feat(android): add interface for messagehandler

android/capacitor/src/main/java/com/getcapacitor/IMessageHandler.java

@@ -0,0 +1,20 @@
+package com.getcapacitor;
+
+public interface IMessageHandler {
+    /**
+     * Posts a message to a JavaScript context. It is expected to be a stringified JSON representation
+     *
+     * @param message The stringified json to be posted.
+     */
+    void postMessage(String message);
+
+    /**
+     * Sends a response message.
+     *
+     * @param call the plugin call, which is attached to the communication
+     * @param successResult the result of the native execution
+     * @param errorResult the error, that might have occured
+     *
+     */
+    void sendResponseMessage(PluginCall call, PluginResult successResult, PluginResult errorResult);
+}

android/capacitor/src/main/java/com/getcapacitor/MessageHandler.java

@@ -11,7 +11,7 @@
  * MessageHandler handles messages from the WebView, dispatching them
  * to plugins.
  */
-public class MessageHandler {
+public class MessageHandler implements IMessageHandler {
 
     private Bridge bridge;
     private WebView webView;
@@ -49,6 +49,7 @@ public MessageHandler(Bridge bridge, WebView webView, PluginManager cordovaPlugi
      */
     @JavascriptInterface
     @SuppressWarnings("unused")
+    @Override
     public void postMessage(String jsonStr) {
         try {
             JSObject postData = new JSObject(jsonStr);
@@ -98,6 +99,7 @@ public void postMessage(String jsonStr) {
         }
     }
 
+    @Override
     public void sendResponseMessage(PluginCall call, PluginResult successResult, PluginResult errorResult) {
         try {
             PluginResult data = new PluginResult();

android/capacitor/src/main/java/com/getcapacitor/PluginCall.java

@@ -19,7 +19,7 @@ public class PluginCall {
      */
     public static final String CALLBACK_ID_DANGLING = "-1";
 
-    private final MessageHandler msgHandler;
+    private final IMessageHandler msgHandler;
     private final String pluginId;
     private final String callbackId;
     private final String methodName;
@@ -33,7 +33,7 @@ public class PluginCall {
     @Deprecated
     private boolean isReleased = false;
 
-    public PluginCall(MessageHandler msgHandler, String pluginId, String callbackId, String methodName, JSObject data) {
+    public PluginCall(IMessageHandler msgHandler, String pluginId, String callbackId, String methodName, JSObject data) {
         this.msgHandler = msgHandler;
         this.pluginId = pluginId;
         this.callbackId = callbackId;