Question

I am working on making an existing iPhone/iPad project backwards compatible down to iPhoneOS 3.0.

My current test device is an iPod Touch with 3.1.3 on it.

The following bit of code is causing problems:

Class gestureRecognizer = NSClassFromString(@"UISwipeGestureRecognizer");

if (gestureRecognizer != nil)
{

    UISwipeGestureRecognizer * leftSwipeRecognizer = [[UISwipeGestureRecognizer alloc] initWithTarget:self 
                                                                                               action:@selector(didSwipeLeft:)];
    leftSwipeRecognizer.direction = UISwipeGestureRecognizerDirectionLeft;
    leftSwipeRecognizer.delegate = self;
    [self.view addGestureRecognizer:leftSwipeRecognizer];
    _leftSwipeRecognizer = leftSwipeRecognizer;
}

According to Apple documentation UIGestureRecognizer is defined starting from iOS 3.2. So I expect Class gestureReconizer to be nil on previous OS version and that the following if to be skipped. However it does not skip. gestureRecognizer is not nil, the code inside the if starts executing and crashes at leftSwipeRecognizer.direction because:

-[UISwipeGestureRecognizer setDirection:]: unrecognized selector sent to instance 0x1e5720

This situation is quite confusing. I guess I am doing everything by the book. I try to check if a class exists before I use it, however the class which shouldn’t be there, is there, fools my test, does not comply with its expected specs, and crashes.

I could, of course put a few respondsToSelector checks here and there to work around this crash, but it wouldn’t be an elegant way to do it.

Any other suggestions?

Answer 1

According to the UIGestureRecognizer class reference, under “Usage Special Considerations”, you actually do need to do an additional respondsToSelector check after checking if the class exists.

This is directly from the documentation:

To determine whether a class is
available at runtime in a given iOS
release, you typically check whether
the class is nil. Unfortunately, this
test is not cleanly accurate for
UIGestureRecognizer. Although this
class was publicly available starting
with iOS 3.2, it was in development a
short period prior to that. Although
the class exists in an earlier
release, use of it and other
gesture-recognizer classes are not
supported in that earlier release. You
should not attempt to use instances of
those classes.

To determine at runtime whether you
can use gesture recognizers in your
application, test whether the class
exists and, if it does, allocate an
instance and see check if it responds
to the selector locationInView:. This
method was not added to the class
until iOS 3.2. The code might look
like the following:

UIGestureRecognizer *gestureRecognizer = [[UIGestureRecognizer alloc] 
    initWithTarget:self action:@selector(myAction:)];

if (![gestureRecognizer respondsToSelector:@selector(locationInView:)]) {
    [gestureRecognizer release];
    gestureRecognizer = nil;
}
// do something else if gestureRecognizer is nil