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Recipe: Using External Screens

There are many ways to use external screens. Take the newest iPads, for example. The second and third generation models offer built-in screen mirroring. Attach a VGA or HDMI cable and your content can be shown on external displays and on the built-in screen. Certain devices enable you to mirror screens wirelessly to Apple TV using AirPlay, Apple’s proprietary cable-free over-the-air video solution. These mirroring features are extremely handy, but you’re not limited to simply copying content from one screen to another in iOS.

The UIScreen class enables you to detect and write to external screens independently. You can treat any connected display as a new window and create content for that display separate from any view you show on the primary device screen. You can do this for any wired screen, and starting with the iPad 2 (and later) and the iPhone 4S (and later), you can do so wirelessly using AirPlay to Apple TV 2 (and later). A third-party app called Reflector enables you to mirror your display to Mac or Windows computers using AirPlay.

Geometry is important. Here’s why. iOS devices currently include the 320×480 old-style iPhone displays, the 640×960-pixel Retina display units, and the 1024×768-pixel iPads. Typical composite/component output is produced at 720×480 pixels (480i and 480p), VGA at 1024×768 and 1280×720 (720p), and then there’s the higher quality HDMI output available as well.

Add to this the issues of overscan and other target display limitations, and Video Out quickly becomes a geometric challenge. Fortunately, Apple has responded to this challenge with some handy real-world adaptations. Instead of trying to create one-to-one correspondences with the output screen and your built-in device screen, you can build content based on the available properties of your output display. You just create a window, populate it, and display it.

If you intend to develop Video Out applications, don’t assume that your users are strictly using AirPlay. Many users still connect to monitors and projectors using old-style cable connections. Make sure you have at least one of each type of cable on-hand (composite, component, VGA, and HDMI) and an AirPlay-ready iPhone and iPad, so you can thoroughly test on each output configuration. Third-party cables (typically imported from the Far East, not branded with Made for iPhone/iPad) won’t work, so make sure you purchase Apple-branded items.

Detecting Screens

The UIScreen class reports how many screens are connected. You know that an external screen is connected whenever this count goes above 1. The first item in the screens array is always your primary device screen:

#define SCREEN_CONNECTED ([UIScreen screens].count > 1)

Each screen can report its bounds (that is, its physical dimensions in points) and its screen scale (relating the points to pixels). Two standard notifications enable you to observe when screens have been connected to and disconnected from the device.

// Register for connect/disconnect notifications
[[NSNotificationCenter defaultCenter]
    addObserver:self selector:@selector(screenDidConnect:)
    name:UIScreenDidConnectNotification object:nil];
[[NSNotificationCenter defaultCenter]
    addObserver:self selector:@selector(screenDidDisconnect:)
    name:UIScreenDidDisconnectNotification object:nil];

Connection means any kind of connection, whether by cable or via AirPlay. Whenever you receive an update of this type, make sure you count your screens and adjust your user interface to match the new conditions.

It’s your responsibility to set up windows whenever new screens are attached and tear them down upon detach events. Each screen should have its own window to manage content for that output display. Don’t hold onto windows upon detaching screens. Let them release and then re-create them when new screens appear.

Retrieving Screen Resolutions

Each screen provides an availableModes property. This is an array of resolution objects ordered from least-to-highest resolution. Each mode has a size property indicating a target pixel-size resolution. Many screens support multiple modes. For example, a VGA display might have as many as one-half dozen or more different resolutions it offers. The number of supported resolutions varies by hardware. There will always be at least one resolution available, but you should offer choices to users when there are more.

Setting Up Video Out

After retrieving an external screen object from the [UIScreens screens] array, query the available modes and select a size to use. As a rule, you can get away with selecting the last mode in the list to always use the highest possible resolution, or the first mode for the lowest resolution.

To start a Video Out stream, create a new UIWindow and size it to the selected mode. Add a new view to that window for drawing on. Then assign the window to the external screen and make it key and visible. This orders the window to display and prepares it for use. After you do that, make the original window key again. This allows the user to continue interacting with the primary screen. Don’t skip this step. Nothing makes end users more cranky than discovering their expensive device no longer responds to their touches:

self.outputWindow = [[UIWindow alloc] initWithFrame:theFrame];
outputWindow.screen = secondaryScreen;
[outputWindow makeKeyAndVisible];
[delegate.view.window makeKeyAndVisible];

Adding a Display Link

Display links are a kind of timer that synchronizes drawing to a display’s refresh rate. You can adjust this frame refresh time by changing the display link’s frameInterval property. It defaults to 1. A higher number slows down the refresh rate. Setting it to 2 halves your frame rate. Create the display link when a screen connects to your device. The UIScreen class implements a method that returns a display link object for its screen. You specify the target for the display link and a selector to call.

The display link fires on a regular basis, letting you know when to update the Video Out screen. You can adjust the interval up for less of a CPU load, but you get a lower frame rate in return. This is an important trade-off, especially for direct manipulation interfaces that require a high level of CPU response on the device side.

The code you see in Recipe 1-8 uses common modes for the run loop, providing the least latency. You invalidate your display link when you are done with it, removing it from the run loop.

Overscanning Compensation

The UIScreen class enables you to compensate for pixel loss at the edge of display screens by assigning a value to the overscanCompensation property. The techniques you can assign are described in Apple’s documentation but basically correspond to whether you want to clip content or pad it with black space.

VIDEOkit

Recipe 1-8 introduces VIDEOkit, a basic external screen client. It demonstrates all the features needed to get up and going with wired and wireless external screens. You establish screen monitoring by calling startupWithDelegate:. Pass it the primary view controller whose job it will be to create external content.

The internal init method starts listening for screen attach and detach events and builds and tears down windows as needed. An informal delegate method (updateExternalView:) is called each time the display link fires. It passes a view that lives on the external window that the delegate can draw onto as needed.

In the sample code that accompanies this recipe, the view controller delegate stores a local color value and uses it to color the external display:

- (void) updateExternalView: (UIImageView *) aView
{
    aView.backgroundColor = color;
}

- (void) action: (id) sender
{
    color = [UIColor randomColor];
}

Each time the action button is pressed, the view controller generates a new color. When VIDEOkit queries the controller to update the external view, it sets this as the background color. You can see the external screen instantly update to a new random color.

Recipe 1-8. VIDEOkit

@interface VIDEOkit : NSObject
{
    UIImageView *baseView;
}
@property (nonatomic, weak)   UIViewController *delegate;
@property (nonatomic, strong) UIWindow *outputWindow;
@property (nonatomic, strong) CADisplayLink *displayLink;
+ (void) startupWithDelegate: (id) aDelegate;
@end

@implementation VIDEOkit
static VIDEOkit *sharedInstance = nil;

- (void) setupExternalScreen
{
    // Check for missing screen
    if (!SCREEN_CONNECTED) return;

    // Set up external screen
    UIScreen *secondaryScreen = [UIScreen screens][1];
    UIScreenMode *screenMode =
        [[secondaryScreen availableModes] lastObject];
    CGRect rect = (CGRect){.size = screenMode.size};
    NSLog(@"Extscreen size: %@", NSStringFromCGSize(rect.size));

    // Create new outputWindow
    self.outputWindow = [[UIWindow alloc] initWithFrame:CGRectZero];
    _outputWindow.screen = secondaryScreen;
    _outputWindow.screen.currentMode = screenMode;
    [_outputWindow makeKeyAndVisible];
    _outputWindow.frame = rect;

    // Add base video view to outputWindow
    baseView = [[UIImageView alloc] initWithFrame:rect];
    baseView.backgroundColor = [UIColor darkGrayColor];
    [_outputWindow addSubview:baseView];

    // Restore primacy of main window
    [_delegate.view.window makeKeyAndVisible];
}

- (void) updateScreen
{
    // Abort if the screen has been disconnected
    if (!SCREEN_CONNECTED && _outputWindow)
        self.outputWindow = nil;

    // (Re)initialize if there's no output window
    if (SCREEN_CONNECTED && !_outputWindow)
        [self setupExternalScreen];

    // Abort if encountered some weird error
    if (!self.outputWindow) return;

    // Go ahead and update
    SAFE_PERFORM_WITH_ARG(_delegate,
        @selector(updateExternalView:), baseView);
}

- (void) screenDidConnect: (NSNotification *) notification
{
    NSLog(@"Screen connected");
    UIScreen *screen = [[UIScreen screens] lastObject];

    if (_displayLink)
    {
        [_displayLink removeFromRunLoop:[NSRunLoop currentRunLoop]
            forMode:NSRunLoopCommonModes];
        [_displayLink invalidate];
        _displayLink = nil;
    }

    self.displayLink = [screen displayLinkWithTarget:self
        selector:@selector(updateScreen)];
    [_displayLink addToRunLoop:[NSRunLoop currentRunLoop]
        forMode:NSRunLoopCommonModes];
}

- (void) screenDidDisconnect: (NSNotification *) notification
{
    NSLog(@"Screen disconnected.");
    if (_displayLink)
    {
        [_displayLink removeFromRunLoop:[NSRunLoop currentRunLoop]
            forMode:NSRunLoopCommonModes];
        [_displayLink invalidate];
        self.displayLink = nil;
    }
}

- (id) init
{
    if (!(self = [super init])) return self;

    // Handle output window creation
    if (SCREEN_CONNECTED)
        [self screenDidConnect:nil];

    // Register for connect/disconnect notifications
    [[NSNotificationCenter defaultCenter]
        addObserver:self selector:@selector(screenDidConnect:)
        name:UIScreenDidConnectNotification object:nil];
    [[NSNotificationCenter defaultCenter] addObserver:self
        selector:@selector(screenDidDisconnect:)
        name:UIScreenDidDisconnectNotification object:nil];

    return self;
}

- (void) dealloc
{
    [self screenDidDisconnect:nil];
    self.outputWindow = nil;
}

+ (VIDEOkit *) sharedInstance
{
    if (!sharedInstance)
        sharedInstance = [[self alloc] init];
    return sharedInstance;
}

+ (void) startupWithDelegate: (id) aDelegate
{
    [[self sharedInstance] setDelegate:aDelegate];
}
@end
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