How to test for the existance of a constant when creating a iPhone universal binary

Question

I am trying to create a universal binary that supports multitasking on the iPhone 4 and can still run on the iPad.

I know how to avoid compile errors for different versions of the iPhone IOS by checking if a class exists by using NSClassFromString and "respondToSelector", but is there a way to check for the existence of constants like UIBackgroundTaskInvalid?

I of course can use #IFDEF, but I want to avoid this.

Answer 1

You do it as follows:

if (NULL != &UIBackgroundTaskInvalid) {
   //do multitasking stuff here
} else {
   // don't do multitasking stuff here.
}

Basically you want to verify that the address of that variable exists.

Update: To be clear, you can't really use an #ifdef for this since you will build with a version of the SDK that contains the symbols.

Answer 2

It's probably sufficient to test for the existence of a method that you know is associated with the constant.

Answer 3

The preferred method to check for multitasking iOS is to see if UIDevice responds to isMultitaskingSupported, like this:

//----------------------------------------------------------------------
// returns YES if multitasking is available (iOS 4.0 or >)
//----------------------------------------------------------------------
BOOL hasMultitasking() 
{
    UIDevice* device = [UIDevice currentDevice];
    if ([device respondsToSelector:@selector(isMultitaskingSupported)]) {
        return [device isMultitaskingSupported];
    }
    return NO;
}

If multitasking is supported then you can use those multitasking related constants, which you should have weak linked to.