Implement OpenGL backend in Android viewer app

tools/viewer/sk_app/android/Window_android.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 "Window_android.h"
-
-#include "VulkanWindowContext_android.h"
+#include "../GLWindowContext.h"
+#include "../VulkanWindowContext.h"
 
 namespace sk_app {
 
 Window* Window::CreateNativeWindow(void* platformData) {
     Window_android* window = new Window_android();
     if (!window->init((SkiaAndroidApp*)platformData)) {
         delete window;
         return nullptr;
     }
     return window;
@@ skipping 18 matching lines @@
 
 void Window_android::setTitle(const char* title) {
     fSkiaAndroidApp->setTitle(title);
 }
 
 void Window_android::setUIState(const Json::Value& state) {
     fSkiaAndroidApp->setUIState(state);
 }
 
 bool Window_android::attach(BackendType attachType, const DisplayParams& params) {
-    if (kVulkan_BackendType != attachType) {
+    if (kVulkan_BackendType != attachType && kNativeGL_BackendType != attachType) {

[jvanverth1, 2016/06/02 17:17:36]

You can probably remove this block -- the conditional always evaluates to false.

[liyuqian, 2016/06/02 18:47:13]

Done.

         return false;
     }
 
+    fBackendType = attachType;
     fDisplayParams = params;
 
-    // We delay the creation of fTestContext until Android informs us that
+    if (fNativeWindow) {
+        initDisplay(fNativeWindow);

[scroggo, 2016/06/02 18:25:17]

nit: Skia code calls member functions using "this", e.g.

this->initDisplay(fNativeWindow);

[liyuqian, 2016/06/02 18:47:13]

Done.

+    }
+    // If fNativeWindow is not set,
+    // we delay the creation of fWindowContext until Android informs us that

[scroggo, 2016/06/02 18:25:16]

When/where does that happen? Should we return false here?

[liyuqian, 2016/06/02 18:47:13]

It happens in initDisplay, which is initiated by kSurfaceCreated event. Derek
originally wrote this function by returning true so I just followed his design.
I'll update the comment to make it more clear.

     // the native window is ready to use.
     return true;
 }
 
 void Window_android::initDisplay(ANativeWindow* window) {
     SkASSERT(window);
+    fNativeWindow = window;
     ContextPlatformData_android platformData;
     platformData.fNativeWindow = window;
-    fWindowContext = VulkanWindowContext::Create((void*)&platformData, fDisplayParams);
+    switch (fBackendType) {
+        case kNativeGL_BackendType:
+            fWindowContext = GLWindowContext::Create((void*)&platformData, fDisplayParams);
+            break;
+
+        case kVulkan_BackendType:
+        default:

[scroggo, 2016/06/02 18:25:16]

Why does default create a vulkan backend?

[liyuqian, 2016/06/02 18:47:13]

Because that's the first backend in Android viewer app?

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

It looks like there are only two enum values. Can you just remove "default"
entirely? That way, if we add another enum, the compiler will catch us if we
forget to add another case.

Another option would be to default to SkASSERT(false); I think you only want to
create a vulkan backend for kVulkan_BackendType

+            fWindowContext = VulkanWindowContext::Create((void*)&platformData, fDisplayParams);
+            break;
+    }
 }
 
 void Window_android::onDisplayDestroyed() {
     detach();
+    fNativeWindow = nullptr;
 }
 
 void Window_android::onInval() {
     fSkiaAndroidApp->postMessage(Message(kContentInvalidated));
 }
 
 void Window_android::paintIfNeeded() {
     if (fWindowContext) { // Check if initDisplay has already been called
         onPaint();
     } else {
         markInvalProcessed();
     }
 }
 
 }   // namespace sk_app