iOS 4.2

Printing

iOS now incorporates support for wireless printing from iPhone and iPad applications. For the most part, the objects provided by UIKit do all of the heavy lifting associated with printing. They manage the printing interfaces, work with your application to render the printable content, and handle the scheduling and execution of print jobs on the printer.

Print jobs submitted by your application are handed off to the printing system, which manages the actual printing process. Print jobs from all applications on a device are queued and printed on a first-come, first-served basis. Users can get the status of print jobs from the Print Center application and can even use that application to cancel print jobs. All other aspects of printing are handled for you automatically by the system.

To simplify the printing process, UIKit provides automatic support for some types of content to make printing easier. Specifically, applications provide printable content in one of three ways:

• An application can hand images or PDF content directly to UIKit and let it manage the printing of those items.

• An application can use print formatter objects to lay out specific types of content over multiple pages. UIKit provides print formatters for plain text, HTML text, and some system views (web views, text views, and map views).

• An application can choose to render the content itself using a print page renderer object. This option gives you the most control over your printed content but also lets you use print formatters to assist with the layout of parts of your content.

For information about how to incorporate the new printing support into your applications, see “Printing” in Drawing and Printing Guide for iOS.

AirPlay

AirPlay is a technology that lets your application stream audio to Apple TV and to third-party AirPlay speakers and receivers. AirPlay support is built in to the AV Foundation framework and the Core Audio family of frameworks. Any audio content you play using these frameworks is automatically made eligible for AirPlay distribution. Once the user chooses to play your audio using AirPlay, it is routed automatically by the system.

For information on how to play audio content, see Multimedia Programming Guide.

Core MIDI

The Core MIDI framework (CoreMIDI.framework) provides a standard way to communicate with MIDI devices, including hardware keyboards and synthesizers. You use this framework to send and receive MIDI messages and to interact with MIDI peripherals connected to an iOS-based device using the dock connector or network.

For more information about using this framework, see Core MIDI Framework Reference.

Weak Linking Support

Applications that link against iOS SDK 4.2 or later can now take advantage of new support for determining the availability of Objective-C classes. System frameworks now tag their classes with availability information that the compiler can use to link weakly to those classes as needed. This new tagging mechanism simplifies the code you need to use to do runtime checking for the availability of the class. Whereas previously, you needed to use the NSClassFromString function to determine whether a class was present, now you can simply check for the existence the class directly using code similar to the following:

if ([UIPrintInteractionController class])

{

   // Create an instance of the class and use it.

}

else

{

   // The print interaction controller is not available.

}

In order to use this feature, you must do the following:

• Build your project using LLVM and Clang. (The standalone GCC compiler does not currently support this feature.)

• Set your project’s deployment target to iOS 3.1 or later.

• Change the linking option for each framework you want to link weakly. To do this, open an inspector for your application target and go to the inspector’s General tab to view the list of frameworks and libraries linked against the target. For frameworks you want to link weakly, change the value in the Type column from Required to Weak.

  If the deployment target for your application is set to a version of iOS that does not contain a given framework (because it was introduced in a later version), set the link type of that framework to Weak. For most other frameworks, you should leave the link type set to Required.

For information about how to perform runtime availability checks for classes and methods, see SDK Compatibility Guide.

Framework Enhancements

The following sections highlight the significant changes to frameworks and technologies in iOS 4.2. For a complete list of all new interfaces available in the system, see iOS 4.2 API Diffs.