[API] Fixed window rendering on resize and white background on start up. (#62)

* Fixed window rendering on resize and white background on start up.

* Code formatting and clean up.

* Fixed attribute modifier.

imgui-app/src/main/java/imgui/app/Window.java

@@ -8,6 +8,7 @@ import org.lwjgl.glfw.Callbacks;
 import org.lwjgl.glfw.GLFW;
 import org.lwjgl.glfw.GLFWErrorCallback;
 import org.lwjgl.glfw.GLFWVidMode;
+import org.lwjgl.glfw.GLFWWindowSizeCallback;
 import org.lwjgl.opengl.GL;
 import org.lwjgl.opengl.GL32;
 import org.lwjgl.system.MemoryStack;
@@ -22,6 +23,7 @@ import java.util.Objects;
  * When extended, life-cycle methods should be called manually.
  */
 public abstract class Window {
+
     private final ImGuiImplGlfw imGuiGlfw = new ImGuiImplGlfw();
     private final ImGuiImplGl3 imGuiGl3 = new ImGuiImplGl3();
 
@@ -90,14 +92,26 @@ public abstract class Window {
         }
 
         GLFW.glfwMakeContextCurrent(handle);
+
+        GL.createCapabilities();
+
         GLFW.glfwSwapInterval(GLFW.GLFW_TRUE);
-        GLFW.glfwShowWindow(handle);
 
         if (config.isFullScreen()) {
             GLFW.glfwMaximizeWindow(handle);
+        } else {
+            GLFW.glfwShowWindow(handle);
         }
 
-        GL.createCapabilities();
+        clearBuffer();
+        renderBuffer();
+
+        GLFW.glfwSetWindowSizeCallback(handle, new GLFWWindowSizeCallback() {
+            @Override
+            public void invoke(final long window, final int width, final int height) {
+                runFrame();
+            }
+        });
     }
 
     private void decideGlGlslVersions() {
@@ -141,26 +155,40 @@ public abstract class Window {
      */
     protected void run() {
         while (!GLFW.glfwWindowShouldClose(handle)) {
-            startFrame();
-            preProcess();
-            process();
-            postProcess();
-            endFrame();
+            runFrame();
         }
     }
 
+    /**
+     * Method used to run the next frame.
+     */
+    protected void runFrame() {
+        startFrame();
+        preProcess();
+        process();
+        postProcess();
+        endFrame();
+    }
+
     /**
      * Method to be overridden by user to provide main application logic.
      */
     public abstract void process();
 
+    /**
+     * Method used to clear the OpenGL buffer.
+     */
+    private void clearBuffer() {
+        GL32.glClearColor(colorBg.getRed(), colorBg.getGreen(), colorBg.getBlue(), colorBg.getAlpha());
+        GL32.glClear(GL32.GL_COLOR_BUFFER_BIT | GL32.GL_DEPTH_BUFFER_BIT);
+    }
+
     /**
      * Method called at the beginning of the main cycle.
      * It clears OpenGL buffer and starts an ImGui frame.
      */
     protected void startFrame() {
-        GL32.glClearColor(colorBg.getRed(), colorBg.getGreen(), colorBg.getBlue(), colorBg.getAlpha());
-        GL32.glClear(GL32.GL_COLOR_BUFFER_BIT | GL32.GL_DEPTH_BUFFER_BIT);
+        clearBuffer();
         imGuiGlfw.newFrame();
         ImGui.newFrame();
     }
@@ -180,6 +208,13 @@ public abstract class Window {
             GLFW.glfwMakeContextCurrent(backupWindowPtr);
         }
 
+        renderBuffer();
+    }
+
+    /**
+     * Method to render the OpenGL buffer and poll window events.
+     */
+    private void renderBuffer() {
         GLFW.glfwSwapBuffers(handle);
         GLFW.glfwPollEvents();
     }