I have looked several links and read the Apple Documentation but I don't get any step-wise instructions to mirror the content of app on an external device. All they explain is how to display a new window for an external screen if recognized.
In my app, I just need to display the current screen being shown in the app on an airplay enabled device. There is a airplay button, clicking on which will check for available external screens and display the content present on the device on that screen.
Well the reason is simple. AirPlay mirroring is enabled by the user from the settings as @Meera mentioned below. The idea to use in code app is to either display selective things (audio/video) on the TV via AirPlay or to use it as a second Window, where the user does see other things. However if you want you can simply send the whole view to the external screen using this code:
// Check for external screen and if found send output there
if ([[UIScreen screens] count] > 1) {
UIScreen *externalScreen = [[UIScreen screens] objectAtIndex:1];
NSArray *screenModes = externalScreen.availableModes;
//set max resolution
externalScreen.currentMode = [screenModes lastObject];
self.window.screen = externalScreen;
}