Using UIScreen to drive a VGA display - doesn't seem to show the UIWindow?

问题

HI there,

I'm trying to use UIScreen to drive a separate screen with the VGA dongle on my iPad.

Here's what I've got in my root view controller's viewDidLoad:

//Code to detect if an external display is connected to the iPad.
 NSLog(@"Number of screens: %d", [[UIScreen screens]count]);

 //Now, if there's an external screen, we need to find its modes, itereate through them and find the highest one. Once we have that mode, break out, and set the UIWindow.

 if([[UIScreen screens]count] > 1) //if there are more than 1 screens connected to the device
 {
  CGSize max;
  UIScreenMode *maxScreenMode;
  for(int i = 0; i < [[[[UIScreen screens] objectAtIndex:1] availableModes]count]; i++)
  {
   UIScreenMode *current = [[[[UIScreen screens]objectAtIndex:1]availableModes]objectAtIndex:i];
   if(current.size.width > max.width);
   {
    max = current.size;
    maxScreenMode = current;
   }
  }
  //Now we have the highest mode. Turn the external display to use that mode.
  UIScreen *external = [[UIScreen screens] objectAtIndex:1];
  external.currentMode = maxScreenMode;
  //Boom! Now the external display is set to the proper mode. We need to now set the screen of a new UIWindow to the external screen
  external_disp = [externalDisplay alloc];
  external_disp.drawImage = drawViewController.drawImage;
  UIWindow *newwindow = [UIWindow alloc];
  [newwindow addSubview:external_disp.view];
  newwindow.screen = external;
 }

回答1:

You need to init your window...

 UIWindow *newwindow = [[UIWindow alloc] init];

回答2:

[newwindow makeKeyAndVisible];?

回答3:

I think your problem is the externalDisplay. Create a viewcontroller outside your code (maybe do a simply Add new file ViewController and put stuff in the .xib) and try it out to make sure that viewcontroller is working before you call it into the external display. Here's your code with my suggested changes -- [mainViewController view] is the viewcontroller you created outside.

//Code to detect if an external display is connected to the iPad.
NSLog(@"Number of screens: %d", [[UIScreen screens]count]);

//Now, if there's an external screen, we need to find its modes, iterate
//through them and find the highest one. Once we have that mode, break out,
//and set the UIWindow.

if([[UIScreen screens]count] > 1) //if there are more than 1 screens connected
                                  //to the device
{
 CGSize max;
 UIScreenMode *maxScreenMode;
 for(int i = 0; i < [[[[UIScreen screens] objectAtIndex:1] availableModes]count]; i++)
 {
  UIScreenMode *current = [[[[UIScreen screens]objectAtIndex:1]availableModes]objectAtIndex:i];
  if(current.size.width > max.width);
  {
   max = current.size;
   maxScreenMode = current;
  }
 }
 //Now we have the highest mode. Turn the external display to use that mode.
 UIScreen *external = [[UIScreen screens] objectAtIndex:1];
 external.currentMode = maxScreenMode;
 //Boom! Now the external display is set to the proper mode. We need to now
 //set the screen of a new UIWindow to the external screen

 UIWindow *newwindow = [UIWindow alloc];

 [newwindow addSubview:[mainViewController view]];
 newwindow.screen = external;

 [newwindow makeKeyAndVisible];
 [newwindow setHidden:NO];
}

回答4:

Just recording this here in case anyone stumbles on this question. I wasn't able to get anything to show up on the second screen until I realized that my app delegate had to retain the UIWindow. It has no natural owner, so if you just do a regular autorelease, the window will be released before it ever displays.

Hope that helps.

回答5:

I've uploaded sample .xcodeproj into github.

I refered to this page mainly.

Thanks a lot. :)

http://github.com/igaiga/iPadDisplayOutSample

回答6:

It needs to be mentioned that the code provided on this page and on the github link by igaiga is merely meant to "move" (NOT CLONE) the view that would normally be on the iPad (or other device).

If you need to clone (aka Mirror) the view and refresh its contents this link is more suitable: http://www.touchcentric.com/blog/archives/123

I hope this helps to clarify the use cases for both sets of code for users just starting to integrate video out capabilities into existing apps.

来源：https://stackoverflow.com/questions/2660473/using-uiscreen-to-drive-a-vga-display-doesnt-seem-to-show-the-uiwindow