How to use dispatch_async in glkview?

问题

I'm learning how to use threading with my iOS Opengl-es app. i tried to modify the opengl game template code:

- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
    glClearColor(0.65f, 0.65f, 0.65f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glBindVertexArrayOES(_vertexArray);

    // Render the object with GLKit
    [self.effect prepareToDraw];

    glDrawArrays(GL_TRIANGLES, 0, 36);

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        sleep(1);
        dispatch_async(dispatch_get_main_queue(), ^{
            // Render the object again with ES2
            glUseProgram(_program);

            glUniformMatrix4fv(uniforms[UNIFORM_MODELVIEWPROJECTION_MATRIX], 1, 0, _modelViewProjectionMatrix.m);
            glUniformMatrix3fv(uniforms[UNIFORM_NORMAL_MATRIX], 1, 0, _normalMatrix.m);

            glDrawArrays(GL_TRIANGLES, 0, 36);
        });
    });
}

It draws a box at 60fps and another at 1fps if it works well, but when i run the code above, it only draws one box.

I searched the web and can't find what is causing the problem.

Thank you very much for any help!

回答1:

The code in the dispatch block isn't going to work. By the time it gets executed, all of the OpenGL state for that frame will have long since been destroyed. If you were to put a call to glGetError() in that block, I'm sure it would tell you the same. You need to make sure that all your drawing code is done in that glkView method for the OpenGL state to be valid. When you do that dispatch, you're essentially shunting the execution of that drawing code out of the scope of that method.

来源：https://stackoverflow.com/questions/10373526/how-to-use-dispatch-async-in-glkview

标签

ios

multithreading

opengl-es