Objective-C Fundamentals

Practical uses of runtime type introspection
175
It’s now common to want to develop an application that can use features specific to
a newer version of the iOS operating system yet still be able to run the application on a
device with an older version of the operating system that doesn’t provide the feature.
Using a number of techniques discussed in this chapter, this goal can often be achieved.
The first technique to be aware of is the macro UI_USER_INTERFACE_IDIOM, which
can be used as follows:
if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) {
... this code will only execute if the device is an iPad ...
}
The UI_USER_INTERFACE_IDIOM macro can be used to determine if the current device
is an iPad or iPhone and to conditionally change the behavior of an application to
make sure the look and feel integrates well with the slightly different usage model of
both platforms.
In other cases, a class may exist in multiple versions of the iOS software development
kit (SDK), yet have new properties or methods added to it. In these scenarios,
you can use the respondsToSelector: message to determine if the device the application
is currently running on provides the specified property or method.
As an example, the version of iOS that introduced multitasking capabilities introduced
a property named isMultitaskingSupported on the UIDevice class to determine
if the current device supports multitasking (devices such as the iPhone 3G can’t
multitask even though they can run the latest version of iOS). If, however, an iPhone
owner hasn’t updated the device to the newest iOS version, it’s possible that attempting
to access this property will cause an exception due to the class not responding to
the selector. The workaround is as follows:
UIDevice *device = [UIDevice currentDevice];
BOOL backgroundSupported = NO;
if ([device respondsToSelector:@selector(isMultitaskingSupported)])
backgroundSupported = device.multitaskingSupported;
In this scenario, the device is assumed not to support background processing, but if
the UIDevice object indicates it responds to the isMultitaskingSupported property,
you query it for the final answer.
In cases in which an entire class has been introduced in a new version of iOS, you can
check for the availability of the class by using a code sample similar to the following:
if ([UIPrintInteractionController class]) {
... make use of the UIPrintInteractionController class ...
} else {
... do something if the UIPrintInteractionController isn't available ...
}
If the UIPrintInteractionController (or any other class placed in a similar code
snippet) isn’t available, the class message responds with nil, which evaluates to false.
Otherwise, it returns a non-nil value and proceeds to execute the code in the first
block of the if statement.