the proxy
+function handleInit(message) {
+  // update the proxy whenever stored settings change
+  browser.storage.onChanged.addListener((newSettings) => {
+    browser.runtime.sendMessage(newSettings.blockedHosts.newValue, {toProxyScript: true});
+  });
+
+  // get the current settings, then...
+  browser.storage.local.get()
+    .then((storedSettings) => {
+      // if there are stored settings, update the proxy with them...
+      if (storedSettings.blockedHosts) {
+        browser.runtime.sendMessage(storedSettings.blockedHosts, {toProxyScript: true});
+      // ...otherwise, initialize storage with the default values
+      } else {
+        browser.storage.local.set(defaultSettings);
+      }
+
+    })
+    .catch(()=> {
+      console.log("Error retrieving stored settings");
+    });
+}
+
+function handleMessage(message, sender) {
+  // only handle messages from the proxy script
+  if (sender.url !=  browser.extension.getURL(proxyScriptURL)) {
+    return;
+  }
+
+  if (message === "init") {
+    handleInit(message);
+  } else {
+    // after the init message the only other messages are status messages
+    console.log(message);
+  }
+}
+
+browser.runtime.onMessage.addListener(handleMessage);