iOS and multiple OpenGL views

Okay, I'm going to answer my own question since I finally found what was going on. It was a small missing line that was causing all those problems.

Basically, to have multiple OpenGL views displayed at the same time, you need :

• Either, the same context for every view. Here, you have to take care not to draw with multiple threads at the same time (i.e. lock the context somehow, as explained on this answer. And you have to re-bind the frame- and render-buffers every time on each frame.
• Or, you can use different contexts for each view. Then, you have to re-set the context on each frame, because other display links, could (and would, as in my case) cause your OpenGL calls to use the wrong data. Also, there is no need for re-binding frame- and render-buffers since your context is preserved.

Also, call glFlush() after each frame, to tell the GPU to finish rendering each frame fully.

In my case (the second one), the code for rendering each frame (in iOS) looks like :

- (void) drawFrame:(CADisplayLink*)displayLink {
  // Set current context, assuming _context
  // is the class ivar for the OpenGL Context
  [EAGLContext setCurrentContext:_context]

  // Clear whatever you want
  glClear (GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);

  // Do matrix stuff
  ...
  glUniformMatrix4fv (...);

  // Set your viewport
  glViewport (0, 0, self.frame.size.width, self.frame.size.height);

  // Bind object buffers
  glBindBuffer (GL_ARRAY_BUFFER, _vertexBuffer);
  glVertexAttribPointer (_glVertexPositionSlot, 3, ...);

  // Draw elements
  glDrawArrays (GL_TRIANGLES, 0, _currentVertexCount);

  // Discard unneeded depth buffer
  const GLenum discard[] = {GL_DEPTH_ATTACHMENT};
  glDiscardFramebufferEXT (GL_FRAMEBUFFER, 1, discard);

  // Present render buffer
  [_context presentRenderbuffer:GL_RENDERBUFFER];

  // Unbind and flush
  glBindBuffer (GL_ARRAY_BUFFER, 0);
  glFlush();
}

EDIT

I'm going to edit this answer, since I found out that running multiple CADisplayLinks could cause some issues. You have to make sure to set the frameInterval property of your CADisplayLink instance to something other than 0 or 1. Else, the run loop will only have time to call the first render method, and then it'll call it again, and again. In my case, that was why only one object was moving. Now, it's set to 3 or 4 frames, and the run loop has time to call all the render methods.

This applies only to the application running on the device. The simulator, being very fast, doesn't care about such things.