Implement OpenGL backend in Android viewer app

tools/viewer/Viewer.cpp

 /*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #include "Viewer.h"
 
 #include "GMSlide.h"
@@ skipping 404 matching lines @@
     Json::Value allSlideNames(Json::arrayValue);
     for(auto slide : fSlides) {
         allSlideNames.append(Json::Value(slide->getName().c_str()));
     }
     slideState[kOptions] = allSlideNames;
 
     // This state is currently a demo for the one without options.
     // We will be able to change the backend too.
     Json::Value backendState(Json::objectValue);
     backendState[kName] = kBackendStateName;
-    backendState[kValue] = fBackendType == sk_app::Window::kVulkan_BackendType ?
-            "Vulkan" : "Other than Vulkan";
+    backendState[kValue] = kBackendTypeStrings[fBackendType];
     backendState[kOptions] = Json::Value(Json::arrayValue);
+    for(auto str : kBackendTypeStrings) {
+        backendState[kOptions].append(Json::Value(str));
+    }
 
     Json::Value state(Json::arrayValue);
     state.append(slideState);
     state.append(backendState);
 
     fWindow->setUIState(state);
 }
 
 void Viewer::onUIStateChanged(const SkString& stateName, const SkString& stateValue) {
-    // Currently, we only recognize the Slide state
+    // For those who will add more features to handle the state change in this function:
+    // After the change, please call updateUIState no notify the frontend (e.g., Android app).
+    // For example, after slide change, updateUIState is called inside setupCurrentSlide;
+    // after backend change, updateUIState is called in this function.
     if (stateName.equals(kSlideStateName)) {
         int previousSlide = fCurrentSlide;
         fCurrentSlide = 0;
         for(auto slide : fSlides) {
             if (slide->getName().equals(stateValue)) {
                 setupCurrentSlide(previousSlide);
                 break;
             }
             fCurrentSlide++;
         }
         if (fCurrentSlide >= fSlides.count()) {
             fCurrentSlide = previousSlide;
             SkDebugf("Slide not found: %s", stateValue.c_str());
         }
+    } else if (stateName.equals(kBackendStateName)) {
+        for (int i = 0; i < sk_app::Window::kBackendTypeCount; i++) {
+            if (stateValue.equals(kBackendTypeStrings[i])) {
+                if (fBackendType != i) {
+                    fBackendType = (sk_app::Window::BackendType)i;
+                    fWindow->detach();
+                    fWindow->attach(fBackendType, DisplayParams());
+                    fWindow->inval();
+                    updateTitle();

[scroggo, 2016/06/02 18:58:37]

nit:
this->updateTitle();
this->updateUIState();

+                    updateUIState();
+                }
+                break;
+            }
+        }
     } else {
         SkDebugf("Unknown stateName: %s", stateName.c_str());
     }
 }