OpenGLES:: GLKView是如何工作的

本篇通过一个AGLKView的示例来深入了解一下上篇中GLKView的工作原理，目的是为了消除GLKView、Core Animation、OpenGLES间交互的神秘感；
本例位于点击此处下载，建议将例子下下来，边看本文解析，边自己实现该例子；

+ (Class)layerClass
{
   return [CAEAGLLayer class];
}

每个UIView都有一个相关联的Core Animation层；Cocoa Touch会调用该方法决定生成一个什么样的层;
CAEAGLLayer会与一个OGE帧缓存共享它的像素仓库；CAEAGLLayer是Core Animation提供的标准层之一；

串行化(Serialization)是计算机科学中的一个概念，它是指将对象存储到介质（如文件、内存缓冲区等）中或是以二进制方式通过网络传输，在iOS中也叫做归档。

- (id)initWithCoder:(NSCoder*)coder
{    
   if ((self = [super initWithCoder:coder]))
   {
      CAEAGLLayer *eaglLayer = (CAEAGLLayer *)self.layer;      
      eaglLayer.drawableProperties = 
         [NSDictionary dictionaryWithObjectsAndKeys:
             [NSNumber numberWithBool:NO], 
             kEAGLDrawablePropertyRetainedBacking, 
             kEAGLColorFormatRGBA8, 
             kEAGLDrawablePropertyColorFormat, 
             nil];          
   }
   return self;
}

该方法是Cocoa Touch用于初始化对象的标准方法之一，Cocoa Touch会自动调用该方法，这是反归档先前归档入一个文件的对象过程的一部分；本例中AGLKView从storyboard文件中加载过程就是反归档；若用代码的话应该是调用“-initWithFrame:context:”方法；
在这两个方法中，kEAGLDrawablePropertyRetainedBacking == NO 表示不使用保留背景，告诉Core Animation层任何部分需要在屏幕上显示的时候都需要绘制整个层的内容；kEAGLColorFormatRGBA8 使用8位来保存每个像素每个颜色的值；这段代码告诉Core Animation不要试图保留任何以前绘制的图像留作以后重用；

// This method sets the receiver‘s OpenGL ES Context. If the 
// receiver already has a different Context, this method deletes
// OpenGL ES Frame Buffer resources in the old Context and the 
// recreates them in the new Context.
- (void)setContext:(EAGLContext *)aContext
{
   if(context != aContext)
   {  // Delete any buffers previously created in old Context
      [EAGLContext setCurrentContext:context];

      if (0 != defaultFrameBuffer)
      {
         glDeleteFramebuffers(1, &defaultFrameBuffer); // Step 7
         defaultFrameBuffer = 0;
      }

      if (0 != colorRenderBuffer)
      {
         glDeleteRenderbuffers(1, &colorRenderBuffer); // Step 7
         colorRenderBuffer = 0;
      }

      if (0 != depthRenderBuffer)
      {
         glDeleteRenderbuffers(1, &depthRenderBuffer); // Step 7
         depthRenderBuffer = 0;
      }

      context = aContext;

      if(nil != context)
      {  // Configure the new Context with required buffers
         context = aContext;
         [EAGLContext setCurrentContext:context];

         glGenFramebuffers(1, &defaultFrameBuffer);    // Step 1
         glBindFramebuffer(                            // Step 2
            GL_FRAMEBUFFER,             
            defaultFrameBuffer);

         glGenRenderbuffers(1, &colorRenderBuffer);    // Step 1
         glBindRenderbuffer(                           // Step 2
            GL_RENDERBUFFER, 
            colorRenderBuffer);

         // Attach color render buffer to bound Frame Buffer
         glFramebufferRenderbuffer(
            GL_FRAMEBUFFER, 
            GL_COLOR_ATTACHMENT0, 
            GL_RENDERBUFFER, 
            colorRenderBuffer);

         // Create any additional render buffers based on the
         // drawable size of defaultFrameBuffer
         [self layoutSubviews];
      }
   }
}

修改视图的上下文会导致先前创建的所有缓存全部失效，需要配置新的缓存；glFramebufferRenderbuffer()用来配置当前绑定的帧缓存以便在colorRenderBuffer中保存渲染的像素颜色；

// Calling this method tells the receiver to redraw the contents 
// of its associated OpenGL ES Frame Buffer. This method 
// configures OpenGL ES and then calls -drawRect:
- (void)display
{
   [EAGLContext setCurrentContext:self.context];
   glViewport(0, 0, (GLint)self.drawableWidth, (GLint)self.drawableHeight);

   [self drawRect:[self bounds]];

   [self.context presentRenderbuffer:GL_RENDERBUFFER];
}

// This method is called automatically whenever the receiver
// needs to redraw the contents of its associated OpenGL ES
// Frame Buffer. This method should not be called directly. Call
// -display instead which configures OpenGL ES before calling
// -drawRect:
- (void)drawRect:(CGRect)rect
{
   if(delegate)
   {
      [self.delegate glkView:self drawInRect:[self bounds]];
   }
}

设置视图的上下文为当前上下文，告诉OGE让渲染填满整个帧缓存，调用“-drawRect:”进行真正给图，然后让上下文调整外观使用Core Animation合成器把帧缓存的像素颜色渲染缓存与其他相关层混合起来；
glViewport()可以控制渲染至帧缓存的子集；

- (void)layoutSubviews
{
   CAEAGLLayer  *eaglLayer = (CAEAGLLayer *)self.layer;

   // Make sure our context is current
   [EAGLContext setCurrentContext:self.context];

   // Initialize the current Frame Buffer’s pixel color buffer 
   // so that it shares the corresponding Core Animation Layer’s
   // pixel color storage.
   [self.context renderbufferStorage:GL_RENDERBUFFER 
      fromDrawable:eaglLayer];


   if (0 != depthRenderBuffer)
   {
      glDeleteRenderbuffers(1, &depthRenderBuffer); // Step 7
      depthRenderBuffer = 0;
   }

   GLint currentDrawableWidth = (GLint)self.drawableWidth;
   GLint currentDrawableHeight = (GLint)self.drawableHeight;

   if(self.drawableDepthFormat != 
      AGLKViewDrawableDepthFormatNone &&
      0 < currentDrawableWidth &&
      0 < currentDrawableHeight)
   {
      glGenRenderbuffers(1, &depthRenderBuffer); // Step 1
      glBindRenderbuffer(GL_RENDERBUFFER,        // Step 2
         depthRenderBuffer);
      glRenderbufferStorage(GL_RENDERBUFFER,     // Step 3 
         GL_DEPTH_COMPONENT16, 
         currentDrawableWidth, 
         currentDrawableHeight);
      glFramebufferRenderbuffer(GL_FRAMEBUFFER,  // Step 4 
         GL_DEPTH_ATTACHMENT, 
         GL_RENDERBUFFER, 
         depthRenderBuffer);
   }

   // Check for any errors configuring the render buffer   
   GLenum status = glCheckFramebufferStatus(
      GL_FRAMEBUFFER) ;

   if(status != GL_FRAMEBUFFER_COMPLETE) {
       NSLog(@"failed to make complete frame buffer object %x", status);
   }

   // Make the Color Render Buffer the current buffer for display
   glBindRenderbuffer(GL_RENDERBUFFER, colorRenderBuffer);
}

任何在接收到重新调整大小的消息时，Cocoa Touch都会调用下面的layoutSubviews；上下文的”renderbufferStorage:fromDrawable”方法会调整视图的缓存尺寸以匹配层的新尺寸；

// This method returns the width in pixels of current context‘s
// Pixel Color Render Buffer
- (NSInteger)drawableWidth;
{
   GLint          backingWidth;

   glGetRenderbufferParameteriv(
      GL_RENDERBUFFER, 
      GL_RENDERBUFFER_WIDTH, 
      &backingWidth);

   return (NSInteger)backingWidth;
}


/////////////////////////////////////////////////////////////////
// This method returns the height in pixels of current context‘s
// Pixel Color Render Buffer
- (NSInteger)drawableHeight;
{
   GLint          backingHeight;

   glGetRenderbufferParameteriv(
      GL_RENDERBUFFER, 
      GL_RENDERBUFFER_HEIGHT, 
      &backingHeight);

   return (NSInteger)backingHeight;
}

通过OpenGLES的glGetRenderbufferParameteriv()方法获取和返回当前上下文的帧缓存的像素颜色渲染缓存尺寸;

接下来要说说AGLKViewController,它使用一个CADisplayLink对象来调度和执行与控制器相关联的视力的周期性的重绘；
CADisplayLink本质上是一个用于显示更新的同步计时器，它能够被设置用来在每个显示更新或者其他更新时发送一个消息；
显示更新频率通常是由嵌入设备硬件决定的，它代表一个帧缓存的内容每秒最多能够被在屏幕上通过的像素显示出来的次数；