{"id":3077,"date":"2014-01-05T20:54:52","date_gmt":"2014-01-05T19:54:52","guid":{"rendered":"http:\/\/t-machine.org\/?p=3077"},"modified":"2014-01-28T14:02:49","modified_gmt":"2014-01-28T13:02:49","slug":"glkit-extended-refactoring-the-view-controller","status":"publish","type":"post","link":"http:\/\/new.t-machine.org\/index.php\/2014\/01\/05\/glkit-extended-refactoring-the-view-controller\/","title":{"rendered":"GLKit Extended: Refactoring the View Controller"},"content":{"rendered":"

If you’ve been following my tutorials on OpenGL ES 2 for iOS<\/a>, by the time you finish Texturing (Part 6 or so) you’ll have a lot of code crammed into a single UIViewController. This is intentional: the tutorials are largely self-contained, and only create classes and objects where OpenGL itself uses class-like-things.<\/p>\n

…but most of the code is re-used from tutorial to tutorial, and it’s getting in the way. It’s time for some refactoring.
\n<\/p>\n

UIViewController vs GLKViewController<\/h2>\n

Recap: Both these classes are provided by Apple; GLKViewController<\/a> is a UIViewController that’s been modified for OpenGL apps. It has the simplest possible interface – a single “update” method that you override, which is called (automatically) once per frame.<\/p>\n

As I explained in the first GL ES 2 tutorial<\/a>, everything in OpenGL revolves around the Draw call. Most engines recognize this, so we’ll subclass GLKViewController and replace:
\n<\/p>\n

    \n
  1. Once per frame:<\/em> call “update”; no intelligence\n<\/ol>\n

    …with:<\/p>\n

      \n
    1. In viewDidLoad:<\/em> Correct setup of EAGLContext (required)\n
    2. In viewDidLoad:<\/em> Pre-Create all the Draw calls needed for the app\n
    3. Once per frame:<\/em> For each Draw call in the list:\n
        \n
      1. …update all the OpenGL per-Draw settings\n
      2. …update the OpenGL Shader settings\n
      3. …render the Draw call\n
      4. …reset any OpenGL state that “must” be turned off immediately after use (e.g. GL_SCISSOR)\n<\/ol>\n<\/ol>\n

        Most of this is identical for all apps – we can create a single base class “GLK2DrawCallViewController”, and put the code in there. A few items will be unique from app to app (e.g. the list of Draw calls actually rendered!); we do those with their own public methods so that you can write a subclass per-app which only overrides the methods it needs to change.<\/p>\n

        GLK2DrawCallViewController.h<\/em>:
        \n[objc]
        \n@interface GLK2DrawCallViewController : GLKViewController<\/p>\n

        @property(nonatomic,retain) EAGLContext* localContext;
        \n@property(nonatomic, retain) NSMutableArray* drawCalls;
        \n[\/objc]<\/p>\n

        This class manages the EAGLContext correctly, and populates the drawCalls – most subclasses can ignore these properties. But some special effects will require you to access the super-class’s data here.<\/p>\n

        [objc]
        \n-(NSMutableArray*) createAllDrawCalls;
        \n-(void) willRenderFrame;
        \n-(void) willRenderDrawCallUsingVAOShaderProgramAndDefaultUniforms:(GLK2DrawCall*) drawCall;
        \n@end
        \n[\/objc]<\/p>\n

        One callback to create the Draw calls – this is essential, if you don’t implement it, nothing will be drawn.<\/p>\n

        The other two are optional, although the one about “willRender….Using…Uniforms” is needed for most non-trivial shaders (any shader where a Uniform is changing from frame to frame – e.g. whenever your 3D objects are moving).<\/p>\n

        Internally, it’s all existing code that I’ve simply moved around.<\/p>\n

        Usage<\/h3>\n

        Usage is exceptionally simple: change your ViewController.h in your app project to extend the new class:<\/p>\n

        ViewController.h<\/em>:
        \n[objc]
        \n@interface ViewController : GLK2DrawCallViewController
        \n[\/objc]<\/p>\n

        …and override the special method for creating Draw calls. Move all your tutorial \/ logic there:<\/p>\n

        ViewController.h<\/em>:
        \n[objc]
        \n-(NSMutableArray*) createAllDrawCalls
        \n{
        \n\t\/** All the local setup for the ViewController *\/
        \n\tNSMutableArray* result = [NSMutableArray array];<\/p>\n

        \t\/** — Draw Call 1: clear the background
        \n\t *\/
        \n\tGLK2DrawCall* simpleClearingCall = [[GLK2DrawCall new] autorelease];
        \n\tsimpleClearingCall.shouldClearColorBit = TRUE;
        \n\t[simpleClearingCall setClearColourRed:0.5 green:0 blue:0 alpha:1];
        \n\t[result addObject: simpleClearingCall];<\/p>\n

        … etc
        \n\treturn result;
        \n}
        \n[\/objc]<\/p>\n

        Source files on Github:<\/em><\/p>\n