CB-11292 fix broken MessageChannel after plugins are recreated

This closes #307

framework/src/org/apache/cordova/CordovaWebViewImpl.java

@@ -135,6 +135,7 @@ public class CordovaWebViewImpl implements CordovaWebView {
         if (recreatePlugins) {
             // Don't re-initialize on first load.
             if (loadedUrl != null) {
+                appPlugin = null;
                 pluginManager.init();
             }
             loadedUrl = url;

test/androidTest/src/org/apache/cordova/test/MessageChannelMultiPageTest.java

@@ -0,0 +1,110 @@
+package org.apache.cordova.test;
+/*
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ *
+*/
+
+
+import android.view.KeyEvent;
+import android.view.inputmethod.BaseInputConnection;
+
+import org.apache.cordova.CallbackContext;
+import org.apache.cordova.CordovaWebViewImpl;
+import org.apache.cordova.PluginManager;
+
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+
+public class MessageChannelMultiPageTest extends BaseCordovaIntegrationTest {
+    private static final String START_URL = "file:///android_asset/www/backbuttonmultipage/index.html";
+
+    @Override
+    public void setUp() throws Exception {
+        super.setUp();
+        setUpWithStartUrl(START_URL);
+    }
+
+    //test that after a page load the cached callback id and the live callback id match
+    //this is to prevent a regression
+    //the issue was that CordovaWebViewImpl's cached instance of CoreAndroid would become stale on page load
+    //this is because the cached instance was not being cleared when the pluginManager was reset on page load
+    //the plugin manager would get a new instance which would be updated with a new callback id
+    //the cached instance's message channel callback id would become stale
+    //effectively this caused message channel events to not be delivered
+    public void testThatCachedCallbackIdIsValid() throws Throwable {
+        Class cordovaWebViewImpl = CordovaWebViewImpl.class;
+        //send a test event - this initializes cordovaWebViewImpl.appPlugin (the cached instance of CoreAndroid)
+        Method method = cordovaWebViewImpl.getDeclaredMethod("sendJavascriptEvent", String.class);
+        method.setAccessible(true);
+        method.invoke(cordovaWebView, "testEvent");
+        sleep(1000);
+
+        //load a page - this resets the plugin manager and nulls cordovaWebViewImpl.appPlugin
+        //(previously this resets plugin manager but did not null cordovaWebViewImpl.appPlugin, leading to the issue)
+        runTestOnUiThread(new Runnable() {
+            public void run() {
+                cordovaWebView.loadUrl(START_URL);
+            }
+        });
+        assertEquals(START_URL, testActivity.onPageFinishedUrl.take());
+
+        //send a test event - this initializes cordovaWebViewImpl.appPlugin (the cached instance of CoreAndroid)
+        method.invoke(cordovaWebView, "testEvent");
+        sleep(1000);
+
+        //get reference to package protected class CoreAndroid
+        Class coreAndroid = Class.forName("org.apache.cordova.CoreAndroid");
+
+        //get cached CoreAndroid
+        Field appPluginField = cordovaWebViewImpl.getDeclaredField("appPlugin");
+        appPluginField.setAccessible(true);
+        Object cachedAppPlugin = appPluginField.get(cordovaWebView);
+        //get cached CallbackContext
+        Field messageChannelField = coreAndroid.getDeclaredField("messageChannel");
+        messageChannelField.setAccessible(true);
+        CallbackContext cachedCallbackContext = (CallbackContext) messageChannelField.get(cachedAppPlugin);
+
+        //get live CoreAndroid
+        PluginManager pluginManager = MessageChannelMultiPageTest.this.cordovaWebView.getPluginManager();
+        Field coreAndroidPluginNameField = coreAndroid.getField("PLUGIN_NAME");
+        String coreAndroidPluginName = (String) coreAndroidPluginNameField.get(null);
+        Object liveAppPlugin = pluginManager.getPlugin(coreAndroidPluginName);
+        //get live CallbackContext
+        CallbackContext liveCallbackContext = (CallbackContext) messageChannelField.get(liveAppPlugin);
+
+        //get callback id from live callbackcontext
+        String liveCallbackId = (liveCallbackContext != null) ? liveCallbackContext.getCallbackId() : null;
+        //get callback id from cached callbackcontext
+        String cachedCallbackId = (cachedCallbackContext != null) ? cachedCallbackContext.getCallbackId() : null;
+
+        //verify that the live message channel has been initialized
+        assertNotNull(liveCallbackId);
+        //verify that the cached message channel and the live message channel have the same id
+        assertEquals(liveCallbackId, cachedCallbackId);
+    }
+
+    private void sleep(int timeout) {
+        try {
+            Thread.sleep(timeout);
+        } catch (InterruptedException e) {
+            fail("Unexpected Timeout");
+        }
+    }
+
+}